A MiniPrimer for Parity Quality Beam

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A Mini-Primer for Parity Quality Beam (as seen
from the Accelerator)

• Outline
• Polarized Beam Experiments
• Parity Experiments (the bar lowers)
• The Imperfect World
• Sources of Problems
• Measurement, Controls Feedback
• Summary

PQB Meeting April 08, 2004 J. Grames
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Why Polarized Beams and Targets?
To learn the significance of particle spin in the
nuclear interactions studied at JLab we take
advantage of preparing the beam and/or target
electrons to be polarized.
Either (beam or target) is polarized if there is
a net difference in the number of spin states
along some physical direction, e.g.,
(N - N-)
Polarization
(N N-)
(9 - 1)
80
(9 1)
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Polarization Experiments
The common technique youll find for learning the
spin physics interaction is to reverse the sign
of the beam (or target) polarization and measure
the relative difference in detected signal
(R - R-)
Aexp
Aphysics Pbeam Ptarget
(R R-)
Flip one or other

• For most experiments the z-component is
  important. This explains why
• Experiments need longitudinal beam polarization.
• The word helicity is used (spin
  parallel/anti-parallel momentum).

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Parity Experiments
Heres the catch. For parity experiments the
experimental asymmetry is very small.
Experiments like G0 and HAPPEX-2 are interested
in measuring asymmetries of order 1-10 ppm. One
of the presently approved experiments in Hall A
would seek something less than 1 ppm. Further
down the road, it becomes even smaller. The
challenge for these experiments is generally
controlling the systematics, as opposed to making
the measurement (spectrometer/detectors/electronic
s).
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The Imperfect World
So, if R or R- changes because of anything other
than the spin physics of the interaction, it is a
false asymmetry. This results in the seemingly
unattainable, golden rule for parity
experiments No beam property other than the
beam polarization should change when the beam
polarization reverses sign.

• But, beam properties do change
• Intensity (first order)
• Position (second order)
• Energy (second order)

• These come in different ways
• Laser light
• Photocathode
• Accelerator

These happen before the electrons are even a beam
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Parity Violation Experiments at CEBAF
Parity violation experiments continue to set the
standard for Polarized Source performance.
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The Polarized Electron Source

• Electrons are produced via photoemission, using a
  laser beam.
• The sign and degree of the electron beam
  polarization is determined by the sign and degree
  of circular polarization of the laser beam.
• The lasers produce linearly polarized light.
  With the application of high voltage (few
  kiloVolts) Pockels cells (electro-optic devices)
  convert the linearly polarized laser beam into a
  circularly polarized laser beam.
• By reversing the Pockels cell voltage the
  helicity of the laser beam, and thus the electron
  beam, is reversed.
• This is the Helicity reversal. Anything that
  changes with this reversal is said to be
  Helicity Correlated.

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What can defy the golden rule?
Cathode
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Quality of Laser Polarization the Photocathode
Photocathode
Purity Even 99.99 circular light is 1.4
linear. When circular light reverses sign linear
light rotates by 90 degrees. High-P
photocathodes have a QE anisotropy, meaning they
emit a different number of electrons in
orthogonal directions defined by the material, so
voila, the intensity can vary by the percent of
linear light.
More QE
Less QE
Uniformity The profile uniformity of the laser
polarization depends on the Pockels cell crystal
material and cell design. Poor uniformity can
result in the centroid of the transverse charge
distribution moving, producing a measured
position difference.
99.93
99.92
99.90
99.95
Laser profile
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Accelerator

• A HC position difference on ANY aperture results
  in a HC intensity asymmetry. (Note we use
  absolute difference for position and relative
  asymmetry for intensity).
• Apertures (Profile Position)
• Emittance/Spatial Filters (A1-A4)
• Temporal Filter (RF chopping apertures)
• Beam scraping monitors.
• Any piece of beampipe!
• The small apertures and tight spots (separation?)

Adiabatic damping of the beam emittance may gain
factors of 10-20 because of the reduction in
amplitude of the beam envelope. Poor optics can
reduce this gain by 10x. Poor optics stability
can vary response between source and user.
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Diagnostics for Measuring HC Beam Properties
BCMs (intensity) and BPMs (position) are the
main diagnostics used. Dedicated parity DAQs
for both G0 and HAPPEX-2 exist in the injector
and in the experimental halls. The beam
properties each period of the helicity reversal
(33 ms). We integrate 10,000 samples to get a
statistically meaningful result. Although it is
intellectually satisfying to measure the parity
beam quality at the injector the diagnostics
measure all beams simultaneously.
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The 3-User Laser Table
All beams have common path
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Laser Beam Controls (common to all lasers)
30 Hz PZT (optics)
Pockels cell (makes circular light)
Steering Lens (positions laser on photocathode)
Insertable halfwaveplate (flips sign of
polarization)
Rotatable half waveplate (nulls analyzing power)
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Laser Beam Controls (independent feedback knobs)
PZT Mirror for Position
IA cell for Intensity
Laser output linearly polarized
Add non-HC elliptical polarization
Analyze light
Add HC elliptical polarization
Commissioning Helicity Correction Magnets for
Position
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Injector Helicity Magnets Installation
(0L01-0L03) January 5-6, 2004
MHE0L03V, MHE0L03H
MHE0L02H
MHE0L01V
110 VAC Isolation Transformer
Grounded cage containing electrically isolated
helicity magnet controls (VME)
Tube protecting Litz magnet wire
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HC Software Controls
The parity experiments want to null the HC
effects at the hall (or further upstream). The
parity users implement their own feedback
algorithm using the HC knobs of the injector.
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Summary Outlook
Parity experiments are different than most
experiments done at Jlab because the experiment
includes the accelerator performance. From the
first Jlab parity experiment (HAPPEX-I) the EGG
and users have worked together on parity issues
concerning the polarized source. Recently, with
G0 and looking forward to more difficult parity
experiments broader involvement of the
accelerator division (CASA) has become
critical. Other electron accelerators have a
longer history of parity violation experiments,
e.g. SLAC or MIT-Bates, however Jlab is poised to
confront some of the most difficult proposed. We
need to continue being more comprehensive for the
present and future parity experiments to be
successful.