I'm trying to use a newer Varian 400 MR console (2010?) with an Oxford AS400 magnet that was formerly part of a Varian Mercury-Plus 400 spectrometer. The problem right now is the RT shim set. The 400 MR shim set (27 shims) fits so tightly in the magnet bore that I'd have to pound it in, and it's about 1.5 cm shorter than the Mercury-Plus shim set. So I'm trying to use the old shims (15 shims) and interface it to the new console. In theory, we could only use 15 of the shims in software and we should be able to get the same homogeneity we had before. We will have to figure out which pin goes with which shim on both ends, and build an interface box. But will this even work? The resistances I measured (without knowing which shims they correspond to) appear to be quite different in the newer shim set (ranging from about 1 ohm to 190 ohms) compared to the old one (5-10 ohms in all 15). How are these resistance differences dealt with in the shim driver circuits? These all seem to be identical circuits. And then the big question is, are these shims matrix shims, and if so where does the matrix live? Is it in software, where it could conceivably be changed, or in a programmable logic chip? And with gradient shim mapping and shim optimization (i.e. gradient shimming), does it matter if the matrix is not right?
All these are interesting questions. We have schematics for the Mercury-Plus, but as you may know Varian refuses to release schematics of the newer instruments.
Of course, we could just energize the 400MR magnet, but we already have a working magnet and the expense of energizing would be prohibitive to this DIY project.
Many thanks for any insights you might have!
Neil
Received on Thu Sep 29 2016 - 06:36:48 MST