Stability and helicity-correlated asymmetries in the polarized ... - MIT

STABILITY AND HELICITY-CORRELATED
ASYMMETRIES IN THE POLARIZED ELECTRON GUN
D. Barkhuff, G.Dodson, M. Farkhondeh, E. Tsentalovich, B. Yang, T. Zwart
MIT-Bates Linear Accelerator Center, USA
The polarized electron source at the MIT-Bates Linear Accelerator Center has been
used for physics experiments since 1985 [1]. The latest source upgrades improved
performance significantly. Presently, the gun produces a high quality beam with an
average current of 120 µA and a life time of 200-300 hours, and has been delivering
average of 2.5 Coulombs per day onto the experimental target for two months.
Experiments studying parity violation [2,3] measure very small asymmetries - of
order of several parts per million (ppm). In order to reduce systematic errors, the
properties of the beam (intensity, size, position, energy) must remain unchanged when
the helicity of polarized electrons is reversed. Also, the stability of the beam must be very
high in order to reduce the width of the measured asymmetries. For example, the
SAMPLE experiment at Bates [3] requires helicity-correlated asymmetry in the beam
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current well below 10 , less than 300 nm in the beam position, and an overall beam
current stability better than 1%. This paper analyzes the most important sources of
instabilities and helicity-correlated asymmetries in the beam.
1. BEAM STABILITY
The stability of the beam is determined by the stability of the initial laser beam, the
stability of the laser beam transport, the stability of the electron beam extraction from a
GaAs crystal and the stability of the electron beam transport. In principle, most of the
problems could be solved with unlimited laser power. In this case an electron gun in the
regime of full space charge saturation can be used, and the electron beam transport would
be the only source of the beam instability.
The Bates polarized injector is used to produce a very high average current (up to
120 µA) and this approach is impractical, so we have to address all mentioned issues.
1.1 Laser stability
The Bates linear accelerator produces ∼18 µs beam pulses at a repetition rate of 600
Hz. The required peak laser power is several Watts. Since no commercial pulse lasers
with such parameters are available, we are using a CW Ti:Sapphire laser pumped by
Argon ion laser. An electro-optical chopper consisting of a pair of crossed linear
polarizers with a λ/2 Pockels cell in between cuts 18 µsec pulses out of a CW beam. The
rest of the beam is dumped in the second polarizer.
Although the Argon laser produces up to 30 W of CW power, the Ti:Sa laser can be
pumped by only 25 W of CW power. A higher power level overheats the crystal and
creates a “thermal lens” effect that changes the optical properties of the laser cavity,

causes beam instability, and affects the mode structure of the laser beam. In order to
minimize the heat load on the crystal, a mechanical chopper has been installed between
the two lasers. The mechanical chopper is a rotating slotted wheel that cuts 250 µs pulses
out of CW beam from the Ar laser. The wheel produces pulses at a repetition rate of 600
Hz and is phase-locked with the pulses firing the electro-optical chopper. The mechanical
chopper reduces the average power on the Ti:Sa crystal by a factor of 7, but keeps the
peak power intact.
Nevertheless, the stability of the laser in this “pulsed” mode with a peak pump power
of 30 W is worse than in CW mode, in both intensity and mode structure. Very careful
tuning is needed to achieve high output power with good intensity stability and mode
structure. The intensity stability of the output beam is usually 1% or better.
Electro-optical stabilizers are used to improve the stability further. Commercially
available stabilizers consist of a Pockels cell followed by a linear polarizer. A beam
splitter after the polarizer splits a small fraction of the beam to a detector for
measurements. A feedback circuit controls the voltage applied to the Pockels cell and
keeps the intensity of the beam that passes through the polarizer constant. A drawback of
these systems is substantial intensity loss (30% or more). We developed a stabilizer with
very low loss (∼10%) using a bipolar 200 Volt amplifier with a gain bandwidth of 100
MHz. Rather than the light intensity, the current of the extracted beam measured by a
toroid is used as a control signal. This suppresses instabilities in laser intensity, extraction
efficiency and the first stage of the electron beam transport combined. The difference
between the toroid’s signal and the preset level is amplified and applied to a Pockels cell.
The stabilizer improves the stability by a factor of five. A stability of 0.2-0.5 % in the
electron beam current is routinely achieved at the end of the injector.
1.2 Laser beam transport and beam extraction
The Quantum Efficiency (QE) of the crystal may vary across the crystal, especially if
the crystal has been used for a long time. The QE may also depend on the density of the
current extracted (current saturation effect [4,5]). The first effect leads to a dependence of
the intensity of the extracted current on the beam position. The second one leads to a
dependence on the beam size.
The beam size variation can only be suppressed by a careful tuning of the laser to a
stable mode structure. In order to avoid beam position instabilities on the crystal, a lens is
placed in the middle of the path between the laser and the gun. (The total length of the
path is ∼20 m.) The lens produces a point-to-point focusing between the laser and the
GaAs crystal, so angular variation of the laser beam does not produce position variation
of the beam on the crystal.
The entire laser transport path is enclosed in pipes to avoid air steering. The section
of the pipe that leads from the laser room into the accelerator vault below is sealed with
flat windows at both ends to suppress a chimney effect.
The properties of the window through which the laser beam enters the vacuum
chamber may affect the laser beam stability. The old version of the window was equipped
with a high-quality flat glass without a wedge. The “normal” reflectivity of the glass is
8%. If the wavelength of the laser beam matches the window thickness, the beams
reflected from the front and back surfaces of the window interfere and the reflectivity
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may be very close to zero. Our laser system has a wide bandwidth ∆ω/ω∼10 and the

