Transcript Why Polarized Beams and Targets?

A Mini-Primer for Parity Quality Beam
(as seen from the Accelerator)
Outline:
1. Polarized Beam Experiments
2. Parity Experiments (the bar lowers)
3. The Imperfect World
4. Sources of Problems
5. Measurement, Controls & Feedback
6. Summary
PQB Meeting
April 08, 2004
J. Grames
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.,
Polarization =
(N+ - N-)
(N+ + N-)
(9 - 1)
(9 + 1)
= 80%
Polarization Experiments
The common technique you’ll 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:
Aexp =
(R+ - R-)
(R+ + R-)
= Aphysics • Pbeam • Ptarget
Flip one or other…
For most experiments the z-component is important. This explains why:
a) Experiments need longitudinal beam polarization.
b) The word helicity is used (spin parallel/anti-parallel momentum).
Parity Experiments
Here’s 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/electronics).
Aexp =
(R+ - R-)
(R+ + R-)
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
These happen before the
• Photocathode
electrons are even a beam…
• Accelerator
Parity Violation Experiments at CEBAF
Parity violation experiments continue to set the standard for Polarized Source performance.
Experiment
Physics
Asymmetry
Total Experiment
Position Difference
Total Experiment
Intensity Asymmetry
HAPPEX-I
13 ppm
10 nm
1.0 ppm
G0
2 to 50 ppm
20 nm
1.0 ppm
HAPPEX-He
8 ppm
3 nm
0.6 ppm
HAPPEX-H
1.3 ppm
2 nm
0.6 ppm
Qweak
0.3 ppm
20 nm
0.1 ppm
Lead
<1 ppm
1 nm
0.1 ppm
The Polarized Electron Source
1.
Electrons are produced via photoemission, using a laser beam.
2. The sign and degree of the electron beam polarization is
determined by the sign and degree of circular polarization of
the laser beam.
3. 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.
4. By reversing the Pockels cell voltage the helicity of the laser
beam, and thus the electron beam, is reversed.
5. This is the “Helicity” reversal. Anything that changes with
this reversal is said to be “Helicity Correlated”.
What can defy the golden rule?
Steering (Position)
Pockels
Cell
PITA (Intensity)
HV
Pockels
Cell
Intervening
Optics
Lensing
Pockels
Cell
Cathode
Quality of Laser Polarization & the Photocathode
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.
Laser profile
More QE
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.
Photocathode
99.92%
99.95%
Less QE
99.93%
99.90%
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.
Diagnostics for Measuring HC Beam Properties
BCM’s (intensity) and BPM’s (position) are
the main diagnostics used.
Dedicated parity DAQ’s 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.
The 3-User Laser Table
All beams have
common path
Laser Beam Controls (common to all lasers)
30 Hz PZT
(optics)
Pockels cell
(makes circular light)
Insertable halfwaveplate
(flips sign of polarization)
Steering Lens
(positions laser
on photocathode)
Rotatable half waveplate
(nulls analyzing power)
Laser Beam Controls (independent feedback knobs)
PZT Mirror for Position
IA cell for Intensity
Laser output
linearly polarized
Add HC
elliptical
polarization
Add non-HC
elliptical
polarization
Commissioning: Helicity Correction Magnets for Position
Analyze
light
Injector Helicity Magnets
Installation (0L01-0L03)
January 5-6, 2004
MHE0L03V,
MHE0L03H
MHE0L02H
MHE0L01V
Tube protecting
Litz magnet wire
Grounded cage containing
electrically isolated helicity
magnet controls (VME)
110 VAC
Isolation
Transformer
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.
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.