Electron Beam Polarimetry at JLab Hall C Dave Gaskell PST 2009 September 7, 2009

Transcription

1 Electron Beam Polarimetry at JLab Hall C Dave Gaskell PST 2009 September 7, Møller Polarimeter 2. Compton Polarimeter 3. Summary

2 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

3 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, Q Weak ) 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, nondestructive measurement of beam polarization 3. Initially, Compton will provide relative measurements of beam polarization, eventually yielding measurements at precision similar to Møller

4 Møller Polarimeter

5 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 Quads for steering Møller events to detectors Lead-glass electron detectors

6 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 M s [µ B ] error Saturation magnetization (T 0 K,B 0 T) ± Saturation magnetization (T=294 K, B=1 T) ±0.002 Corrections for B=1 4 T ± Total magnetization ±0.002 Magnetization from orbital motion ± Magnetization from spin ±0.004 Target electron polarization (T=294 K, B= 4 T) ±

7 Hall C Møller Acceptance Optics designed to maintain similar acceptance at detectors independent of beam energy Møller events Detectors 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]

8 Hall C Møller Systematics - Idealized 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 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 2 o 0.06 B-field strength 5% 0.03 Spin polarization in Fe 0.25 Total 0.47%

9 Hall C Møller Systematics G0 Experiment Source Uncertainty dasy./asy. (%) Systematic error budget from G0 Forward Angle E beam = 3 GeV Nominal I beam = 40 µa Møller measurements taken every other day dp/p = 1.32% Likely could have improved this number not a limiting systematic for G0 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 2 o 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%

10 Møller Reconfiguration Q Weak and 12 GeV Some re-design of the Møller was required to make it 12 GeV ready Insert 2 nd 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 3 rd quad Q1 Q2 Q3 11 GeV configuration Detectors Q2 Not used for Q Weak

11 Hall C Møller Systematics - Q Weak Systematic error budget for Q Weak 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 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 2 o 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%

12 Hall C Møller at High Beam Currents In general Møller polarimeter limited to low beam currents to avoid foil depolarization Fe 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 Operating Temp. ΔP ~ 1% for ΔT ~ deg.

13 Møller Raster Using a circular raster with radius of 2 mm, can run up to 10 to 20 µa without significant heating effects Experiments (especially Q Weak ) run at significantly higher currents 150 µa! Møller running up to 100 µa (or higher) desirable abs(p e ) (%) P0:! 2 /" = Moller raster d=2 mm I (!A) Møller current dependent tests during G0 forward angle (2003) Calculated target heating vs. raster size at 20 µa

14 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 Studies

15 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 Q Weak (2010) beamline redesign compatible with high current running with kicker magnet Kicker system not totally necessary for Q Weak, but would provide nice high current cross-check of polarization

16 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

17 Hall C Compton Polarimeter Compton polarimeter provides: Continuous, non-destructive measurement of polarization under experiment running conditions Independent cross-check of Møller polarimeter Components 1. Laser: Low gain (~ ) 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

18 Coherent VERDI-10 Laser and Low Gain Cavity Low gain, external cavity (low loss mirrors) Hall C will use high power CW laser ( nm coupled to a low gain, external cavity 1-2 kw of stored power Laser locked to cavity using Pound-Drever-Hall (PDH) technique

19 Low gain cavity tests Gain 100 cavity linewidth=400 khz Gain 300 cavity linewidth = 175 khz

20 Pure CsI crystal CsI Photon Detector 10 x 10 x 30 cm 3, 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 Sum of Samples (within time window) hsumwindow Entries Mean 7151 RMS Photon beam energy from 22 to 40 MeV (Q Weak endpoint = 46 MeV) variable intensity

21 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

22 Compton Polarimeter Systematics Systematic errors based on HAPPEX-II in Hall A using zero-crossing technique Source da/a (%) Dipole Bdl, detector pos E beam (10-3 ) 0.10 Detector efficiency A background 0.02 Radiative corrections 0.25 P laser 0.35 Cuts, beam spot size 0.5 Total 0.70 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

23 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

24 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

25 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, nondestructive 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

26 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

27 Compton Dipole 1st dipole delivered to MIT detailed mapping at JLab this week! MIT Dipole Excitation Results (hall probe at magnet pole gap center) I (A) B (gauss)