wavelength of the laser light is unstable in this range with a characteristic time of few
seconds. The phase advance between the light reflected from the front and from the back
surface of the window ∆ϕ=2π(∆ω/ω)(2d/λ) (d=4mm is a thickness of the window) varies
from 0 to 2π, and the reflectivity of the window changes from 8 to 0%. The problem was
solved by installing a slightly wedged glass in the window.
1.3 Electron beam transport
The electron beam passes through the chopper at the entrance of the accelerator. The
capture efficiency is 30-40% and strongly depends on the beam position on the chopper
collimator. Therefore, beam position variation results in intensity instability of the beam.
In the area in and immediately after the gun the beam energy is low, and the beam is
especially vulnerable to the influence of external magnetic fields. In order to protect the
beam from dc magnetic fields the beam line is wrapped with sheets of µ-metal. All ion
pumps in vicinity of the beam line are equipped with soft iron shields. The residual
magnetic field in the beam area is less than 1 Gauss.
Any equipment in the vicinity of the gun may produce an electromagnetic field at a
frequency of 60 Hz, so all power supply transformers, the main sources of this field, are
covered with soft iron shields. A major source of 60 Hz field was a separate isolation
transformer which supplies the gun equipment with ac power. The transformer produced
a field of few tens of mGs inside the gun chamber and in the beam line. This ac field was
sufficient to move the beam as much as 2-3 mm at the end of the beam line. The problem
was significantly reduced by moving the transformer about 6 m away from the gun.
The stability of all steering elements in the beam line has been analyzed and only
power supplies that meet these stability requirements are used. The controlling
electronics is installed inside screened cabinets to suppress interference from external
radio-frequency signals.
Any insulator inside the beam line exposed to the beam may be a source of
instabilities. A stream of scattered electrons could charge the insulators up to a voltage
that is sufficient to steer the beam. When the charge on the insulator reaches a critical
value a discharge occurs, and the beam position changes instantaneously. To avoid this
effect, all insulators must be hidden from the beam behind metal screens.
2. HELICITY-CORRELATED ASYMMETRIES
A helicity-correlated intensity asymmetry originates in an imperfect symmetry of the
laser transport system. Circularly polarized light with left (right) helicity is produced by
applying a positive (negative) voltage to a λ/4 helicity Pockels cell (HPC). Since a small
fraction of linearly polarized light always remains in the beam, any asymmetry for S and
P reflection in the laser transport system leads to a helicity-correlated asymmetry in the
beam intensity.
A transport system containing reflecting mirrors or prisms may produce a helicitycorrelated
positional asymmetry as well. We believe that the effect originates in different
distributions of the linear polarization fraction in the beam profile for the left and right
helicities, combined with an imperfect symmetry of the transport system. (We thank

Douglas Beck for the suggested explanation.) The effect may produce a positional
difference as large as several microns. The HPC itself could produce an angular
asymmetry for the left and right helicities due to a piezo effect.
In order to suppress these asymmetries the HPC was placed at the end of the laser
transport line, after the last reflecting mirror. This step virtually eliminated the positional
asymmetry associated with the transport line, and careful tuning of the HPC reduces the
HPC-induced asymmetry to well below 300 nm.
A correction Pockels cell (CPC) is used to control the remaining intensity
asymmetry. A λ/10 wave plate installed in front of the CPC produces a small fraction of
circular polarization in the laser beam. In this configuration a small voltage applied to the
CPC changes the fraction of the beam passing through the polarizer after the CPC. The
CPC power supply produces negative pulses for one helicity and positive pulses for
another in order to equalize their intensities. The power supply is controlled by a slow
feedback system that measures the helicity-correlated intensity difference of the electron
beam at the end of the accelerator. Although the accuracy of these measurements is
several ppm in a half-hour run, the intensity difference averaged over 1-2 days run is
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usually below 10 . The angle of rotation of the λ/10 wave plate defines the sensitivity of
the beam intensity to the CPC voltage, and we set it to about 50 ppm/Volt. This method
allows the control of the helicity-correlated intensity of the beam with a very small
differential voltage on the Pockels cell, and the positional asymmetry produced in the
CPC due to piezo effect is vanishingly small.
In order to suppress a residual positional asymmetry the SAMPLE collaboration
developed a correction device. It includes a piezo-driven optical flat. The angle of the flat
relative to a laser beam can be controlled in a helicity-correlated way with a very high
frequency (several kHz) and a helicity-correlated laser beam shift of up to 1 µm in any
direction can be produced. This controlled shift compensates the residual positional
asymmetry. Preliminary results indicate that with the device positional asymmetries
averaged over 24 hour run are reduced from ≈200 nm to ≈50 nm.
Summary: with implementation of improvements discussed in this paper, exceptional
quality and stability of the beam have been achieved. The beam parameters meet or
exceed the demanding specifications of the SAMPLE experiment. To date, more than 80
Coulombs have been delivered to the SAMPLE target and the experiment is nearly
finished.
References
[1] G.D.Cates et al, NIM A 278, 293 (1989).
[2] P.A.Soulder et al, Phys. Rev. C 65, 694 (1990).
[3] B.Mueller et al, Phys. Rev. Lett. 78, 3824(1997).
[4] M.Farkhondeh et al, Proc. of 7th Intern. Workshop on Polarized Gas Targets and
Polarized Beams, Urbana, IL, August 1993, AIP 421, p.240.
[5] A.S.Jaroshevich et al, Proc. of 7th Intern. Workshop on Polarized Gas Targets and
Polarized Beams, Urbana, IL, August 1993, AIP 421, p.485.