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User name V. Sulkosky
Log entry time 19:01:18 on November 01, 2008
Entry number 245995
This entry is a followup to: 245303
keyword=Analysis of Bulls eye Scan
I have concluded my analysis of the Harp scans and CODA runs taken for
the bulls eye scan on October 29th.
I have been analyzing the BPM and Harp data starting with the Coulomb Sum
Rule experiment, which performed a few bulls eye scans as well as some
data taken this past June. The short summary is that the Harp A X
position compared to the EPICS BPMA X position is now shifted by about
1 mm. This can be seen in the first attached figure (top left-hand
corner). Prior to the recent problem with Harp A, the various data for
Harp A versus the EPICS readout were very consistent. The blue data
points are from the recent scan, and the red points are accumulated
data from CSR and this past June.
There are also some smaller shifts seen in the other positions, but they
are within 500 um. For Harp A Y and Harp B X, the difference is at the
300 um level and probably not significant. I did take into account the
survey results from October for the harps. Compared to earlier surveys,
the Harp B X and Y offsets changed by about 200 microns. The offsets
for Harp A changed well within 100 microns.
The first figure shows the plots with a fit to the earlier data. The
second figure shows the plots with a fit to the current data. During
the Coulomb Sum Rule experiment, the coupler for Harp B was slipping.
This explains the large outlier points for BPMB B greater than -5 mm.
This problem was fixed in March and the new data lies reasonably close
to the data I acquired back in June. Due to the nature of the analysis,
issues in the absolute X position DO NOT translate into a problem in the
Y direction. In fact, the Harp A Y position shows very good agreement
with the earlier data with a small systematic shift (0.28 mm).
This analysis makes the assumption that the EPICS BPM information is
stable. From my earlier analysis, this appears to be a safe assumption,
but we know things change over time.
At the next major down, I think we should have a complete survey of both
Harps. I am afraid we might not be able to rely on them for an absolute
measure of the beam position to better than 0.5 to 1.0 mm at this point
in time. For the time being, I can recalibrate the BPMs from the ADC
readout using the Harps, but I will use the EPICS position for the
BPMA X position instead of the Harp results.
A copy of this log entry has been emailed to: brads,jiang,moffit,jpchen