Electron Beam Polarimetry at JLab – Hall C
Dave Gaskell PST 2009
September 7, 2009
1. Møller Polarimeter 2. Compton Polarimeter 3. Summary
JLab Polarimetry Techniques • Three different processes used to measure electron beam
polarization at JLab – Møller scattering: , atomic electrons in Fe (or
Fe-alloy) polarized using by external magnetic field – Compton scattering: , laser photons scatter from
electron beam – Mott scattering: , spin-orbit coupling of electron
spin with (large Z) target nucleus • Each has advantages and disadvantages in JLab environment
Method Advantage Disadvantage
Compton Non-destructive Can be time consuming, systematics energy dependent
Møller Rapid, precise measurements Destructive, low current only
Mott Rapid, precise measurements Measures at 5 MeV in the injector only – not the experimental hall
Electron Polarimetry in Hall C • Hall C presently has a single device for measuring the electron
beam polarization Møller polarimeter
– Although systematic precision is in principle <1%, smallest quoted uncertainty for a particular experiment is dP/P=1.3%
• Upcoming experiments (most notably, QWeak) require knowledge of beam polarization to dP/P=1%
• Hall C strategy for achieving 1% polarimetry
1. Use existing Hall C Møller polarimeter to measure absolute beam polarization to <1% at low beam currents
2. Build new Compton polarimeter to provide continuous, non-destructive measurement of beam polarization
3. Initially, Compton will provide relative measurements of beam polarization, eventually yielding measurements at precision similar to Møller
Møller Polarimeter
Basel-Hall C Møller Polarimeter • 2 quadrupole optics maintains constant tune at detector plane • “Moderate” (compared to Hall A) acceptance mitigates Levchuk
effect still a non-trivial source of uncertainty • Target = pure Fe foil, brute-force polarized out of plane with 3-4 T
superconducting magnet • Total systematic uncertainty = 0.47% [NIM A 462 (2001) 382]
Superconducting solenoid
Lead-glass electron detectors
Quads for steering Møller events to detectors
Hall C Møller Target • Fe-alloy, in-plane polarized targets typically
result in systematic errors of 2-3% – Requires careful measurement of
magnetization of foil
• Hall C uses a pure Fe saturated in 4 T field
– Spin polarization well known 0.25% – Temperature dependence well known – No need to directly measure foil
polarization
Effect Ms[µB] error
Saturation magnetization (T0 K,B0 T) 2.2160 ±0.0008
Saturation magnetization (T=294 K, B=1 T) 2.177 ±0.002
Corrections for B=14 T 0.0059 ±0.0002
Total magnetization 2.183 ±0.002
Magnetization from orbital motion 0.0918 ±0.0033
Magnetization from spin 2.0911 ±0.004
Target electron polarization (T=294 K, B= 4 T) 0.08043 ±0.00015
Hall C Møller Acceptance Møller events
Detectors
Optics designed to maintain similar acceptance at detectors independent of beam energy
Collimators in front of Pb:Glass detectors define acceptance One slightly larger to reduce sensitivity to Levchuk effect [broadening of angular correlation due to electron Fermi motion]
Hall C Møller Systematics - Idealized Source Uncertainty dAsy./Asy. (%)
Beam position x 0.5 mm 0.15
Beam position y 0.5 mm 0.03
Beam direction x 0.15 mr 0.04
Beam direction y 0.15 mr 0.04
Q1 current 2% 0.10
Q2 current 1% 0.07
Q2 position 1 mm 0.02
Multiple Scattering 10% 0.12
Levchuk effect 10% 0.30 Collimator positions 0.5 mm 0.06
Target temperature 50% 0.05
B-field direction 2o 0.06
B-field strength 5% 0.03
Spin polarization in Fe 0.25
Total 0.47%
Systematic error budget from NIM A 462 (2001) 382
This systematic error budget applies to a specific measurement (at low beam current) at a fixed point in time Ignores potential time dependence of polarization Measurements not taken under same conditions as experiment
Hall C Møller Systematics – G0 Experiment Source Uncertainty dAsy./Asy. (%)
Beam position x 0.5 mm 0.15
Beam position y 0.5 mm 0.03
Beam direction x 0.15 mr 0.04
Beam direction y 0.15 mr 0.04
Q1 current 2% 0.10
