Diagnostics of accelerator performance under the impact of electron cloud effects H. Fukuma, KEK DIPAC2005, Lyon, 6th June, 2005 1.
Download ReportTranscript Diagnostics of accelerator performance under the impact of electron cloud effects H. Fukuma, KEK DIPAC2005, Lyon, 6th June, 2005 1.
Slide 1
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 2
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 3
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 4
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 5
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 6
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 7
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 8
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 9
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 10
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 11
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 12
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 13
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 14
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 15
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 16
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 17
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 18
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 19
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 20
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 21
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 22
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 23
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 24
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 25
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 26
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 27
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 28
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 29
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 30
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 31
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 32
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 33
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 34
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 35
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 36
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 37
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 38
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 39
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 40
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 41
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 42
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 43
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 2
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 3
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 4
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 5
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 6
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 7
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 8
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 9
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 10
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 11
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 12
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 13
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 14
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 15
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 16
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 17
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 18
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 19
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 20
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 21
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 22
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 23
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 24
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 25
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 26
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 27
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 28
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 29
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 30
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 31
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 32
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 33
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 34
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 35
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 36
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 37
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 38
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 39
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 40
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 41
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 42
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.
Slide 43
Diagnostics of accelerator
performance under the impact of
electron cloud effects
H. Fukuma, KEK
DIPAC2005, Lyon, 6th June, 2005
1. Introduction
2. Characteristics of electron cloud
3. Electron cloud effects and cures
4. Diagnostics
1) Measurement of electrons
2)Beam measurement
5. Summary
6. Acknowledgments
In this talk I referred materials which appears journals, proceedings of conferences and
workshops as possible as I can.
Phys. Rev., N.I.M., EPAC, PAC, DIPAC, Two-Stream, ECLOUD02, ECLOUD04 ...
1. Introduction
Many (primary) electrons are generated in accelerators.
Electron/positron rings : photoelectrons produced by synchrotron radiation.
Proton rings : electrons produced by a proton hitting chamber wall, collimator,
and stripping foil for charge exchange injection,
phtoelectrons(i.e. LHC).
If charge of a beam is positive, primary electrons receive a kick from the
beam toward the center of beam pipe and hit the opposite wall, then
secondary electrons are produced.
Maximum secondary electron yield(SEY) ∼ 1.4 for Cu, 2 for SUS
If a condition is satisfied, amplification of electrons occur (beam induced multipacting).
Primary and secondary electrons form a group of electrons called an electron cloud.
•Long bunch proton
Trailing edge muitipacting
Captured electrons
released
secondary
electrons
Many electrons are produced at the tail of the bunch.
M. Pivi and M. Furman, at
ECLOUD02.
2. Characteristics of electron cloud
1)Spatial distribution
•Spatial distribution is strongly affected
by magnetic field.
•Electron density is typically 1012 - 1013m-3.
Simulation for SuperKEKB(e+) [upgrade plan of KEKB]
(H. Fukuma and L. F. Wang, at PAC2005.)
cloud density
two strips
peak at the center of
chamber
drift(field free)
bend
confined in the vicinity
to the chamber wall
eight peaks on the
chamber wall
quad
solenoid (60G)
2)Energy distribution
APS(e+)(measurement)
PSR(p)(measurement)
SPS(p)(measurement)
Field free - Strip pick-ups
Strip pick-up Distribution (a.u.)
Field free - Retarding Field Detector (fit)
RFD Distribution (a.u.)
Low energy (<100eV) electrons are
dominant.
Field free
0
100
80 eV
200
300
400
500
Electron Energy (eV)
600
700
800
SPS(field-free region)(simulation)
3)Time distribution
•Electron cloud is built up along a
bunch train.
•Trailing edge multipacting can occur.
•Electrons can be trapped in a quad and a
sextupole.
Trapping in quad and sextupole(simulation)
bunch
F. Zimmermann and G. Rumolo, ECLOUD02.
PSR(simulation)
trapped
electrons
bunch
L. F. Wang et al., Phys. rev. ST-AB
Time resolved measurement is
important to study the electron cloud.
M. Pivi and M. Furman, ECLOUD02.
3. Electron cloud effects and cures
1)Electron cloud have many effects on the accelerators.
•Pressure rise
by gas desorption by electron bombardment
•Electrical noise to instrumentations
•Beam induced multipacting
•Heat load on a cold chamber wall (LHC)
by electron bombardment
•Tune shifts
by Coulomb force of the electron cloud
•Coupled bunch instability
by long/medium wake force of the electron cloud
•Single bunch (strong head-tail) instability
by short range wake force of the electron cloud
Beam size blowup
2)Following cures have been taken or are under consideration.
a) Reduction of the number of primary electrons
•Ante-chamber
b) Reduction of the number of secondary electrons
•Processed chamber surface by TiN or NEG(TiZrV) coating.
