Here is the story: Our 7T, 183mm horizontal bore magnet recently had an
unfortunate encounter with a heat gun, which attached itself to the front
of the magnet. The magnet did not quench, and boil-off rates were normal.
We detected no obvious damage to the magnet. To get the heat gun off,
Bruker field service lowered the field to a point where it could be
pulled off, then raised the field back to 99.2 amps, nominally 7T.
At this point we went searching for a proton signal at ca. 300MHz and
could not find one! We tried three different probes with various (large)
samples in them and also looked at ca. 46MHz for a deuterium signal of
D20, all without success. Fortunately we have a vertical bore 300 some
meters from the horizontal, and stringing together some cables we
verified that the horzontal 7T console could produce a signal from the
vertical magnet, ie the problem didn't lie with the console (or probe
interface).
Based on all of the above, the problem seems to be something with the
magnet. We thought perhaps the signal could be parked far from 300, but
have searched over dozens of MHz from 305 on down without success. The
size of the phantoms we used would allow for some change in the position
of the field center. Is it possible that the field homogeneity has
gotten incredibly bad? We do not have current in the cryoshims (nothing
to shim on), but originally we were able to easily detect a broad line from
H2O without cryoshims when the magnet was first brought to field in 1991.
Separate tests indicate that the power supply is working correctly, ie
actually delivered 99 Amps to the magnet. The boil-off rates are still
normal. If the magnet had been seriously damaged I would have expected
higher boil-off or a quench. Any ideas as to what is going on? Please
post ideas to me and I will post to the Net any solution we come up with.
Jeff de Ropp
UCD NMR Facility
jsderopp@ucdavis.edu
Phone: 916-752-7677
FAX: 916-752-3516