This is my promised summary of the responses I got regarding very low
temperature (-150 C) experiments on a Varian. Thanks to all who responded.
The majority of experience was on older probes and most went down to -130 C
with little problem. The newer BB probes w/o gradients seem to be fine down
to -120 C, but care must be taken. The bottom line is that -150 C is
possible, but quite difficult and is best done with a specially designed
probe and the following should be kept in mind (this also applies to all
low-temp experiments):
- All gas including spin, eject, bearing, etc. must be anhydrous N2.
- Solvent choice is limited (mixtures of diethyl or dimethyl ether and
acetone or THF, or Freon-type solvents) and since T1's can be very short at
these temperatures, locking/shimming is very difficult.
- Getting down to these low temperatures requires a specialty dewar or
running a FTS chiller in tandem with a dewar.
- There is a real risk of freezing the O-rings at the top and bottom
flanges of the magnet, which could create a 'Challenger' disaster. That is,
the magnet could quench. To minimize this risk, you can either get the
Varian heated upper barrel (this is a commercial version of Charlie Frye's
solution) or you can use Charlie's approach which is to wrap the top (and
bottom if there is any space to do so) with heat tape and set it to 'warm to
the touch'. Also, it is advisable to vent the VT gases away from the magnet
using a tube from the top of the upper barrel.
- Turn up the shim coil and probe cooling gas (in this case, it's a warming
gas) to prevent the magnet and shim stack from freezing.
Dan
Received on Tue Feb 03 2009 - 05:18:06 MST