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

2000

4000

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!