In response to Rich's suggestion to pump a failing magnet while
cold, I would like to point out that there are safety valves to protect
against accidental loss of vacuum if the power fails during the cold
pump-out. These are simply solenoid-activated gate valves which are
normally closed. They are wired to the pump power mains so that if there is
a loss of electric power, the gate will fall, closing the valve. These
valves are available from manfacturers of high vacuum equipment like
Alcatel, Leybold, and Varian.
In my experience, cold-pumping a magnet is usually not very
effective. The magnet, while filled with liquid helium, is an excellent
cryopump, exceeding the pumping speed of just about any commercially
available turbopump system. Also, it is not possible to bring a turbopump
very close to a large NMR magnet without having serious eddy current
problems. Thus, there usually has to be at least a one meter metal hose
between the magnet and the pump, and this can seriously degrade pumping
efficiency. In particular, it is very hard to pump helium gas out of a cold
magnet cryostat (or a warm one, for that matter). Since the presence of
traces of helium gas in the cryostat (due, for example, to an O-ring seal
frozen during a helium fill) is often the cause for degraded magnet
cryogenic performance, removing this helium will usually be necessary to
restore the magnet to specifications.
Provided that proper safety devices and practices are used, pumping
a cold magnet doesn't present a great hazard and it's not a very expensive
procedure, but we haven't seen much benefit from cold pumping our magnets.
In particular, pumping our 400/104 superwidebore magnet had no benefit at
all in restoring its liquid helium hold time to specifications, and pumping
our 1.9T/30 cm horizontal magnet provided only a few days of reduced
boiloff. Ultimately, the magnet continued to degrade, and a complete
overhaul similar to the one Rich described was necessary.
Regards,
Ken Fishbein
Facility Manager, NMR Unit
NIH/NIA/GRC
Ph. (410) 558-8512
FAX (410) 558-8323 or 8173
E-mail: fishbein@vax.grc.nia.nih.gov
Address: Ken Fishbein
NIH/NIA/GRC
4940 Eastern Avenue
Baltimore, MD 21224 USA