# Presentation: 512951

## The MIT-Bates Compton Polarimeter

**The MIT-Bates Compton Polarimeter**

- _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*

- Presented at Workshop on EIC Polarimetry
- Brookhaven National Laboratory
- November 8, 2002

## OUTLINE

**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

## Compton Polarimeters For Storage RingsBelow 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

## Compton Polarimeter Analyzing Power

**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

**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 ************  E******e ************  1 GeV, 0.40 ************  P******e ************ 0.80, 1 mA ************ I******m ************ 200 mA, 5 min ************ ************ ************ 50 min***

- SHR Typical
- Parameters

## Bates Large Acceptance Spectrometer Toroid

**Bates Large Acceptance Spectrometer Toroid**

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

## Polarization in Bates South Hall Ring

***Polarization in Bates South Hall Ring***

***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 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

## MIT-Bates Compton Polarimeter

- CsI detector

- Laser hut

- Remotely controlled
- mirrors

- Electron beam

- Interaction
- Region

- Scattered
- photons

- Injection Line

- Ring
- Dipole

- 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

## Compton Polarimeter Laser

**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 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 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 *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

## 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

## Scattered Gamma Ray Line

**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 without
- absorbers, 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

**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 HPC
- change

- 20 Hz

##  Analysis entails subtraction of background from signal as function of gamma ray energy

-  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

**Polarization Calculation**

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

## Systematic Error Investigation

**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-related
- laser steering plus tight
- collimation produces false
- asymmetry

- 1” Collimator

- AmPS Systematic Errors

- 0.5” Collimator

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

## 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

**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

## 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

- <date/time>

-  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

## SHR Polarization 

**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.