The MIT-Bates Compton Polarimeter

1. The MIT-Bates Compton Polarimeter Presented at Workshop on EIC Polarimetry Brookhaven National Laboratory November 8, 2002 South Hall Ring Compton Polarimeter Collaboration T. Akdogan, D. Dutta, M. Farkhondeh, W.A. Franklin, M. Hurwitz, J.L. Matthews, E. Tsentalovich, W. Turchinetz, T. Zwart, MIT-Bates Linear Accelerator Center E. Booth, Boston University W. Lorenzon, University of Michigan

2. OUTLINE • 1. Laser Backscattering Below 1 GeV • 2. Description of Bates Compton Polarimeter • Bates Electron Beam • Laser and Laser Transport • Calorimeter • Data Acquisition and Electronics • Data Analysis • 3. Recent Results • 4. Relevant Considerations for EIC Polarimeters

3. Compton Polarimeters For Storage RingsBelow 1 GeV • Analyzing powers a few percent or less • Interaction mechanism varies with gamma energy • Wide kinematic cone for scattered photons • Lots of background in low energy photon region • Beam lifetime less than 1 hour Difficult conditions for polarimeter operation, but backscattering remains best method for nondestructive measurement of beam Low energy polarimeters address some issues which higher energy devices do not face

4. Compton Polarimeter Analyzing Power • Compton polarimetry is well established as a method of measuring electron polarization at high energy accelerators • As electron energy falls, Compton scattering analyzing power diminishes • New challenges exist in • applying laser backscattering • technique for polarimeters • at energies below 1 GeV • Pioneering work at NIKHEF • for this energy regime 532 nm laser light HERA EIC Bates • New efforts underway at MIT-Bates and Mainz. Bates seeks absolute • polarization measurement good to 2-3% for experiments with BLAST.

5. MIT-Bates Linear Accelerator Center • Polarized electrons obtained from photoemission source • Beams up to 1 GeV achievable with Bates Linac and Recirculator • South Hall Ring stores beam for CW operation SHR Typical Parameters 300 MeV  Ee  1 GeV, 0.40  Pe  0.80, 1 mA  Im  200 mA, 5 min    50 min

6. Bates Large Acceptance Spectrometer Toroid New spectrometer for studying light nuclei with stored beams and internal targets

7. Polarization in Bates South Hall Ring COMPTON POLARIMETER • Electrons injected with longitudinal • polarization (controlled by Wien filter) • Internal target inside BLAST Spectrometer • Full Siberian Snake used to preserve • polarization in Ring • Compton polarimeter, separated from • internal target by 22.5o bend, measures • longitudinal projection of beam polarization • Commissioning RF dipole to allow • spin reversal with beam stored in Ring Spin Flipper South Hall Ring

8. MIT-Bates Compton Polarimeter Injection Line Laser exit Ring Dipole • Design based on AmPS polarimeter • Compton polarimeter located upstream of BLAST target to reduce background from bremsstrahlung • Laser resides in shielded hut with 20 m flight path to Int. Region (IR) • IR is a 4 m long straight section • Laser trajectory varied remotely to optimize overlap with electron beam • Dipole chambers modified to allow laser to enter and exit IR • Photons detected with pure CsI Interaction Region Electron beam Remotely controlled mirrors Scattered photons Laser line CsI detector Laser hut Target

9. Compton Polarimeter Laser • Coherent Verdi Laser employs vanadate crystal with lithium • triborate crystal for frequency doubling in compact formation • Laser is diode-pumped and requires little input power • Internal feedback stabilizes output • Single mode cavity allows possibility of efficient frequency doubling • Purchased May 1999, significant improvements in peak powersince • Laser Parameters • Power Range: 10 mW- 5 W • Duty Cycle: Continuous wave • Wavelength: 532 nm • Divergence: < 0.5 mrad • Diameter: 2.25 mm • Linear Polarization: ~ 0.99

10. Laser Optics • Set of 3 lenses provide focusing over 20 m flight path to IR • Laser chopped by rotating slotted wheel at 10 Hz • allowing background measurements (duty cycle adjustable) • Circular polarization imparted by Helicity Pockels Cell (HPC) allowing for rapid helicity reversal • 4 mirrors downstream of HPC arranged in phase-compensated manner to preserve polarization • Final mirror inside vacuum system • Laser scanned by 2 rotary mounts with 2 axes of rotation (controlled through EPICS) • Laser position monitored by cameras at entrance and exit windows to vacuum enclosure • Circular polarization measured periodically • Considering options for feedback system

11. Interaction Region • IR contains steering coils and triplet • of quadrupole magnets • Beam Position Monitors run continuously • Tune electron beam first to remove • steering from quads, especially vertically • Most background from rest gas in • beam pipe • Laser spatially constrained to intercept • electron beam at angle < 2 mrad • Backscattered gamma trajectory defined • largely by electron trajectory • Energy versus angle correlation of • gamma rays dependent on electron energy

