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I have added some new stuff to <a
href="http://faculty.cua.edu/sober/HallD/mapping">http://faculty.cua.edu/sober/HallD/mapping</a>
.<br>
I have pasted the text below -- if you are interested in the
figures, go to the web page.<br>
I can talk about it on Monday after ~12:35 (I teach until 12:25).<br>
Dan<br>
<hr size="2" width="100%"><b><br>
</b><b>New field analysis results -- 23 October 2014</b><br>
<br>
<b>B(at NMR probe)/E0 and B(at NMR probe)/Current using scaled
field (Table and plots)</b><br>
<br>
By ray-tracing through the measured and Tosca fields, I have
calculated the scale factor needed to multiply the field to give a
full-energy deflection of 13.400 degrees.<br>
<br>
By interpolation in the field maps to the nominal position of
the NMR probe (x=-12.8 cm, y=-283.0 cm) I have calculated the NMR
value required to steer the full energy beam to the dump at E0 = 12,
13.6 and 6 GeV, corresponding to the nominal map fields of 1.5, 1.7
and 0.75T. Note that the interpolated field at the probe position
at 1.7T differs by about 9 gauss from the actual NMR reading
recorded in the mapping data files -- not a big effect (0.05%), but
bothersome.<br>
<br>
The Tosca field results vary significantly (by more than 0.1%)
from the measured field. This is presumably related to the fact
(noted earlier) that the measured fields increase slightly with x,
while the Tosca fields do not -- probably because the poles deflect
slightly when the field is on.<br>
<br>
<b> Comparison of B vs y at different x with average B and B at
NMR probe position (plots)</b><br>
<br>
The large difference in B(at NMR)/E0 at 6 and 12 GeV seen in the
previous plot is due to a substantial difference in the field shape.
The square data point on each plot shows the interpolated field at
the NMR probe position. At 0.75T this is very close to the maximum
field in the gap, while at 1.5T and 1.7T it is close to a minimum
versus x. The horizontal line shows the average field along the line
x=0.<br>
<br>
The breaks in B seen at the transition between Configurations 1
and 3 (near y=0) are not as big as they look at first glance --
typically 2 or 3 gauss on each curve. I have not made any attempt to
smooth things at that level.<br>
<br>
The increased non-uniformity of B at 0.75T is surprising, but I
saw similar effects with the Hall B tagger magnet. My interpretation
is that it is due to the fact the permeability begins to decrease at
low excitation, so that the lowest-energy field configuration is not
necessarily the most uniform.<br>
<br>
<b>Shifts of E/E0 at the focal plane for 13.6 GeV and 6 GeV
relative to 12 GeV (plot)</b><br>
<br>
This plot shows that the energy calibration of the tagger focal
plane as a function of E/E0 is essentially independent of E0. Using
the 1.5T-derived map for all E0 will give an error of less than 3
MeV, which is less than the resolution of any microscope or
fixed-array channel.<br>
<br>
<br>
<div class="moz-signature">-- <br>
<font color="#ff0000"><i>Daniel Sober<br>
Professor<br>
Physics Department<br>
The Catholic University of America<br>
Washington, DC 20064<br>
Phone: (202) 319-5856, -5315<br>
E-mail: <a class="moz-txt-link-abbreviated" href="mailto:sober@cua.edu">sober@cua.edu</a></i></font><br>
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