Q2 current 1% 0.07
Q2 position 1 mm 0.02
Multiple Scattering 10% 0.12
Levchuk effect 10% 0.30
Collimator positions 0.5 mm 0.06
Target temperature 50% 0.05
B-field direction 2o 0.06 0.37
B-field strength 5% 0.03
Spin polarization in Fe 0.25
Leakage 30 nA 0.2
High current extrap. 1%/40 uA 1.0 Solenoid focusing 100% 0.1
Elec. DT. 100% 0.04
Charge measurement 0.02
Monte Carlo Statistics 0.28
Unknown accelerator changes 0.5
Total 1.32%
Systematic error budget from G0 Forward Angle Ebeam= 3 GeV Nominal Ibeam = 40 µA Møller measurements taken every other day
dP/P = 1.32%
Likely could have improved this number – not a limiting systematic for G0
Møller Reconfiguration – QWeak and 12 GeV Some re-design of the Møller was required to make it “12 GeV” ready Insert 2nd large quad for higher energy Region between first and second quads about 30 cm smaller Special pipe required so Møller events do not scrape exiting 3rd quad
Q1 Q2 Q3 Detectors
Q2 Not used for QWeak
11 GeV configuration
Hall C Møller Systematics - QWeak
Source Uncertainty dAsy./Asy. (%)
Beam position x 0.5 mm 0.32
Beam position y 0.5 mm 0.02
Beam direction x 0.15 mr 0.02
Beam direction y 0.15 mr 0.01
Q1 current 2% 0.10
Q2 current 1% 0.17
Q2 position 1 mm 0.18
Multiple Scattering 10% 0.01
Levchuk effect 10% 0.20
Collimator positions 0.5 mm 0.06
Target temperature 50% 0.05
B-field direction 2o 0.14
B-field strength 5% 0.03
Spin polarization in Fe 0.25
Elec. D.T. 100% 0.04
Solenoid focusing 100% 0.10
Total 0.57%
Systematic error budget for QWeak with new Møller configuration low current running only applies to a particular measurement, not polarization for the experiment
dP/P = 0.57%
Comparable systematics at 11 GeV – new configuration will not impact performance
Hall C Møller at High Beam Currents
ΔP ~ 1% for ΔT ~ 60-70 deg.
Operating Temp.
Fe Foil Depolarization
In general Møller polarimeter limited to low beam currents to avoid foil depolarization
This can be mitigated in 2 ways:
Use raster to increase effective beam size
Use fast kicker at low duty cycle to maintain low “average” beam current on target, dwell time short enough to keep effects of instantaneous heating small
Møller Raster
I (!A)
ab
s(P
e)
(%) P0: !
2/" = 0.85
Moller raster d=2 mm68
70
72
74
76
0 2 4 6 8 10 12
Using a circular raster with radius of 2 mm, can run up to 10 to 20 µA without significant heating effects Experiments (especially QWeak) run at significantly higher currents – 150 µA! Møller running up to 100 µA (or higher) desirable
Møller current dependent tests during G0 forward angle (2003)
Calculated target heating vs. “raster size” at 20 µA
Kicker Studies • Since 2003, have been pursuing
studies with a fast kicker magnet and various iron wire/strip targets
• Most successful tests in 2004 – Short test – no time to optimize
polarized source – Tests cannot be used to prove
1% precision • Took measurements up to 40 µA
– Machine protection (ion chamber) trips prevented us from running at higher currents
– Lesson learned: need a beam tune that includes focus at Møller target AND downstream
• Demonstrated ability to make measurements at high currents – good proof of principle
Kicker Summary • Since 2004 tests, there have been no
opportunities to perform more kicker tests • We have had difficulty building target
appropriate for kicker running without “wrinkles”
– Wrinkles increase uncertainty in target polarization, difficult to quantify
• Will attempt further tests during QWeak (2010)
beamline redesign compatible with high current running with kicker magnet
• Kicker system not totally necessary for QWeak, but would provide nice high current cross-check of polarization
Compton Polarimeter
Hall C Compton Group 1. JLab Hall C 2. MIT 3. University of Virginia 4. Mississippi State University 5. University of Winnipeg 6. Yerevan Physics Institute
Hall C Compton Polarimeter
Components 1. Laser: Low gain (~100-200) cavity pumped with 10 W green laser 2. Photon Detector: CsI from MIT-Bates Compton polarimeter 3. Electron Detector: Diamond strip detector 4. Dipole chicane (MIT-Bates) and beamline modifications
