Transcript Precise Beam Energy Measurements

Precise Beam Energy
Measurements
Michele Viti
Outlook
• ILC
– General overview
– Why precise beam energy measurement
• T474
–
–
–
–
Introduction
BMPs
Magnets
First results
• Comtpon
– Introduction
– Basic idea and basic layout of the apparatus
– Some feasibility studies
• Conclusions
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ILC
•30 Km electrons/positrons linear accellerator
•Total energy in the cms 500 Gev (upgradeable 1 Tev)
•High luminosity (2*10^34 /cm^2*s)
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ILC: goals
• ILC is a machine for very precise
measurements and test of many theories:
– Higgs mass
– Extra Dimension
– Supersymmetry
– Dark Matter
– Ultimate unification
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ILC: Precise Top Mass
Measurements
•
Many Standard Model depends
strongly on the value of the Top
Mass.
•
Unique situation at ILC
(Perturbative QCD applicable)
•
Well understood background,
clean experimental environment
•
Best direct measurement of the
top mass will be at ttbar threshold
– Vary the beam energy (Precise
Beam Energy Measurements)
–
Count number top-antitop events.
M t 50MeV

 3 104
M t 175GeV
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Basic Requirements for Beam
Energy Measurements
•
•
•
•
In order to make a precise
measurement of the top quark
mass we need to know some
“input” parameters very well such
the mean energy of the bunch as
well the luminosity spectrum
We need to have a fast, precise
and non-destructive monitor for
beam energy
Direct measurement of energy at
the IP is very difficult. We want to
measure the beam energy
upstream, downstream the IP plus
a slow monitoring at the IP
Relative Energy precision required
for upstream measurements
Eb
 104
Eb
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Magnet Chicane Energy
Spectrometer
BPM
L
BPM
offset d

BPM
magnets
Eb 
ce

 Bdl , with   arctg
d
L
•Electrons are deflected in this chicane and the offset in the mid-chicane is
antiproportional to the energy.
•Measuring this position with some special devices (Beam Position Monitor, BPM)
together with integrated B-field we have access to the beam energy
•Methode well tested used at LEP with a precision of 1.7 104
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Magnet Chicane Energy
Spectrometer
• At LEP was measured the bend angle of the beam
passing through a single magnet.
• The system was calibrated at the Z resonance where the
energy was determined using resonant depolarization,
no absolute angle measurement.
• Since the possibility to collect events the requirements
on BPM resolution was not too strictly (ca 1 micron).
• In our case since we plan to have in the mid chicane an
offset of 5 mm we require a BPM resolution of 500 nm.
• We want to monitor the Integrated B-field with a relative
precision of 50 ppm
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End Station A
• Characteristic:
– Parasitic with PEP II
operation
– 10 Hz and 28.5 GeV
– Bunch charge, bunch
length energy spread
similar to ILC
• Prototype components of
the Beam delivery
System and interaction
Region.
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End Station A
Beam Parameters at SLAC ESA and ILC
Parameter
SLAC ESA
ILC-500
10 Hz
5 Hz
Energy
28.5 GeV
250 GeV
Bunch Charge
2.0 x 1010
2.0 x 1010
Bunch Length
300-500 mm
300 mm
Energy Spread
0.2%
0.1%
Bunches per train
1 (2*)
2820
- (20-400ns*)
337 ns
Repetition Rate
Microbunch spacing
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T474
• At the End Station A (ESA) a 4 magnet chicane
energy spectrometer is commisioned.
• The goal is to demonstrate the stabilty and the
resolution of the system.
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T474 Collaboration,running
• Instituts involved: SLAC, U.C. Berkeley, Notre
Dame, Dubna, DESY, RHUL, UCL, Cambridge
• FY06 running:
– January (4 days): commissioning steering BPMs
– April(2 weeks): commissioning cavity BPMS,
optimization digitization and processing
– July(2 weeks): commissioning interferometer and
stabilty data taken with frequent calibrations
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T474 running
• FY2007 running:
– March(3 weeks): Commissioning and
installation magnets: first chicane data!!!
– July(2 weeks): Additional new BPM in the
centre of the chicane.
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BPMs
• The T474 BPMs are resonant cavities.
• The beam passing through the cavities excites
the eigenstates of the cavity (stationary waves).
• All the modes with a node in the center of the
cavity has the strongest dependence on the
offset of the beam (dipole,quadrupole,…)
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BPMs
n
L
R
C
Z
Circuital representation of n-th
mode of the cavity. Of course the
elements are not lumped!!!
The energy stored oscillates
between electrical and magnetic
field. The coupling with some output
and the cavity itself absorb power
(resistance R).
In principle we can
couple the electrical
field with an antenna.
P
Electric
coupling
Monopole signal
much higher than
dipole signal.
We need to select
to dipole mode!!!
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BPMs
To select the dipole
mode we couple the
cavity with waves
guides where the
coupling is strong with
the dipole mode and
weak with monopole.
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BPMs used in ESA
• ILC linac prototype cavities
• 36 mm aperture, 2.859 GHz
• low Q (~ 500)
• good monopole suppression
• x/y polarizations in same cavity
• Middle BPM on x/y mover system
• Referenced to downstream Q
• Rectangular cavities, Q, x and y
• Polarizations separated
• 2.856 GHz, high Q ~ 3000
• 20 mm aperture (0.8 “)
Q
X
Y
SLAC cavities
ILC cold linac prototypes
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BPMs results (2006 runs)
BPM
Precision(X)/micron
Precision(Y)/micron
1,2
1.64
4.71
3
0.49
0.50
4
1.26
1.12
5
0.59
0.44
9
0.28
0.34
10
0.16
0.20
11
0.28
0.25
It was also studied the stability of the parameters of the cavities (frequency and
decay time) and of the calibration constant, finding them very stable over 48
hours
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BPMs results (2006 runs)
Stability of the BPM3 (linac style
BPM). The green lines represents
an interval of =- 100 nm
Stability of the BPM10 (old cavities)
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Magnets 10D37
Plan view of the BPM based magnetic chicane
- four 10D37 bending magnet included.
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Magnets 10D37
•H magnet
•Actual length 94 cm
•Maximum B-Field 1
Tesla
•Working Point ca 0.11
Tesla*meter at 150
Amps
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Our goal
In order to measure the energy with a
resolution of 100 ppm or better, it’s required
to have an integrated B-field measurements
with a resolution of 50 ppm or better.
We want to study stability, reproducibility
and monitoring.
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Instruments
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•
•
•
•
NMR,Hall probe, B field measuremtns
Flux Gate, low B field measurements
Moving Wire, integratred B field
Flip Coil, integratred B field
Transductor, current
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Data taking and experimental setup
• Calibration of Hall using NMR in very uniform
magnet
• Calibration of the flip coil using one of the
magnet 10D37
• We took several kind data mainly:
– Stability and reproducibility run
– Current scan in large and small range
– Z/X mapping (at 150 Amps and 0 Amps)
• Before the run the magnet was standardizied
with 3 cycle between -200,200 Amp
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Stability runs
18 hours stability runs
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24 hours stability runs
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Calibration NMR-Flip Coil
Current scan between 140-150, 1
Amp step in order to calibrate the
NMR probe to the flip coil
Bdl predicted via
NMR minus Bdl
measured via flip coil.
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Energy Scan in March Run
Present spectrometer scheme
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Energy Scan in March Run
Run-1699: magnet scan with energy
feedback setpoint at each setting
I = -150 A : -200 -> 200 MeV in 5 steps
from nominal value
•E-average =28.58 GeV; possible to see energy scan.
•Beam deflection value calculated by taking into account BPM4 and zygo data.
•BPM 1,2 and 3,5 were used to predict the bunch position in the BPM4.
•BdL-integral was predicted using the Hall-pobe data and obtained before
dependences between Hall and BdL.
•Range of the energy changing is about 350 MeV instead of 400 MeV.
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Compton Backscattering
• At LEP it was possible to use together with
magnet spectrometer the resonant
depolarization  cross check!!!
• At ILC not possible to use resonant
depolarization, need to find something
else
• Studying the feasibility of an energy
spectrometer based on compton
backscattering (CBS) events
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Compton (Back)scattering
Elastic scattering between electrons and
photons.
Common situation is when the electron is a
binded to an atom (but we can consider it
free) and interact with a photons, which
looses energy
A kind of opposite situation we have
when electrons is moving: the
photons gain energy and is
scattered towards the direction of
the incoming electrons
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Compton Backscattering
Example of energy
spectrum for scattered
photons.
max 
2
2
m

