Log entry time 20:24:09 on July 3,2004

Entry number 127859

We are preparing a "data dump" also, so the general descriptions 
given here can be checked (and further correlations explored) by 
other observers.  

--------------------------------------------------------------------

Started study at the last running point (1085, 1050), about 
(1.55mm, 2.19mm) from the electrostatic center of the cathode.
There is nothing special about those numbers, but just so you know.

We performed an RHWP scan @ QWV and at -200 V (PITA offset), for
both IHWP IN and OUT.

At nominal QWV, the charge asymmetry response showed non-unreasonable 
offsets (few hundred ppm) and very small 4-theta modulation terms 
(<30 ppm) which suggests that at QWV we still have a very well-tuned
Pockels cell. With the PITA offset, the 4-theta term becomes very large
(2000 ppm) as it should.  The 4-theta and 2-theta term properly swap
sign with IHWP flip.

As far as position differences, we observed about what we expected 
(not that we were happy about it). At QWV, there were large position 
differences offsets (several microns), with very small modulation 
amplitudes.  The modulation amplitudes became significant (>1 micron at 
some BPMs) with the PITA offset, but as a rule were smaller than the
offset position differences.

The position differences at some bpms appeared to modulate with a shape
that looked precisely like the charge asymmetry, which had me worried
about a pedestal error tying Aq to pos difs.  Other tests (including
IA scans) reassured me that we were fine in this regard, so we do 
trust this data.

The pos dif behaviour is consistent with large steering effects (or 
polarization gradients which analyze upstream of the RHWP), although
have absolutely no idea why we should have such an effect. The fact
that the PITA offset produces a pos dif modulation, suggests that there
is a significant gradient in the analyzing power of the cathode. We've 
seen such gradients before, but we don't know off-hand whether this effect
is larger than before.

We checked the results from the Hall A beam using the Hall C laser (the IA
cell was used at every point to zero Aq, so that systematic error was 
minimized).  The Hall C laser showed larger modulation amplitudes than
the Hall A laser, which suggests that the average linear polarization
of this laser on cathode was larger than the Hall A with a 200 V PITA offset.
More surprisingly, the offsets (that didn't modulate with RHWP) were 
also larger.  We don't have a model to suggest why.  With IHWP
removal, the modulation roughly switches sign and the offsets don't 
(just like for the A laser).

Then we ran the Hall C laser to the slit at about 20 uA, and took 
20 uA of Hall A beam to the 45 MeV dump.  This allowed us to look 
for the effect of the Hall C beam on cathode, while measuring just
the A beam in the 0L (and 0I05) regions.  At each point in the 
waveplate scan, we zeroed the Hall C beam with the IA cell (the 
IA settings were found during the previous scan) so that any affect
on the Hall A beam wasn't some simple "charge limit" type affect. 

Position differences were relatively unaffected.  The charge asymmetry,
surprisingly, was affected.  Below we've included figures of the charge 
asymmetry from a RHWP scan without C beam (top) and with C beam (bottom).
The amplitudes and phases of the standard 2- and 4-theta fits change 
significantly.  We didn't have transmission problems through the slit
before or after this run, so it would be pretty surprising if this was
just a weird clipping effect. The Hall C beam charge asymmetry, measured
at these RHWP positions and IA settings in the previous run, jumped around
at the couple hundred ppm level.  We have no model to explain the very 
different A-beam charge asymmetry with the C laser on the cathode.

Before going on to a comparison of the several cathode spots: we 
observed a very large position difference "4-peak" splitting.  This
splitting was as large as 18 um, and was observed in both x and y, 
even for runs with the PITA slope as small as 3.5 ppm/V.  A more 
detailed study is needed to see if these splitting scaled with PITA
slope.  Significant position difference splitting has been seen before; a
more careful comparison to past results should be made to see if this
effect is consistent in magnitude.

A comparison was made with several cathode spots.  PITA scans and
RHWP scans were made at 3 other spots on the cathode.  One of these was 
very close to the electrostatic center (which was also very close to
one of the spots we used during the Helium experiment). Analyzing
power varied by about 10% between the different spots.

The position difference behaviour was slightly different for each spot, 
with the dominant effect being the offset which was not modulated by 
RHWP angle. They all had some amount of Delta-y offset, but more 
variation was seen in Delta-X (one spot even had very little 
Delta-x offset).

The most glaringly weird thing we saw was the charge asymmetry at the
spot near the electrostatic center of the cathode.  This area had very 
low QE, so our laser power was at maximum. There may have been a strong
QE slope at this point, due to the deep hole we were near.  The
charge asymmetry offset was unaccountably huge (1800 ppm) and the
2- and 4-theta terms changed significantly as well.  The position 
differences weren't obviously much worse from this spot.