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User name silwal
Log entry time 13:41:25 on November 10, 2009
Entry number 298758
keyword=photocathode rotated by ~90 degrees in the injector between the end of HAPPEX III and start of PVDIS
Excerpts from Joe Grames' email:
The photocathode was rotated about 1 week ago, between the end of
HAPPEX-III and the start of PVDIS. I removed the photocathode from
the gun chamber on 11/2 to heat it and following activation on 11/3 put
it back in the gun chamber. Based upon before/after "QE scans" the
photocathode appears rotated by approximately 90 degrees. Consequently,
we are not at the same location on the photocathode - even though we
are using the same x/y steering lens value now as before the gun work.
Another point worth knowing is that during the last power outage the
hardware which provides remote control of the IA waveplates was
damaged. We didn't think this was a problem for you considering you
use PITA instead of the IA, but if ever you're interested to change the
IA slope it cannot be done remotely. We plan to fix during the winter
shutdown, after PVDIS.
At the same RHWP setpoit as we had during HAPPEX, 1200, the current PITA
slope is about 26-30 ppm/V, which is much larger than ~4ppm/V that we had
during HAPPEX running. The current PITA position difference slopes are
about 0.017 and 0.025 um/V, compared to about 0.02-2.0 um/V during HAPPEX
The feedback has been turned on for the past two days, and the charge
asymmetry is well behaved. The position differences are below 200 um.
Position differences from run # 13864 (production run from owl 10Nov09)
bpm4ax Mean -0.125524 +- 0.0980392 Width 18.4912
bpm4ay Mean -0.138012 +- 0.107152 Width 20.2099
bpm4bx Mean -0.137445 +- 0.105362 Width 19.8724
bpm4by Mean -0.15233 +- 0.111537 Width 21.037
bpm12x Mean 0.0100528 +- 0.0453234 Width 8.54847
Usually we would prefer to run with orientation that has a small amount
of analyzing power, which is just big enough to accommodate the charge
feedback. A lot of analyzing power makes us more sensitive to higher
order effects, namely position differences and spot size differences.
Despite the much larger PITA slope (hence the analyzing power) at
present, the position differences seem to be under control (at least to
the extent we care about for PVDIS).
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