Transcript Document

Summary of Workshop on Precision
Electron Beam Polarimetry
Newport News
June 9-10, 2003
workshop summary by
Dave Gaskell, Jefferson Lab
Richard Jones, Connecticut
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.
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)
Workshop Agenda I
• Monday, June 9
– Bob Michaels (JLab)
– Ingo Sick (Basel)
– Andrei Afanasev (JLab)
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Physics Intro
The Hall C Moller Polarimeter
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
Workshop Agenda II
• Tuesday, June 10
– Wolfgang Lorenzon (UMich)
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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
Summary:
Why precision polarimetry?
• Approved experiments requiring high precision (<2%)
polarimetry
– Lead Parity (1%)
– Qweak (1%)
• Future (12 GeV) experiments that require precision
polarimetry
– Moller
– DIS-Parity
• More are undoubtedly on the way...
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.
Moller Polarimetry II
• Hall A: Eugene Chudakov and Vladimir Luppov
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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
Compton Polarimetry I
• Compton polarimetry provides a continuous, nondestructive 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.
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 > 2.5 GeV
– shorter wavelengths needed to control systematics
– an upgrade from an IR to green laser is planned for lead-parity
experiment Ebeam< 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
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
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?
Impact on Hall C
• a proto-type kicker+wire 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.