Transcript THE SCIENCE BEHIND THE CEBAF ACCELERATOR Andrew Hutton Director of Operations Jefferson Lab Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for.

THE SCIENCE BEHIND THE
CEBAF ACCELERATOR
Andrew Hutton
Director of Operations
Jefferson Lab
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Hall A Collaboration meeting, December 10, 2002 1
Outline
• Review of the basic accelerator layout
• Injector
• making electrons
• creating electron bunches
• Linacs
• How does RF acceleration work
• Setting up the RF
• Secrets of the Arcs
• Emittance and Optics notions
• Special features
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Hall A Collaboration meeting, December 10, 2002 2
Summary Accelerator Description
• Beam performance objectives:
• ≥ 5.7 GeV, 200 µA, CW
• 3 simultaneous beams, independent energy and current adjustment
• Beam polarization≥ 75%
• Parity quality
• Design concept: recirculating, superconducting CW Linac
• CW for high beam quality
• RF superconductivity for efficiency
• 5-pass beam re-circulation gives multi-energy operation
• Isochronous, achromatic beam recirculation arcs
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Hall A Collaboration meeting, December 10, 2002 3
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Hall A Collaboration meeting, December 10, 2002 4
Continuous Electron Beam Accelerator Facility
2 gain switched
diode lasers @ 499 MHz
1 G0 laser @ 31 MHz
Df= 120
0.6 GeV linac
(20 cryomodules)
1497 MHz
67 MeV injector
(2 1/4 cryomodules)
1497 MHz
A
B
RF separators
499 MHz
C
B
A
Pockels cell
C
A
Gun
B
C
Double sided
septum
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Hall A Collaboration meeting, December 10, 2002 5
Making Electrons
• Photo-electric Gun
• Shine laser light on a semiconductor wafer
• Photo-electric effect kicks out an electron
• same effect as solar cells which make electricity from the sun
• Because the laser can be pulsed, electron beam is also pulsed
• Use three separate lasers, one for each Hall
• provides greater flexibility
• Cathode material is strained Gallium Arsenide
• Laser light of 780nm gives weakly polarized beams ~35%
but high Quantum Efficiency (QE~1%)
• Laser light of 840 nm gives strongly polarized beam ~75%
but low QE~0.2%
Thomas Jefferson National Accelerator Facility
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Polarized photoinjector
2 identical horizontal guns installed in 1998
Gun 2
Gun 3
Oct 2000
to
Jan 2001
Feb 2001
to
Mar 2002
Gun service required once per year.
Both guns provide high polarization (>70%).
Thomas Jefferson National Accelerator Facility
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Source perspective
A 100 keV beam from either gun
is deflected 15° by a magnet to a
common pre-accelerator beamline.
Two laser tables straddle the
beamline and provide a direct
optical path to the cathode.
Thomas Jefferson National Accelerator Facility
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Hall A Collaboration meeting, December 10, 2002 8
Diode Laser
• Diode lasers – like in your CD player
• Requires additional amplifier to obtain ~100 mW of light
• Highly reliable
• Pulse length δ ~20 ps
But
• Amplifier does not fully switch off between pulses
• Creates “bleed-through”
• Have tried to pulse amplifier with some success – still a problem
• We always probably use diode laser for Hall B
• Presently using diode laser for Hall A
Thomas Jefferson National Accelerator Facility
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Hall A Collaboration meeting, December 10, 2002 9
Ti-Sapphire Laser
• High power without additional amplifier
• Switches off cleanly – unmeasurable bleed-through
• Early laser had noise on the beams
• Led to trips of the safety system
• A Ti-Sapphire laser from Time–Bandwidth Inc. in Switzerland
(Tiger) was ordered for the G0 experiment at 31.2 MHz
• Uses a different, proprietary method for locking to RF frequency
• Has operated flawlessly
• Can create pulses with δ from 20ps to 70ps
• We have ordered another Tiger laser for 499MHz operation
• Expect good performance
• Will be used for future, high current Hall A experiments
Thomas Jefferson National Accelerator Facility
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Dynamic laser configuration
Re-re-re-re-configuration. . .
