[Halld-tagger] updated vertical focal surface calculation

[Richard Jones]

Dear colleagues,

Please find linked below an excel spreadsheet containing my focal 
surface calculations for the dispersion plane and vertical plane in the 
tagger.  I have now included the effects of the quadrupole and also the 
non-trivial effects of the fringe fields of the dipole. 

http://gryphn.phys.uconn.edu/halld/tagger/magnet/excel-fp
http://gryphn.phys.uconn.edu/halld/tagger/magnet/excel-fp/excel-fp.xls 
(spreadsheet with graphics)
http://gryphn.phys.uconn.edu/halld/tagger/magnet/excel-fp/excel-fp-model.pdf 
(my hand-written notes)

In order to recover the good vertical focus at the microscope that Yang 
showed in his model, I had to push the quad as far upstream as possible 
and increase its field by almost a factor 2.  There is still plenty of 
headroom in the magnet, because it is designed for a maximum of 20 T/m 
and I am running it at 4.7 T/m.  I also noticed that the quadrupole 
symmetry axis is shifted from the center of the beam pipe by more than 1 
cm in gxtwist.  Alex, did you notice that?  When we resimulate it, we 
might want to shift it to the correct alignment with the beam pipe at 
the same time.

Alex, you can change any of the alignment numbers in green (top of first 
worksheet in excel-fp.xls, highlighted in green) and see the changes 
instantaneously reflected in the figure.  As we expected, the dispersive 
plane focal surfaces are insensitive to the quadrupole field, but the 
y-focus is sensitive to it.

-Richard Jones
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