•Grooved surface
•Beam scrubbing
c) Confinement electrons in the vicinity to the chamber wall
by weak solenoid field (B factories)
d) Stabilizing beam
•Bunch by bunch feedback (i.e. damper)
•Landau damping by sextupoles and octupoles.
4. Diagnostics
Characteristics of the electron clouds are studied not only by the direct
measurement of the electrons but also by the measurement of beam
behavior affected by the electron clouds.
1) Measurement of electrons
•Pressure gauge
flux of electrons
•Simple electrode
flux of electrons
•Retarding field analyzer
flux and energy distribution of electrons
•Electron sweeper
flux and energy distribution of electrons
•Strip detector
flux, spatial and energy distribution of electrons
•Microwave transmission(indirect)
flux of electrons
2)Beam measurement
a)Dipole Bunch oscillation
•Pickup electrode
•Turn by turn BPM
•Streak camera
b)Transverse beam/bunch size
•Interferometer
•Gated camera
•Streak camera
•Bunch by bunch luminosity monitor (indirect)
c)Tune
•Tune meter
1) Measurement of electrons
•Pressure gauge
Simple and spreads around a ring. Global sampling of the electron cloud in the ring
is possible.
PSR(LANL)
PEPII(e+)(SLAC)
Nonlinear pressure rise with the beam current in the
straight section of the ring.
HV probe to look at the voltage developed across a
100 kΩ resistor in series with the pump. The ion
pump pulse tracks the retarding field analyzer signal.
R.J. Macek et al., at PAC2003.
beam current
By removing the permanent magnets from the ion
pump, ions pump is sensitive only to the electrons
entering the pump from the beam chamber.
A.Kulikov et al., at ECLOUD04.
•Simple electrode
PEPII
Ante-chamber
Arc 7A Electrode
Solenoid
Electrode
beam current
SR fan
A.Kulikov et al., at ECLOUD04.
•Planar retarding field analyzer(RFA)
Pioneered at APS(ANL).
Measure flux and energy distribution of
electrons.
Retarding grid to select the electron energy.
Electrons whose energy larger than retarding
voltage are collected.
First derivative of collector current is
proportional to energy distribution.
shielding grid
retarding grid
collector
Unperturbed measurement of electron cloud
owing to shielding grid.
APS(e+)(ANL)
Graphite-coated collector in order to
lower the SEY.
Mu-metal wrapped around the tube for
shielding of magnetic fields.
Calibrated by an electron gun.
R. A. Rosenberg, at Two-Stream00.
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6, 034402 (2003)
•Transmission of electrons
Angular distribution affects to the
transmission.
deteriorate the energy
resolution.
Measured transmission for a
mono-energetic electron beams
I
dI/dV
perpendicular
injection
I
dI/dV
gun
RFA
30˚30˚
V
e-
dI/dV
monotonic increase
Al
V
R. A. Rosenberg, K. C. Harkey, N.I.M., A453(2000)
•Some data taken at APS
collector current/beam current
Beam induced multipacting
Electron energy distributions
collector current/beam current
collector current/beam current
Electron cloud buildup and saturation
bunch
spacing
derivative of (a)
bunch
spacing
ten bunches
K. C. Harkay, R. A. Rosenberg, PRST-AB, 6,
034402 (2003)
PSR
Time-resolved measurement
collector signal
Effect of repeller(i.e. retarding grid)
•Fast electronics connected to the collector.
•Chassis placed about 1 meter below the beam line.
•Collector signal connected to the electronics input
by 1.2 m of 93 Ω cable.
•Gain changing attenuators (1, 0.1, and 0.01).
•Over-voltage clamps to protect sensitive
components.
•50 Ω cable to a digital oscilloscope.
A. Browman, at ICFA Two-Stream 2000.
R.J. Macek et al., at PAC2003.
KEKB(e+) (KEK)
Pump port
Electron Monitor
Micro Channel Plate
Area of collector：6x5x5.9/4~40 mm2
• Electrons with an energy larger than the repeller
voltage and with an almost normal incidence
angle are measured.
Electron Monitor with Micro Channel Plate
• Planner collector : DC measurement
• Micro Channel Plate(MCP)'s collector :
K. Kanazawa et al., PAC2005.
Time-resolved measurement
Idea to measure the electron density near beam (K. Kanazawa)
Energetic electrons are produced near the bunch because of strong beam kick.
