Ray Tracing at Different Magnet Excitations

D. Sober         5-August-2013

I have done some raytracing through Tosca fields at different excitations, always adjusting the incident energy to 10 ppm precision to give exactly the same full-energy deflection.

Results at the two extremes:
Compared to the same E/E0 at 1.5 Tesla (12.00 GeV):

0.6 Tesla	(E0 = 4.79 GeV)	delta x along focal plane is < 0.21 mm, delta theta_x < .007 degrees
1.7 Tesla	(E0 = 13.57 GeV)	delta x along focal plane is < 1.85 mm, delta theta_x < .023 degrees

To put the 1.85 mm in perspective, note that at the ends of the Microscope region, the transverse shifts (compared to the 2mm fiber width) are

"3 GeV"=>9 GeV photon	1.21 mm * sin(12.2 deg) = 0.23 mm
"4 GeV"=>8 GeV photon	1.85 mm * sin(10.8 deg) = 0.35 mm

which I would say is not a problem.

Assuming that the saturation effects of the real magnet are comparable to the "standard steel" used in Tosca, the excitation effects should be very small.

I propose that after mapping at 1.5 T we try maps at the highest and lowest excitations, and immediately look at the ratio of (B/NI)2/(B/NI)1 versus x and y and compare with the Tosca predictions to see if there are big differences. If not, then there should be no need for a full set of excitations. Other opinions?