Summary of Workshop on Precision Electron Beam Polarimetry Newport News June 910, 2003

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Summary of Workshop on Precision Electron Beam
PolarimetryNewport NewsJune 9-10, 2003

• workshop summary by
• Dave Gaskell, Jefferson Lab
• Richard Jones, Connecticut

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Polarimetry Workshop

• from the Qweak Cost and Technical Review
• High precision polarimetry is a lab--wide
  concern at JLab. ...JLab should consider
  sponsoring a workshop of polarimeter experts to
  examine the prospects and techniques for
  achieving such high precision polarization
  measurements.
• JLab, University of Connecticut, and Hampton
  University jointly sponsored such a workshop this
  summer.

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Workshop held June 9-10 at JLab

• 15 speakers over 2 days
• significant contribution from non-local experts
  (SLAC, HERMES, Basel)
• about 60 participants overall
• organizing committee
• Dave Gaskell (JLab)
• Richard Jones and Kyungseon Joo (Connecticut)
• Cynthia Keppel (Hampton)

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Workshop Agenda I

• Monday, June 9
• Bob Michaels (JLab) Physics Intro
• Ingo Sick (Basel) The Hall C Moller Polarimeter
• Andrei Afanasev (JLab) Radiative Corrections for
  Moller and Compton Asymmetries
• Vladimir Luppov (UMich) Storage of Polarized
  Atomic Hydrogen
• Eugene Chudakhov (JLab) Moller Polarimetry with
  Atomic Hydrogen Targets
• Dave Mack (JLab) Alkali Atom Moller
• Chen Yan (JLab) Iron Wire Basel Moller Plus
  Beam Kicking System for Higher Beam Current
  Operation
• Joe Grames (JLab) Accelerator Tools for Improving
  Polarimetry
• M. Poelker (JLab) The JLab Polarized Source

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Workshop Agenda II

• Tuesday, June 10
• Wolfgang Lorenzon (UMich) The Longitudinal
  Polarimeter at HERA
• Mike Woods (SLAC) SLD Compton Polarimeter
• David Lhuillier (CEA-Saclay) Compton Polarimetry
  at JLab Hall A
• Townsend Zwart (MIT Bates) Electron Polarimetry
  at MIT Bates
• Simon Taylor (MIT) Polarimetry for the Mainz A4
  Experiment
• Richard Jones (UConn) Hall C Compton
  Polarimeter Preliminary Design
• Dave Armstrong (W M) Summary

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SummaryWhy precision polarimetry?

• Approved experiments requiring high precision
  (lt2) polarimetry
• Lead Parity (1)
• Qweak (1)
• Future (12 GeV) experiments that require
  precision polarimetry
• Moller
• DIS-Parity
• More are undoubtedly on the way...

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Moller Polarimetry I

• High precision requires good knowledge of target
  polarization
• Typically targets are iron or iron-alloy foils so
  measurements are invasive
• Hall C Ingo Sick and Chen Yan
• brute force magnetization of iron foil to
  saturation using 4 T solenoid
• beam heating of foil limits to low currents (2-10
  mA)
• could be overcome with slow foil rotation (?)
• Chen Yan suggests iron wire kicker magnet to
  get to higher currents and do continuous
  measurement.

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Moller Polarimetry II

• Hall A Eugene Chudakov and Vladimir Luppov
• replace iron foil with polarized atomic hydrogen
  target
• saturate magnetic field at 8T
• Ptarget 80 (vs. about 8 for iron foil)
• target is thin so it would allow non-invasive
  measurement
• target is hard to build
• Dave Mack look at other options
• alkali atoms
• good for doing in-situ diagnostics
• one day for 1 measurement

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Compton Polarimetry I

• Compton polarimetry provides a continuous,
  non-destructive measurement.
• Compton polarimetry works better at higher beam
  energies where the Compton asymmetry is quite
  large.
• Lorenzon and Woods
• Even at much higher energies (HERMES and SLAC)
  achieving 1 systematics is not trivial.
  Dominant systematic seems to be characterization
  of detector response.

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Compton Polarimetry II

• Lhuillier Hall A Compton is closing in on 1
  measurement.
• small asymmetry, low beam current requires
  high-gain laser cavity
• coincident electron detection allows good
  characterization of photon detector
• at lower energies, the systematics grow
• so far 2 precision achieved for Ebeam gt 2.5 GeV
• shorter wavelengths needed to control systematics
• an upgrade from an IR to green laser is planned
  for lead-parity experiment Ebeamlt 1 GeV
• Jones Qweak is working on Compton for Hall C
• preliminary chicane design and optics solution
  exists
• considering going with a pulsed laser option
  would give more options for choice of high-power
  laser
• choices are between green and UV laser

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Accelerator

• Polarized Source (Poelker)
• The source configuration is constantly changing -
  pay attention
• Polarization is sensitive to spot on
  photocathode, quantum efficiency
• Measure the polarization often, and if possible
  under running conditions
• New Ti-Sapphire lasers will improve bleedthrough
  situation
• Accelerator Tools (Grames)
• Spin-dance can provide useful cross check of Hall
  polarimeters
• 5-MeV Mott will be back in action some time soon
• Transmission polarimeter might be nice complement
  (used at Bates and Mainz)
• High gun current polarimetry may help nail down
  current dependence without sending large currents
  to the Hall

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Points of Discussion

• Is a continuous measurement of the polarization
  really necessary? Perhaps that is overkill?
• How can we make Compton polarimetry work better
  at low energies?
• How can we make Moller polarimetry work better at
  high currents?

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Impact on Hall C

• a proto-type kickerwire target is being
  installed this fall
• Ingo Sick is testing the feasibility of rotating
  an iron foil in an inhomogeneous magnetic field.
• It became clear that a carbon copy of the Hall A
  Compton will not work for Qweak
• A design that decouples the laser from the magnet
  chicane has been developed - this will allow us
  to take advantage of improvements in laser
  technology.