Compton polarimeter provides: Continuous, non-destructive measurement of polarization under experiment running conditions Independent cross-check of Møller polarimeter
Laser and Low Gain Cavity
Hall C will use high power CW laser (10 W) @ 532 nm coupled to a low gain, external cavity 1-2 kW of stored power
Laser locked to cavity using Pound-Drever-Hall (PDH) technique
Coherent VERDI-10
Low gain, external cavity (low loss mirrors)
Low gain cavity tests
Gain 100 cavity linewidth=400 kHz
Gain 300 cavity linewidth = 175 kHz
CsI Photon Detector Pure CsI crystal • 10 x 10 x 30 cm3, slightly tapered from MIT-Bates polarimeter • Decay time: 16 ns (1000 ns), yield 2000 γ/MeV (5% of NaI) Read out • 250 MHz sampling ADC with integrated accumulators (developed for Hall A Compton by Hall A/Carnegie Mellon University) HIγS tests • Photon beam tests performed at HIγS facility at Duke
hsumwindowEntries 1823342
Mean 7151
RMS 1960
0 5000 10000 15000 20000 250000
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6000
8000
10000
hsumwindowEntries 1823342
Mean 7151
RMS 1960
Sum of Samples (within time window)
Photon beam energy from 22 to 40 MeV (QWeak endpoint = 46 MeV) variable intensity
Electron Detector Diamond strip detector built by Miss. State, U. Winnipeg 4 planes of 96 strips 200 µm pitch
Planes at Universities testing ongoing
Readout using CAEN v1495 boards Should be able to read out either in event mode or in scaler mode DAQ test stand work in EEL
Compton Polarimeter Systematics
Source dA/A (%) Dipole Bdl, detector pos. 0.03 Ebeam (10-3) 0.10 Detector efficiency 0.1-0.2 Abackground 0.02 Radiative corrections 0.25 Plaser 0.35 Cuts, beam spot size 0.5 Total 0.70
Systematic errors based on HAPPEX-II in Hall A using “zero-crossing” technique
Integrate response from asymmetry zero crossing
Crucial that zero-crossing in electron detector acceptance Hall C Compton designed with this in mind; zero crossing ~ 1 cm from beam
Compton Polarimeter at 12 GeV Operation at 11 GeV requires: 1. Changing chicane geometry
57 cm drop becomes 13 cm 2. Changing pole face of dipole
Low energy poles: nominal field = 5.5 kG
High energy poles: field=12 kG
Compton Detectors at 12 GeV • Space for photon detector becomes quite small (e.g. 13 cm
separation between back-scattered photon and nominal beam path) final solution still under discussion
• Diamond detector should work fine as-is (may need to be slightly larger) – with good beam tune, ok down to 3 GeV
Scattered electron deflections for 12 GeV configuration
3 GeV 11 GeV
Summary • Novel, out-of-plane, saturated iron target allows Hall C
Møller polarimeter to make sub-1% polarization measurements at low beam currents – Work ongoing to increase useful current range
• New Compton polarimeter will allow continuous, non-destructive polarization measurements
• 6 GeV 12 GeV upgrade path for both systems straightforward
• Halls A and C are learning from each other – Hall A will incorporate Hall C style Møller target – Hall C Compton builds on experience, success of
Hall A Compton
Compton Chicane
• Chicane design and engineering by MIT • Compton dipoles being fabricated by Buckley Systems (now)
One dipole delivered directly to JLab for mapping Other dipoles delivered to MIT for “test fit” and assembly with
whole chicane • Delivery of all dipoles, stands, vacuum channels by December • Design of interaction region and electron detector still in progress,
although at advanced stage
Compton Dipole MIT Dipole Excitation Results
(hall probe at magnet pole gap center)
I (A) B (gauss)
0 12
10 537.2
20 1055.4
30 1574.8
40 2095.2
50 2617.4
60 3140.6
70 3664
80 4185.4
90 4704.8
100 5222.4
110 5737.8
120 6250.6
130 6758.4
140 7259
150 7746
160 8205.4
170 8612
180 8949.6
190 9234
200 9484.8
210 9714.4
220 9928.8
230 10131.6
240 10325.8
250 10517.8
0
2000
4000
6000
8000
10000
12000
0 50 100 150 200 250
1st dipole delivered to MIT – detailed mapping at JLab this week!