The retarding bias defines the observed volume around the beam from which
observed electrons come.
From the electron current per bunch and the retarding voltage, the cloud
density near the beam just before the arrival of the bunch can be estimated.
Cloud Density in NEG coated chamber and
in TiN coated chamber
bunch
r
observed volume
electron
RFA
K. Kanazawa et al., PAC2005.
•Electron sweeper
A variant of RFA.
Originally designed at LANL to measure electrons surviving passage of the bunch gap.
The device consists of RFA and a pulsed electrode which sweeps electrons into RFA.
PSR
H.V. : ≈1 kV, rise time : 15~ 20ns
Collection region
R.J. Macek et al., at PAC2003.
R. Macek et al., at ECLOUD02.
•Strip detectors
SPS(p)(CERN)
Originally developed at CERN to measure spatial and energy distribution of electrons.
Measurement can be applied at drift space, inside bend and even inside quad.
The copper strips on a MACOR allow the collection of the electrons from the electron
cloud.
Three versions: strip detector, strip pick-up detector, retarding field strip detector.
spatial and energy dist.
spatial distribution energy dist.
Another variants: room temperature type, cold type, variable aperture type.
J.M. Jimenez, at ECLOUD02.
J. M. Jimenez et al., LHC Project Report 634
Strip detector
top view
Strip pick-up detector
top view
side view
to apply a voltage
Integration interval of strip signal : 2 - 255ms.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD02.
J.M. Jimenez, at ECLOUD04.
Two strips/pole are seen.
by courtesy of J. M. Jimenez
•Microwave transmission measurement
SPS
The experiment aims at measuring electron cloud induced modulation of TE
wave guide modes.
Electron cloud leads to small phase shift of TE wave.
Phase shift is modulated with the SPS revolution freq. (43kHz).
FM sidebands
Observation
cloud
Actually, strong amplitude modulation
was found.
Study is continued.
T. Kroyer et al., at ECLOUD04.
PEPII
HOM generated at collimators in the upstream section of the straight.
A spectrum Analyzer pick-up located at the end of the straight.
spectrum
analyzer
collimator
HOM
solenoid
Solenoids(~20 m) at the beam pipe between the collimators and the
Analyzer pick-up can be switched “off” and “on” creating and
removing the electron cloud in the beam pipe.
pump current indicating
electron cloud
No noticeable modulation of the HOM amplitudes with the
density of the electron cloud was found.
A.Kulikov et al., at ECLOUD04.
2)Beam measurement
•Pickup electrode
Connected to spectrum analyzer
Oscillation spectrum of coupled bunch instability by electron cloud.
W
8 bunches
KEK-PF(e+)
BEPC
Long/medium range wake
Izawa et.al., Phys. Rev. Lett. 74, 5044 (1995)
Y.Z.Guo et al., Phys. Rev. ST - AB,
5, 124403 (2002)
•Turn-by-turn beam position monitor
Measure centroid position of all bunches in every turns.
Time evolution of oscillation modes
Custom multiplex/demultiplex IC
Growth rate of oscillation modes
KEKB
Use filter board for bunch-by-bunch feedback system
Filter board
ADC
BPM signal
M. Tobiyama and E. Kikutani,
Phys. Rev. ST Accl. Beams 3,012801(2000).
LER horizontal oscillation
time
bunch
time
mode
Change of mode spectrum w/wo solenoid field.
S. S. Win et al., at ECLOUD02.
DAFNE(e+)
Fast horizontal instability at positron ring
Pulse generator HP8116A :
control of feedback on/off , start trigger of DAQ.
Oscilloscope Lecroy LC574A :
data storage up to 500MHz sampling.
Caused by electron cloud?
A. Drago et al., at EPAC04, C. Vaccarezza et al., ECLOUD04.
Synchrotron radiation monitor
•Interferometer
Measure average transverse beam size.
Diffraction-free measurement.
KEKB
f
y
I(y,D)
D
f(y)
J.W. Flanagan et al., at EPAC2000.
Beam blowup at KEKB
2a
R
van Citterut-Zernike's theorem
T. Mitsuhashi, at DIPAC01.
:visibility
H. Fukuma et al., HEACC2001.
•Gated camera
Measure transverse size of each bunch.
No simultaneous measurement of bunches due to low repetition
rate (several 10 Hz).
Resolution limited by diffraction.
PEPII
Gated camera by Princeton Instruments.
R. L. Holtzapple et al., at Two-Stream01.
PEPII
ver.
hor.
bunch id along a train
Dramatic growth of the bunch size after the ion
gap in both planes.
