BXS Re-calibration Page: 1 of 3
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BXS Re-calibration *
June 20, 2007
Early in the commissioning it was noticed by Cecile
Limborg that the calibration of the BXS spectrome-
ter magnet seemed to be different from the strength
of the BXO1/BX02 magnets. First the BXO1/BX02
currents were adjusted to 135 MeV and the beam
energy was adjusted to make the horizontal orbit
flat. Then BXO1/BX02 magnets were switched off
and BXS was adjusted to make the horizontal orbit
in the spectrometer line flat, without changing the
energy of the beam. The result was that about 140-
141 MeV were required on the BXS magnet. This
measurement was repeated several times by others
with the same results.
It was not clear what was causing the error: mag-
net strength or layout. A position error of about
19 mm of the BXS magnet could explain the differ-
ence. Because there was a significant misalignment of
the vacuum chamber in the BXS line, the alignment
of the whole spectrometer line was checked. The vac-
uum chamber was corrected, but the magnets were
found to be in the proper alignment. So we were
left with one (or conceivably two) magnet calibration
Because BXS is a wedged shaped magnet, the bend
angle depends on the horizontal position of the in-
coming beam. As mentioned, an offset of the beam
position of 19 mm would increase or decrease the
bend angle roughly by the ratio of 135/141. The
figure of 19 mm is special and caused a considerable
confusion during the design and measurement of the
BXS magnet. This is best illustrated in Figure 1
*Work supported in part by the DOE Contract DE-AC02-
76SF00515. This work was performed in support of the LCLS
project at SLAC.
which was taken out of the BXS Traveler document.
The distance between the horizontal midplanes of the
poles and the apex of the beam path was chosen to be
19 mm so the beam is close to the good field region
throughout its entire path. Thus it seemed possible
that there was an error that resulted in the beam
not being on this trajectory, or conversely, that the
magnetic measurements were done on the wrong tra-
jectory and the magnet was then mis-calibrated. Me-
chanical measurements of the vacuum chamber made
in the tunnel indicated that the vacuum chamber was
in fact in the proper position with respect to the mag-
net not 19 mm off to one side so the former
possibility was discounted. Review of the Fiducial
Report and an interview with Keith Caban convinced
me that there was no error in the coordinate system
used for magnet measurements.
I went and interviewed Andrew Fischer who did
the magnetic measurements of BXS. He had exten-
sive records, including photographs of the setups and
could quickly answer quite detailed questions about
how the measurement was done. Before the inter-
view, I had a suspicion there might have been a sign
flip in the x coordinate which because of the wedge
would result in the wrong path length and a miscali-
bration. Andrew was able to pin-point how this could
have happened and later confirmed it by looking an
measurement data from the BXG magnet done just
after BXS and comparing photographs. It turned
out that the sign of the horizontal stage travel that
drives the measurement wire was opposite that of the
x coordinate in the Traveler, and the sign difference
wasn't applied to the data. The origin x 0 was
set up correctly, but the wire moved in the opposite
SLAC National Accelerator Laboratory, Menlo Park, CA 94025
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Welch, J. BXS Re-calibration, report, November 24, 2010; United States. (digital.library.unt.edu/ark:/67531/metadc1014042/m1/1/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.