Re: Critical probe position?
R.Shoe (rshoe@tinman.unl.edu)
Tue, 11 Oct 94 09:16:30 CDT
In most magnets, the probe position is VERY Critical, due to the fact
that the observe coil MUST be centered in the center of the field,
most importantly the fields from the shim coils (both supercon and
Room Temp shims). In order to serve their function, the higher-order
shims (i.e. > than zero order) "should" have a null effect at the center
of the sample, and should "shape" the field from the center, outward.
(imagine the symmetry of a "p" or "d" orbital. IF the observe coil
is not properly centered, adjustments to the shim currents will effectively
alter the "center" field...or...moving "Z1" will have the effect of moving
"Z0". If it's bad enough, you could lose your field-frequency lock every
time you adjust Z1 or Z2.
Centering the field is generally the FIRST thing that is done in
preparation for shimming a newly commissioned magnet. Often, this is
done using a small, spherical water sample, and the center is mapped
out using the Z1 field gradient.
I hope that your new probe is not too short to make it to the center!
Good Luck!
Richard Shoemaker, Ph.D. Phone--(402) 472-6255
Instrumentation Director, Chemistry FAX---- -9402
University of Nebraska-Lincoln