The MIT-Bates Compton Polarimeter

South Hall Ring Compton Polarimeter CollaborationT. 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

Presented at Workshop on EIC PolarimetryBrookhaven National Laboratory

November 8, 2002

OUTLINE

1. Laser Backscattering Below 1 GeV2. Description of Bates Compton Polarimeter

• Bates Electron Beam • Laser and Laser Transport• Calorimeter• Data Acquisition and Electronics• Data Analysis

3. Recent Results4. Relevant Considerations for EIC Polarimeters

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 backscatteringremains best method for nondestructive measurement of beam

Low energy polarimeters address some issues which higher energy devices do not face

Compton Polarimeter Analyzing Power

• Compton polarimetry is well established as a method of measuring electron polarization at high energy accelerators

• New efforts underway at MIT-Bates and Mainz. Bates seeks absolute polarization measurement good to 2-3% for experiments with BLAST.

• 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

Bates

EIC

HERA

532 nm laser light

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

300 MeV Ee 1 GeV, 0.40 Pe 0.80, 1 mA Im 200 mA, 5 min 50 min

SHR TypicalParameters

Bates Large Acceptance Spectrometer Toroid

New spectrometer for studying light nuclei with stored beams and internal targets

Polarization in Bates South Hall Ringe

South Hall Ring

COMPTON POLARIMETER

Spin Flipper

• 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 frominternal target by 22.5

o bend, measures

longitudinal projection of beam polarization• Commissioning RF dipole to allow spin reversal with beam stored in Ring

CsI detector

Laser hut

Remotely controlledmirrors

Electron beam

InteractionRegion

Scatteredphotons

Injection Line

RingDipole

Target

Laser exit

Laser line

• 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

MIT-Bates Compton Polarimeter

Compton Polarimeter Laser

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

• Coherent Verdi Laser employs vanadate crystal with lithiumtriborate 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 power since

Laser Optics

• Set of 3 lenses provide focusing over 20 m flight path to IR• Laser chopped by rotating slotted wheel at 10 Hzallowing 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

Interaction Region

• IR contains steering coils and tripletof quadrupole magnets• Beam Position Monitors run continuously• Tune electron beam first to removesteering 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 definedlargely by electron trajectory• Energy versus angle correlation ofgamma rays dependent on electron energy

Scattered Gamma Ray Line

• Calorimeter located 10 m from exit window to reduce background • Movable collimators 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

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 withoutabsorbers, up to 90 mA so far with absorbers

Compton Polarimeter Data Acquisition Criteria

• 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

Fast and reliable data acquisition system is very important

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

500 kHz

20 Hz

0.1 Hz

Read at HPCchange

20 Hz

• Analysis entails subtraction of background from signal as function of gamma ray energy• Normalization of backgroundobtained from scalers or from bremsstrahlung tail of energy spectrum

Polarization Calculation

• Asymmetries formed as function of energy from subtracted yields• Fit asymmetry data with function representing polarimeter analyzing power

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

Example: Helicity-relatedlaser steering plus tight collimation produces falseasymmetry

1” Collimator

AmPS Systematic Errors0.5” Collimator

I. Passchier et al, NIM A414, 446 (2000)

Backscattered Photon Polarization669 MeV Electrons

Recalibrate Gain and Offset

-1.5-1.0

-0.50.00.5

1.01.5

0 4 8 12 16

Photon Energy (MeV)

Pho

ton

Pol

ariz

atio

n

-0.06

-0.03

0.00

0.03

0.06

Ab

sorb

er

Asy

mm

etry

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

Consistency Checks

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

Advantage of Spin Reversal

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)

•Accuracy of polarization extraction significantly enhanced by changing direction of electron polarization

• Leads naturally to the cross-ratio method for calculating the average asymmetry

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

• 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

V.S. Morozov et al, PRST AB 4, 104002 (2001)

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)

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

Compton pol. Pol vs stored Current

20

40

60

80

0 20 40 60 80 100

Injected current (mA)

Po

lari

zati

on

(%

)

Tune 1

Tune 2

Absorber In

Absorber out

12

34 5 6

1 2 3

4

65

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

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

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

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

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.

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.

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.

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?

Other EIC Polarimeter Considerations

Calorimeter

• Stopping power

• Response time

• Segmentation

• Resolution

• Cost

Laser • Frequency

• Pulse Structure

• Power

• Optics

• Cost

Many other considerations in polarimeter design including:

Should have good Monte Carlo. Early start helps.

DAQ System

• Rates

• Signal Process

• Analysis

• Synchronization

• Cost

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.