3 end-stations makes for a
dynamic physics program
which requires that the laser
table be configurable for
beam qualities:
Intensity (power)
G0
Future
HAPPEx2
Polarization (wavelength)
RF (1497, 499, 31.1875)
Parity (Independent)
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Making Bunches
• Beam from 100 keV gun sent through 499 MHz chopper
cavity (one third of accelerating frequency)
• Rotates beam in circle of ~1.5 cm radius
• Slits at 240°, 0° and 120° degrees allow bunches of electrons to pass
• Slits are individually controlled to regulate currents for Halls A, B & C
• The three beams are recombined by another 499 MHz chopper cavity
• Creates 110 picosecond bunch
Hall A
Hall B
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Bleed-Through
• To reduce “bleed-through”, Hall B uses about 20 µA beam
current from the cathode and rejects all but a few nA at the
slits
• Improves rejection of leakage from Hall A and Hall C beams
But
• Makes Hall B current very sensitive to position movement at the slit
• Have installed a fast feedback to stabilize position
But
• BPMs read position of the average currents for Halls A, B & C
• Displaces low current beam if all the beams are not exactly coincident
• Cannot stabilize against laser spot movements
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Making Bunches (cont)
• Beam then goes through buncher section
• Decelerates front of bunch, accelerates back of bunch
• After a drift space, bunch is compressed (shorter) – 5 picosecond
• but energy spread in the bunch is increased
• Beam then goes through capture which continues process
• Final bunch length – 1 picosecond (3/10 of a millimeter)
• Phases of Injector Rf must be stable to within ~1 ps
• Can achieve this for short periods of time
• Best in summer and winter
• Injector very unstable in spring and fall – sun-up and sun-down
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Measuring Bunch Length
• Measure the time of arrival of the bunch at a high frequency
cavity (5.988 GHz)downstream of the buncher
• Move the phases of the chopper cavities and re-measure the
time of arrival
• Plot the time of arrival as a function of the phase
• Each point is the center of gravity of the beam
• By moving the center of gravity, the result mimics the behavior of
particles distributed along the bunch
• This measurement is accurate to about 15 femtoseconds
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Hall A Collaboration meeting, December 10, 2002 15
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Hall A Collaboration meeting, December 10, 2002 16
Wave acceleration
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How does RF acceleration work?
• Imagine a surfer riding a wave
• Get on the wave at right time and right speed - acceleration
• Get on the wave at wrong time or wrong speed, deceleration, wipe-out
• A good surfer will speed up to catch the wave and will then start to move
across the wave to avoid overtaking the wave and wiping out
• Matching the phase velocity and the group velocity
• Now imagine an electron traveling in a radio-frequency (RF)
wave
• If it arrives at exactly the right time it will gain energy
• If it arrives at the wrong time it will gain less energy
• But If it is traveling near the speed of light, it will not speed up or slow
down – it will gain or lose mass as it gains energy
• No wipe -out, but not the right energy at the end
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Hall A Collaboration meeting, December 10, 2002 18
How does RF acceleration work? cont.
• RF acceleration comes from multiple, independent klystrons, each feeding
one 5-cell cavity
• The trick is to ensure that as a bunch arrives at each cavity, the fields are
correct to continue acceleration
• The waves (fields) in each cavity have to be in phase
• Phases have to be set up correctly
• Phases have to stay correct
• Both of these are a challenge
• Each klystron is fed from the same phase reference, but . . . . . .
• must be accurate to a few picoseconds
• klystrons are up to half a mile away from the source
• Thermal problems, stability problems in repeaters . . . .