R. L. Holtzapple et al., at Two-Stream01.
KEKB
J.W. Flanagan et al., EPAC00.
•Streak camera
Measure longitudinal and transverse distribution of each bunch.
Simultaneous measurement of several bunches is possible.
Resolution limited by diffraction.
Head-tail motion would be detectable.
Resolution measurement
(H. Ikeda)
KEKB
Streak camera : Hamamatsu Photonics C5680
Reflective optics in order to avoid
chromatic aberration(by T. Mitsuhashi)
1. Change the beam size by the orbit bump at a sextupole.
2. Measure the beam size by the streak
camera and the interferometer.
Eliminate a band pass filter
to increase light intensity.
3. Fit the data to
resolution=199m
3.8m at collision point
(Calculation assuming diffraction 190m)
KEKB LER
1000 bunches, 4 bucket spacing, beam current 1000mA
Train head
Longitudinal
Tail
Ve rtical
Solenoid
on
bunch train
Solenoid
off
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch which indicates head-tail motion is not clearly observed.
H. Ikeda et al. at Factories03.
•Tune meter
Measure bunch by bunch tune shift by the electron cloud along a bunch train.
Indirect estimate of the average density of the electron cloud around the ring.
density of electron cloud
Buildup of electron cloud can be seen.
Tune
RHIC(p)(BNL)
tail
A single beam position monitor (BPM) in
each plane recorded the injection oscillations
of the last incoming bunch.
Typically 1024 turns were recorded and the
tunes are obtained from a fast Fourier
transform of the coherent beam oscillations.
W. Fischer et al., Phys. Rev., ST-AB,
5, 124401 (2002)
head
FIG. 1. (Color) Coherent tunes measured along a train
of 110 proton bunches with 105 ns spacing in the
Yellow ring. Because of coupling both transverse tunes
are visible.
Tune along a bunch train at KEKB LER
KEKB
•Gated tune meter
Select a specific bunch by a high speed gate.
solenoid on/off
Two GaAs switches connected in a series improve the
isolation.
A pair of two switches are used to avoid a ringing at on/off
transition.
T. Ieiri et al., Phys. Rev. ST AB, 094402 (2002).
•Bunch-by-bunch luminosity monitor
Luminosity drop in “ long” mini-trains
Estimate of vertical size of each bunch.
PEPII
Detect gamma by radiative Bhabha scattering.
Medium to generate Cherenkov light is a high
quality fused silica.
A.Kulikov et al., at ECLOUD04.
Signal fed to shift register(SRG)
which is shifted at bunch freq..
After counting 32 bunches
the data in the SRG are
shifted out to a 32ch scaler.
The procedure is repeated
up to requested number of
turns.
beam
Stan Ecklund et al., N. I. M. A, 463 (2001), 68.
KEKB
Zero Degree Luminosity Monitor(ZDLM)
Detecting recoil electrons emitted by radiative
Bhabha scattering to the zero-degree angle and
deflected by Q magnetic fields because intensity
of radiative gamma is not enough due to thick
chamber wall.
J. Flanagan et al., at PAC2005
Logic analyzer
I.P.
Pb glass : 50x70x190 mm
by courtesy of S. Uehara
5. Summary
•The electron clouds produce various effects such as pressure rise, beam
induced multipacting, heat load, tune shifts, coupled bunch instability, beam
size blowup and so on which often limit the performance of existing
accelerators and would be a potential threat in under constructing and future
accelerators such as intense neutron sources and damping rings of linear
colliders.
•Many dedicated or standard instrumentations have contributed to understand the
electron cloud effects and give the data for a benchmark of simulation programs.
•The effort to refine and develop the diagnostics should be continued further to
improve the performance of existing accelerators and to predict performance of
future accelerators.
An example of challenges for experimentalists is the measurement of the electron
cloud in magnetic fields at beam location.
A question : Are electrons accumulated at the center of quad
where there is no magnetic field ?
Why magnetic field ?
•In B factories, most drift space has been covered by solenoids.
•In some machines (BEPCII, DAFNE, PSR, SPS...) a large fraction of
rings is already occupied by magnets.
Why at beam location ?
•Beam instabilities, especially a single bunch instability, are largely
affected by the electron cloud near a beam.
Measurement would be difficult because detectors are usually located on a
chamber wall (i.e., peripheral) while motion of electrons is affected by magnetic
fields by the time they reach at the detectors.
6. Acknowledgments
•I thank all authors of the papers where I referred in this talk.
•I apologizes those who performed the experiments or the
measurements which were not mentioned in this talk due to
lack of my knowledge.