40
, Emin    max 
1

4 0
Maximum energy for
scattered photons
(minimum energy for
scattered electrons) well
defined
m2
This 2 quantities give us access to the energy of the incoming beam 
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Previous experiences
• CBS already used in 2 facilities:BESSY I/II
and VEPP-4M.
• Measuring the spectrum of the scattered
photons.
• Not possible at ILC: problem of calibration,
need to accumulate statistic.
 New approach needed
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Basic layout
•Photons provided by a laser
•All scattered particles strongly collimated in forward region
•Need a magnet to separate them
•The bend angle is related to the energy
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Basic Layout
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Energy measurement
• X 0 is the center of gravity of the scattered photons, we can measure
with a dedicate detector
• X beam, position of beam, possible to measure with BPMs
• X edge, position of the electrons with minimum energy, we can measure
with a dedicate detector, possibly the same used for the photons
Ebeam
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m 2  X edge  X beam


40  X beam  X 0   sr
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


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Accuracy
Stastical error calculated for some input parameters:
•10^6 scattered events
•50 micron beam size (in x)
•0.15% energy spread, 250 GeV beam energy
•Bdl=0.84 T*m
•Distance magnet-detector= 25m
Relative error on energy measurement calculated assuming accuracy on beam position 500 nm, accuracy on photon
center of gravity 1 micron
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Simulation
Photon center of gravity
Edge electrons position
0.5 +- 0.7 micron
145.795 +- 0.006 mm
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Laser Properties
Considering the beam parameters listened
Above, to reach 10^6 scattered events we
need a laser with this properties:
– Wavelength = 1.064 micron (infrared YAG laser), 532
nm (green YAG laser)
– Waist size in x = 100 micron
– Pulse length = 10 ps (3 mm)
– Crossing angle = 8 mrad
– Pulse energy = 0.04 Joule (infrared), 0.1 Joule
(green)
– Repetition rate = 3 MHz
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Detectors
• We want to use the same detectors for electrons
and photons:
– Smearing for the distribution of photons ca 200-300
micron
– Smearing of edge electrons 60 micron
We need a detector with a resolution of 20-30 micron
– High granularity (20-30 micron)
– Good radiation hardness
• We have 2 basic options
– Silicon detector
– Quarz fiber detector
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Conclusions
• A lot of activies in this sector!!!
• A prototype for the magnet chicane
successfully built at SLAC with first good
results
• Ongoing studies on Compton
Backscattering to measure the energy,
very promising!!!
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