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Superconducting Cavities
• Use superconducting niobium cavities to create the RF fields
for acceleration
• Very low losses on the cavity walls
• cavity resonance very sharp (Qext ~ 106)
• sensitive to vibration (microphonics)
• Cooled by superfluid liquid Helium @ 2 K so heat transfer is easy
• no bubbles to vibrate the cavity
• Helium temperature (pressure) changes the tuning
• keeping all of the cavities in tune requires automated software
• CEBAF Acceleration System
• 330 cavities in 41 1/4 cryomodules, installed and functional
• Gradient limit and Q twice as good as specification
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CEBAF SRF Cavities
CEBAF 5-cell
cavities operate
at 1497 MHz
with an active
length of 50 cm
each
There are eight
cavities per
cryomodule
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Linac Cryomodules
CEBAF has 42¼
cryomodules
with a total
active length of
169 meters
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Superconducting Cavity Treatment
• Gradient specification of CEBAF cavities was 5 MV/m
• Average gradient of superconducting cavities as installed was
7.3 MV/m
• Two approaches to improve performance of cavities,
•
Helium processing and waveguide vacuum processing
• Carried out in situ
• Aim to reduce field emission
• Average gradient of superconducting cavities is now 7.76
MV/m
• Limit is established with CW beam under standard operating conditions
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Hall A Collaboration meeting, December 10, 2002 23
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Hall A Collaboration meeting, December 10, 2002 24
FSD Trips
• Fast Shut Down (FSD) trips are triggered by RF arcs and
protect the SRF cavities from arc damage
• ¿Caused by charging up of ceramic widows?
• ¿Caused by “three dimensional“ gas discharges?
• Strongly dependent on accelerating gradient
• Weakly dependent on total linac beam current
• Pushing energy to:
• 5.7 GeV  10% hit in availability (just acceptable)
• 6 GeV  20% hit in availability (unacceptable)
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Improving RF arc trip rate
• Installed >50 stub tuners to improve match
• Reduces klystron power for same beam power
• Helium processed all cryomodules (some twice)
• Reduces electron emission in cavities
• Improved algorithm for calculating set points
• Optimize for gradient, current, cryogenic load, etc
• Problem is inherent, getting close to limit
• This year, will try to reduce time to reset trip
Thomas Jefferson National Accelerator Facility
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Hall A Collaboration meeting, December 10, 2002 26
6
12%
5
10%
4
8%
3
6%
2
4%
1
2%
0
0%
Trip penalty
Weekly Average
Energy
Thomas Jefferson National Accelerator Facility
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Hall A Collaboration meeting, December 10, 2002 27
Trip Penalty
Linac Energy GeV
FSD Trip Rate Versus Energy
October 99 – June 01
Ponderomotive Force
• Ponderomotive force of RF fields tends to lower cavity
resonant frequency
• RF fields have energy, tends to push outwards, makes cavity bigger
• Cavity tuner changes to maintain correct operating frequency
• In a few cavities (so far)
• If cavity trips off at high gradient, resulting frequency shift is so large
• Cavity goes out of resonance
• Cannot be switched back on
• This is the most serious problem for RF control of the new
cryomodules for 12 GeV
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Linac RF Operation
• Operation of the linacs requires sophisticated high-level software
• Automated Cavity Tuning
• Make it resonate at exactly the right frequency
• Automated Cavity phasing
• Make all the cavities resonate in phase
• Linac Energy Management
• Set up exactly the right energy
• KREST
• Set up exactly the right phase for each cavity
• MOMOD
• Maintain the right phase for each linac
Thomas Jefferson National Accelerator Facility
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Hall A Collaboration meeting, December 10, 2002 29
Automated Cavity Tuning
• Sweep Mode (Exact tuning)
• The cavity frequency is modulated over a range of ± 200 Hz in steps of 5 Hz.
• Find the typical response of a resonant system to harmonic excitation
• The measured de-tuning angle
•
as a function of modulating frequency is
compared to the predicted curve to determine the resonance frequency

f f 
  0
 = a tan 2Q L 0
f0 

• as well as the phase offset
• Autotrack (locks cavity on frequency)
• Uses the phase offset
operating frequency
determined above to keep the cavity tuned to the
Thomas Jefferson National Accelerator Facility
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Hall A Collaboration meeting, December 10, 2002 30
Linac Auto-Phasing
• Purpose: Precise cavity phasing is achieved by maximizing the Linac
energy.
