[Halld-tagger] Some new tagger field map results

[sober]

I have added some new stuff to http://faculty.cua.edu/sober/HallD/mapping .
I have pasted the text below -- if you are interested in the figures, go 
to the web page.
I can talk about it on Monday after ~12:35 (I teach until 12:25).
Dan
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*
**New field analysis results -- 23 October 2014*

*B(at NMR probe)/E0 and B(at NMR probe)/Current using scaled field 
(Table and plots)*

     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.

     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.

     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.

*   Comparison of B vs y at different x with average B and B at NMR 
probe position (plots)*

     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.

     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.

     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.

*Shifts of E/E0 at the focal plane for 13.6 GeV and 6 GeV relative to 12 
GeV (plot)*

     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.


-- 
/Daniel Sober
Professor
Physics Department
The Catholic University of America
Washington, DC 20064
Phone: (202) 319-5856, -5315
E-mail: sober at cua.edu/
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