Laser's Phase Influence on the Polarization
Description of the effect
It turned out that the polarization measured may depend on the slit-attenuator configuration
in the injector. HAPPEX was running with the slit retracted (completely open) and no laser attenuation
while the Moller measurements typically used configurations with the slit tight and no/low attenuation.
The latter was done in order to minimize cross-talks from the lasers of Halls B,C running
at a phase opposite to Hall A laser. Additional important parameter of the polarized gun
is "phase". With a narrow slit the "phase" defines what time phase of the laser RF pulse
comes through the slit, which may vary from the maximum of the pulse to a tail,
while with the slit retracted practically all the pulse passes through whatever is
the "phase" parameter. The "phase" was readjusted 2-3 times per week.
The influence of the phase adjustment on the polarization was measured at the end of May at the conditions as follows:
However, since the beam current, laser power, attenuation and slit parameters have been recorded, one can reconstruct the final beam attenuation. Typically, for all the measurements the beam current, the laser power and the laser attenuation were the same. The beam current drops sharply at the edges (see plots 1) and 4)) by a factor of about 5-10, and should drop even sharper if a narrower slit is used, which was the case for the Moller measurements. Therefore, it is unlikely that the phase was accidentally chosen somewhere on the slope, but rather it was either on the top or on the tail. The polarization suffers a 4% drop when the beam current drops by a factor of 5. For a given slit parameter one can decide whether it was the top or the tail using the plot 5).
In order to filter the measurements done for HAPPEX, looking for those likely
been done on the tail, we used 3 values, stored for every run: the beam current,
the laser attenuation and the slit parameter. We had no calibration of the beam
current dependence on the laser power and therefore could not take it into account.
Fortunately, for the most of the HAPPEX measurements the laser power stayed the same,
Assuming that no other parameter
but the laser attenuation and the slit could change the beam current, we
found a factor of the "additional" beam attenuation, attributed to the phase
position off the peak, normalizing to a run definitely taken on the peak.
The definite "tail" run had this factor of about 0.2, consistent
with the the plot 1). This "attenuation factor" is used to select the measurements
taken on the tail. The best we can do is to reject these measurements.
Summary
A potential source of systematic errors was found, associated with the fact that
Moller polarimeter typically runs at a beam current of about 1% of the current
used by the experiments. There are two reasonable ways to reduce the current
to such a low value:
Concerning the measurements already taken, for the HAPPEX run of Apr-May 1999
one is able to reconstruct whether the measurement was taken on the plateau
or on the tail. For earlier runs it is not possible. However, it is just
for that HAPPEX period the injector might be tuned in a way that the "phase"
was off the plateau, since the experiment demanded as high beam current
as possible and an asymmetric phase tune increased the current by 1-2uA,
or so, incorporating the late in time tail of the RF laser structure.
More details
There is an additional information about the injector and laser phase.
In 1999 strained GaAs photo-cathode was used in the injector. In 2009 super-lattice photo-cathode
is used in the injector. In the super-lattice photo-cathode electrons are produced in more thin
layer and phase profile is different.
Injector slit sketch is presented on the plot below. Angle 60o is Hall A laser phase range
for fully open slit.
Profile of beam current vs laser attenuator dependence is shown on the plot below:
For the laser phase study with Moller polarimeter it is the best to have MCC
turn OFF the prebuncher, which can push the head/tail of the bunch to
different temporal locations. Unfortunately, it will affect other Halls beam condition.
Laser phase study on the injector is free of this effect.
2009 Results
In October 2009 new laser phase study with supper-lattice photocathode was done.
Hall A laser phase setting for HAPPEX-III running and for the Moller measurements was the same.
All measuremens were done with the prebuncher turned ON.
No cross-talk from the lasers B and C was found.
Preliminary results are presented on a plot which shows:
a) The Moller coincidence rates depending on the phase for the slit parameters of 14.6, 15.45 and 23.7.
b) The beam current in Hall A for the slit parameters of 14.6, 15.45 and 23.7. For safety reasons at the beam current
higher than 1uAmp the Moller target was moved out of the beam. Moller coincidence rate and beam polarization was not
measured at high current. Different shape and shift of curves at slit parameter 15 and 23 can be explained (by M. Poelker)
with the prebuncher effect.
c) The beam polarization dependence of laser phase. There are a few points (174o, 163.8o, 148o and 144o) with large
statistics. These points give the same polarization in wide laser phase range.
d) This plot shows Moller results for different combinations of slit and attenuator. Every color is corresponded to the
beam polarization was measured with Moller polarimeter at the same day but with different slit/attenuator settings.
It is seen that for the laser phase 163.8o Moller results stay the same in wide range of slit/attenuator settings.
Dependence of the beam current of slit parameter at the same laser phase and attenuator is shown on a
plot .
To do list
In order to finish Hall A laser phase study the next measurements should be done: