So I've got these big coils. ~12" diam, ~150 turns per side*. Big coils here.The homogeneity of these coils sets the high field signal linewidth (LW). LW*signal is ~constant. About 1/2 of the coils can have the LW improved by a little mechanical shimming of the three mechanical support points. (mostly the top one.)
~5-10% of them fail** just a crappy LW that I can't fix. So I had this idea of a current shim (resistor is shunt with one coil.)*** which I may have talked about before.
Anyway this time a ~2k ohm resistor in shunt with one of the ~5 ohm coils (copper) decreased the LW five fold, from twice too bad, to twice better than good enough.
Is there any reason not to ship this? With notes about the current shim. There is the change in copper resistance, ~ +/- 10%. (which doesn't matter so much.)
George H.*The coils are taller than wider ~10%, because of some paper I read about Helmholtz coils years ago. (I can find a reference if someone cares.) ** I've got a rather arbitrary spec. LW (Rb87) @ 5MHz RF (~4 G) is less than 1 mG. *** this is adding some small magnetic dipole, I have to adjust the 'zero' field, but it's small.