Hello all,
thank you very much for the large amount of numerous
comments.
There are two facts to summarize.
1. The problem described is not very uncommon.
Especially older Oxford magnets seem to be sensitive.
The problem is the attachment of the coil using the
nitrogen neck tubes. Their length can change and as
a consequence the position of the magnet coil.
Some people suggested to leave that neck tube
open, which is positioned symmetrically between
the two helium neck tubes.
2. As a second prerequisite for the problem described
either the main coil shows a large Z2 gradient or the
cryoshims are not correct (or both). The room temperature
shim system is only able to correct cryoshim problems
for one selected position of the main coil (including
the cryoshim coils). This problem can be minimized
via field mapping. The specialist for this procedure seems
to be Piotr Starewicz or David Hillenbrand at
1-978-671-0811 ext 232 from Resonance Research
(hint from Klaas Hallenga).
Once again thanks to all who responded.
Rainer
Received on Mon May 21 2001 - 11:04:01 MST