One of our magnets recently began to boil off liquid helium at an
accelerated rate following a liquid helium fill, and I have been trying to
diagnose and remedy this problem. Here are the symptoms:
1. Magnet's boil-off rate met specifications (100 days hold time)
for about 18 months prior to a particular helium fill. Immediately after
this fill, the hold time dropped to about 60 days.
2. Since the fill after which the helium boil-off rate jumped, the
helium loss rate has remained steady at the new, higher value despite
several subsequent helium refills. That is, the boil-off is too rapid but
is not accelerating.
3. The gas-cooled shield temperature (ca. 52 K) has not changed
since the magnet was commissioned.
4. A helium leak detector did not locate any leaks on the exterior,
accessible surfaces of the magnet at a sensitivity of 1.0 X 10-10 mbar L/s.
5. After pumping the magnet for about six hours, the pressure at
the mouth of the pump (about 1 meter from the magnet) was 5 X 10-6 torr.
After the magnet was re-evacuated in this way, the helium gas flow rate
from the magnet fluctuated wildly for several days, then settled down to
the same value as before repumping. The liquid boil-off rate is still
abnormally high, but is not getting worse.
Here are my questions:
1. Would it make sense to repump the magnet for a longer time (e.g.
several days)?
2. Could the accelerated boil-off rate be due to some problem other
than faulty vacuum?
3. If repumping would help, where can we rent (or borrow) the
appropriate apparatus, including:
a) Direct-drive roughing pump
b) Turbomolecular pump
c) Vacuum sensors and meter(s) to cover a range of at least
ATM to 10-7 torr (preferably sensors which can function in fringe
field very close to magnet for accurate pressure measurement)
d) Normally-closed, electrically-actuated solenoid valve
with manual reset to isolate magnet in case of power failure
e) Non-magnetic KF fittings and hose (ca. 1 meter length)
I appreciate any advice that you can offer for resolving this problem.
Sincerely,
Ken Fishbein
Facility Manager, NMR Unit
NIH/NIA/GRC
Ph. (410) 558-8176, 8388, 8169
FAX (410) 558-8173
E-mail: fishbein@vax.grc.nia.nih.gov
Address: Ken Fishbein
NIH/NIA/GRC
4940 Eastern Avenue
Baltimore, MD 21224 USA