• Arc is used to measure energy changes
• The higher the energy, the heavier it is, the harder it is to bend so it travels on
the outside of the bend
• Phase of an individual cavity is changed by ±30°.
• Initial beam position and beam position changes are recorded.
• Crest phase is found from
f crest =
•
•
f 00


y  y
Df
 tan
 a tan 
2
 y   y   2y 0 
0: initial phase setting, y0: beam position at
= 0±∆
0, y±: beam position at
Reproducibility better than 1°, Phasing time 2 min/cavity
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Linac Energy Management (LEM)
• For a given energy at the end of North or South Linac and
available cavities:
• Optimizes the cavity gradient distribution using individual cavity
characteristics
• Calculates energy profile along the linacs
• Calculates Linac quad values consistent with calculated energy profile
• Downloads and sets RF, Quads (including hysteresis) and skew quads
• Required input
• Maximum gradient permitted for each cavity
• List of available cavities
• Integrated into the maintenance log
• Most useful number – energy overhead available (usually 6 – 10%)
Thomas Jefferson National Accelerator Facility
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Hall A Collaboration meeting, December 10, 2002 32
Global Procedures
• Krest (intrusive)
• Modulate the phase of each cavity
• Observe the change in energy at a BPM in the Arc where there is
dispersion (particles with different energies have different orbits)
• Move the cavity phase until energy does not change with modulation
• Repeated for each cavity
• Each cryomodule is done by hand at start-up
• Momod (parasitic)
• Modulate the phase of the RF phase reference for each Linac using
two different frequencies ~800 Hz during accelerator operation
• Move the cavity phase until energy does not change with modulation
• Linac is now on crest
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What is Emittance?
• The beam size in an accelerator can be separated into two parts:
• The emittance or phase space which is a property of the beam
• The beta functions or R matrix elements which describe the focusing
• In an accelerator, the emittance is defined by the gun characteristics and
cannot be improved later – but it can be degraded
• For CEBAF, emittance is the extent of the beam in six dimensions:
• X, X’, Y, Y’, δE, δL
• As the beam is accelerated, the transverse emittance should be adiabatically
damped (reduced proportionally to the energy)
• As the beam is accelerated the transverse energy of the beam stays constant but
the longitudinal energy increases. It is like pulling on a rubber band – it gets thinner
Thomas Jefferson National Accelerator Facility
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Hall A Collaboration meeting, December 10, 2002 34
Basic Accelerator Optics
• DIPOLES
• magnets that bend the beam (usually horizontally, sometimes vertically
and sometimes at an angle) They define the shape of the machine
• Higher energy electrons will be bent less (like a prism in optics)
• The edges of the beam have fields that are not uniform, so they focus
as well (like aberrations due to non-flat prism faces)
• QUADRUPOLES
• magnets that focus the beam (like a lens in optics)
• main difference with an optical lens is that if the lens focuses in the
horizontal, it will defocus in the vertical and vice versa
• “Strong focusing” if the quadrupoles are sufficiently strong (and close
together) overall they will focus in horizontal and vertical
• the result is rather like a periscope in optics and is stable
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Hall A Collaboration meeting, December 10, 2002 35
The R Matrix
The R Matrix relates the output angles and positions from a beamline to the
input angles and positions.