12. Scattered Gamma Ray Line • Calorimeter located 10 m from exit window to reduce background • Movablecollimators used to eliminate background from beam pipe • Thin windows minimize attenuation of backscattered flux • Variable thickness stainless steel absorbers used to regulate rate • Permanent sweep magnet eliminates most charged particles • Veto scintillator rejects remaining charged particles • Scintillator hodoscope provides some position information • Pure CsI detector offers combination of resolution and speed

13. Calorimeter • Max. gamma energy changes rapidly over electron energy range in SHR (3-35 MeV) • Asymmetry is a strong function of energy • Need versatile calorimeter with high stopping power and rapid response • Pure CsI crystal (4”x4”x10”) backed by 3” phototube with UV window chosen • Base contains transistors allowing rates up to ~ 300 kHz with very stable gain • Unsegmented calorimeter eliminates gain matching, analog summing • Calorimeter modeled using GEANT • Data taken with up to 30 mA stored without • absorbers, up to 90 mA so far with absorbers

14. Compton Polarimeter Data Acquisition Criteria Fast and reliable data acquisition system is very important • Rapid digitization and high readout speed for high event rates • External triggering, single event mode • Energy resolution of ADC • Rapid spin sorting capability • Insensitivity to electronic crosstalk • Need synchronized ADC and scaler data • Integration with analysis package and BLAST event stream • Ability to include EPICS (slow controls) information • Possibility of pulse-shape discrimination/ pile-up rejection

15. Data Acquisition Overview • VME-based system • Use VTR10012 Flash ADC for digitization • Encode logic information in second ADC channel • Issue interrupt to read ADC, scalers at helicity state change • DMA allows rapid transfer to MVME177for sorting into histograms • Data transferred over network every 10 sec to Linux work station • Oscilloscope mode to set ADC parameters and test signal processing Read at HPC change 500 kHz 20 Hz 0.1 Hz 20 Hz

16. Polarization Calculation • Analysis entails subtraction of background from signal as function of gamma ray energy • Normalization of background • obtained from scalers or from • bremsstrahlung tail of energy spectrum • Asymmetries formed as function of • energy from subtracted yields • Fit asymmetry data with function • representing polarimeter analyzing power

17. Systematic Error Investigation • Small analyzing power makes systematic error reduction crucial • Potential false asymmetries from many sources including helicity-related laser steering, electronic crosstalk, luminosity asymmetry, kinks in electron trajectory, etc. • Need accurate modeling of shape and magnitude of analyzing power and good energy calibration for calorimeter • Monitor laser circular polarization AmPS Systematic Errors 0.5” Collimator 1” Collimator Example: Helicity-related laser steering plus tight collimation produces false asymmetry I. Passchier et al, NIM A414, 446 (2000)

18. Calorimeter Energy Calibration Techniqes • Energy calibration largest systematic uncertainty at AmPS • Document position of Compton edge and asymmetry zero crossing • Monitor stability continuously with pedestal and pulser High circular polarization of backscattered photons can be used in conjunction with absorption magnet to yield enhanced asymmetry with well-defined zero crossing for energy cal. purposes

19. Consistency Checks • Measure asymmetries with • unpolarized beam • Check for sign reversal in • asymmetry when polarization • direction changed at source • Change laser helicity with • half-wave plate • Quality of fit to asymmetry data • Verify polarimeter calibration • versus independently standard • (e.g. polarized e-p elastic • scattering asymmetries) • Monitor stability of energy • calibration with LED pulser in • calorimeter light guide

20. Advantage of Spin Reversal • Accuracy of polarization extraction • significantly enhanced by changing direction of electron polarization • Leads naturally to the cross-ratio method for calculating the average asymmetry Crucial advantage: Factors associated purely with the spin-state of the laser or with the spin state of the beam, but not with both will cancel exactly (e.g. luminosity difference due to PITA)

21. Spin Flipping in Bates South Hall Ring • Adiabatic spin flip possible through application of rf magnetic field • Prototype rf dipole successfully tested • in South Hall Ring • Spin-flip efficiency greater than 90% • achieved V.S. Morozov et al, PRST AB 4, 104002 (2001) • Improved ferrite dipole for higher spin flip efficiency designed and built at Universityof Michigan • Efficient dipole could permit spin flip with period of order 1 minute

22. SHR Polarimeter Commissioning Timeline History • Polarimeter installed in Bates South Hall Ring (2000) • First polarization asymmetries measured in South Hall Ring and prototype spin flipper tested (Winter, 2001) • BLAST Spectrometer commissioning initiated (June, 2002) • Polarization measurements at 850 MeV (August, 2002) • Study of polarization versus stored current, time, and tune (Oct, 2002) • Upcoming Plans • Testing of spin flipping rf dipole (November, 2002) • First asymmetry measurements with polarized internal target (Late 2002) • Spin dynamics tests (2003)

