[Halld-tagger] Tagger magnet excitation studies

[Daniel Sober]

I have finally gotten around to analyzing Yang's Tosca midplane maps at 
excitations of 6, 9, 12, 14, 15, 16 and 17 kilogauss.  The results are 
posted at http://faculty.cua.edu/sober/HallD/Excitation .

The most informative single plot is probably the one labeled "Plot vs B" 
(http://faculty.cua.edu/sober/HallD/Excitation/Bnorm_vs_B.pdf). If the 
permeability were infinite, the field per ampere-turn would be constant 
at about 0.83 gauss/amp-turn.  The figure shows that the saturation 
effects are already significant at the operating field of 15 kG.  If you 
look at the "fine" plots of B/NI versus z, you will note that the 
saturation effects are not uniform through the magnet.  For B>15 kG, the 
field at the ends of the magnet (z = ±300 cm) is slightly higher than 
the field at the center (z=0), while at lower fields the z-dependence is 
negligible.
Conclusions for mapping:
1.  If there is any possibility of running above 12 GeV, we must map at 
the highest conceivable field.  (What is the expected power supply limit?)
2.  To allow for running at beam energies other than 15 kG, we should 
map the magnet at 16, 15, 14, 12 and 6 kG.  Some of these maps may be 
made at coarser granularity if time is short.
3.  Note that the Tosca calculations were made using the "default steel" 
permeability table, so we must make measurements at several excitations 
to test the reliability of the Tosca calculations.

I will be traveling in Europe from June 13-July 14.  See you next month.
Dan
-- 
/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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