Xout
R11
R12
R13
R14
Xin
R21
R22
R23
R24
X'in
Yout
R31
R32
R33
R34
Yin
Y'out
R41
R42
R43
R44
Y'in
X'out
=
The matrix for an uncoupled beamline would have the cross-terms zero
If the beam is kicked horizontally, it will oscillate in the horizontal plane but
should never show a vertical oscillation
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Hall A Collaboration meeting, December 10, 2002 36
CEBAF Optics
• LINACS
• A Linac (LINear ACcelerator) is straight – no bends
• In CEBAF, electrons of very different energies travel together
• There is a regular array of equi-spaced quadrupoles
• They focus strongly for the lowest energy pass, more weakly for the
highest pass – but still “strong” focusing
• ARCS
• At the end of the Linacs the beams of different energies are separated
by the “spreaders”
• There is then a regular circular Arc section (actually five of them)
• At the end of the Arcs the beams of different energies are brought
together by the “recombiner”
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Hall A Collaboration meeting, December 10, 2002 37
Achromatic & Isochronous
• Bunches must be transported around the arc and all of the
electrons must arrive at the other end at the same place and
time
• Achromatic – electrons of different energies arrive at the
same place
• From the Greek “a” without, “khroma” – color
• Isochronous – electrons of different energies take the same
time
• From the Greek “isos” – equal, “chronos” – time
• The Arc was designed to be both achromatic and Isochronous
Thomas Jefferson National Accelerator Facility
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Hall A Collaboration meeting, December 10, 2002 38
Principle of Isochronicity
Recombiner
Linac
Spreader
Arc
Arc
Spreader
Linac
Recombiner
Isochronous if path length difference in Arcs equals
path length difference in spreaders and recombiners
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Hall A Collaboration meeting, December 10, 2002 39
Types of Emittance Increase
• Filamentation
• In the longitudinal plane, RF focusing in circular machines causes particles of
different energies to rotate around the RF bucket at different speeds. This can
cause an effect like an egg beater resulting in real emittance dilution. This
effect is not relevant for CEBAF
• Longitudinal variation of Emittance
• If the front of the beam is not in the same position as the back of the beam, the
projection of the beam on a screen is apparently enlarged (think of the beam
as a banana – the projection is thicker than the cross-section of the banana).
This effect is usually not a problem at CEBAF.
• Emittance Projection
• If the beam is strongly X-Y coupled, the projections of the phase space onto
the visible axes X, X’, Y, Y’ can all be increased – this has been a problem at
CEBAF.
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Coupled Beams
• Beam coupling refers to a coupling between horizontal and
vertical oscillations in the beam
• Under normal (uncoupled) conditions, the horizontal and
vertical motions of the beam are independent
• Note that a quadrupole has the effect of focusing in one plane and
defocusing in the other plane so there is a correspondence between
the two planes
• When the beams are coupled, an oscillation in one plane
couples into the other plane, like two coupled oscillators
• This can lead to an apparent increase in the beam emittance
• Note that Louiville’s theorem says that phase space (emittance) is
conserved, so the increase is only apparent – but very real!
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Uncoupled Emittances
X’
X
In the absence of coupling, the product of the projections
of the phase space area on the X and X’ axes is a constant
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Coupled Emittances
X’
X
In the presence of coupling, the product of the projections
of the phase space area on the X and X’ axes is a never a
constant and is usually much larger than when uncoupled
Thomas Jefferson National Accelerator Facility
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Causes of Coupling
• Point coupling in the Injector
• Most likely candidates are the counter-wound solenoids, the cryounit
and the cryomodules
• Uniform Coupling in the Linacs
• Produced by skew fields in the SRF cavities due to asymmetry in the
HOM couplers
• Corrected by the skew quadrupoles
• Point Coupling in the Arcs
• Produced by mis-steered beams going through fringe fields in dipoles
• Most serious in the spreaders and recombiners
• Goal is to reduce coupling as much as possible
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Summary
• Keeping the accelerator tuned up is a demanding, delicate
task
• The operators usually have a Batchelor degree in Physics (some are
studying for their Master’s)
• They are trained on the job for six months to become an Operator
• It takes another 2-3 years to become Crew Chief
• They try incredibly hard to deliver quality beam to Users
• There are new capabilities all of the time
• We are all learning as we go
• Please – give the Operators a break and complain to me if
you are unhappy
• That’s my job!
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Hall A Collaboration meeting, December 10, 2002 45