23. Preliminary Polarization Measurements at 850 MeV • First polarization data for SHR with strained GaAs photocathode in source • Expected sinusoidal dependence observed on injection orientation, but • Magnitude low compared with Mott and Transmission Pol. Measurements • Lower than expected polarization lifetime • Significant sensitivity of polarization to injected beam current, betatron tune

24. 6 5 4 1 2 3 6 4 5 1 3 2 SHR Polarization Study at 7 mA • Plot shows predicted betatron and spin resonance lines • Study polarization close to expected spin resonance • Take multiple runs at each tune point with different polarization from source • Polarization in SHR averages to 0.73 +/- 0.01, matching well with 0.75 from transmission polarimeter

25. Summary of Low Current Results Injection Tests at 7 mA • Compton polarimeter delivers reproducible results under reproducible beam conditions • Full polarization transport has been observed from source to South Hall Ring • Polarization stable as beam circulates in Ring • No evidence of polarization loss clearly observed during tune scan

26. SHR Polarization Study at 70 mA • Vary tune using SHR quads • Avoid suspected resonances, but tune much more spread out • Polarization results vary much more than at low currents • Large polarization loss for low values of vertical betatron tune

27. Polarization as function of storage time • For nominally good tune, get reasonably high initial polarization • For large injection currents, polarization reduction observed over course of storage time • Betatron tune also changes with storage time for large currents, but extent of correlation unclear • Polarization decay time varies somewhat for different fills • Polarization reduction seems to be in a discrete step (crossing resonance line), rather than continuous loss

28. Summary of High Current Results • Compton polarimeter functions at currents up to 85 mA with aid of absorbers, higher currents possible. • Full polarization has been observed over initial 10 minutes. • Asymmetries are often reduced. The degree of reduction is correlated with betatron tune at injection. • Asymmetry reduction not instrumental. • Polarization is reduced over time as beam circulates in Ring. This reduction is correlated with shifting of the tune. • More data and improved analysis are needed to quantify accurately the dependence of polarization on storage time and beam current.

29. Implications for Experiments • Polarization can’t be assumed constant in SHR! Polarimeter data must be monitored continuously. • Best solution is to minimize polarization changes by optimizing tune, keeping fills short, beam lifetime long. • Polarization may need to be characterized as function of beam current, storage time, and betatron tune. • Polarimeter data stream should be synchronized with BLAST data stream. Qualitatively similar polarization results observed at NIKHEF. Significant dilution of beam polarization for large stored currents limited accuracy of experiments I. Passchier, Ph.D. Thesis, 2000.

30. Observations for EIC Polarimetry • A laser backscattering polarimeter for EIC will have substantially higher analyzing power than Bates. Asymmetry measurements at high precision appear possible. • Need fast DAQ and segmented calorimeter to cope with 1A currents. • Lack of electron spin flipping with radiative polarization design makes systematic error reduction crucial. Minimization of false asymmetries and modeling of detector must be done carefully. • Need as many consistency checks as possible. Multiple polarimeters suggested for EIC, especially if no independent check from low energy polarimeter possible. • Beam tuning will require feedback from polarimeter, especially for self-polarizing ring design. • Collider experiments will need a polarimeter capable of investigating polarization profile of beam.

31. Considerations for EIC design • Location of polarimeters and proximity to Interaction Point • Length of polarimeter interaction region • Laser trajectory and crossing angle • Calorimeter position • Electron beam optics for polarimeter interaction region • Electron beam tuning and diagnostics for interaction region • Detection of electron from Compton scattering? • Can spin flipping of electrons be included in machine design?

32. Other EIC Polarimeter Considerations Many other considerations in polarimeter design including: Calorimeter • Stopping power • Response time • Segmentation • Resolution • Cost Laser • Frequency • Pulse Structure • Power • Optics • Cost DAQ System • Rates • Signal Process • Analysis • Synchronization • Cost Should have good Monte Carlo. Early start helps.

33. Conclusions • MIT-Bates has commissioned a laser backscattering polarimeter for high current operation during internal target experiments with BLAST at energies below 1 GeV. • Polarimeter provides precise results for a typical Ring fill. • Systematic error control and calibration uncertainty at level needed for initial BLAST experiments. • Polarimeter has performed important measurements demonstrating sensitivity of polarization to beam tune. This fast feedback capability is crucial for experiments. • Polarimeter for Electron-Ion Collider will confront many of issues addressed at Bates. Designing polarimeter in conjunction with electron ring is highly advantageous.