- Both seal surfaces and the indium should be kept as clean
- Acetone does an adequate job cleaning surfaces and indium.
- When making a seal out of a wire of indium, cut the wire
to length, beveling the edges to make the joint. The freshly
cut edges will stick together readily.
- Not all sealing schemes make reliable indium seals. An improperly
designed seal geometry can often be sealed initially, and then
after a couple of days, will open up. For example, a seal between
a knife edge and a flat. It initially worked, but after a couple
of days, the indium crept inwards, over the knife edge, and a
leak opened up. The problem was solved by machining washers which
prevented the indium from flowing inwards.
- A good rule of thumb for determining the number of fasteners
needed to secure an indium seal is - as many as you can possibly
fit on the surface!
- Indium seals can be reliable down to very low temperatures.
The fill tube was reliably sealed to the LHe reservoir
with 0.125 inch indium.
- Indium can be reformed into useful seals after use. Successfully
extruded indium wire from scrap indium. First clean the indium
with acetone. The extrusion mechanism consisted of a hollow cylinder
with a bolt screwed into one end, a plunger and a stand. The
bolt had a hole drilled lengthwise through which the indium was
extruded. A 0.070 inch hole yielded indium with a 0.079 inch
diameter. Use a hydraulic press to extrude the indium. Smaller
diameter indium required larger forces (500 pounds for 0.125 inch
wire, 2000 lbs for 0.080 inch wire).
AMMRL General Comments about Loss of Vacuum
and Cold-pumping -- A quench start was reported for a Varian 200 MHz "R2D2"
type magnet due to low LHe levels.
Symptoms noted were
high He gas flows (increased from ~ 4 cc/hr to ~ 20 cc/hr) and
extreme lock problems due to the decreasing field (2H
lock frequency loss rate of ~ 300 Hz/hr).
They were able to arrange
for same-day delivery of LHe. At NMSU delivery
of liquid helium takes a week. After filling the magnet, the field
stabilized. Net result was 230 ppm loss of field. After adjusting
NMR frequencies, the system was able to meet all specs, including
Cold-pumping magnet Dewars there clearly are different
experiences: sometimes it helps, sometimes it doesn't. When somehow
He diffuses into the vacuum space from the He can, cryopumping
by the liquid He does not do the job but external cold pumping
will remove the He gas. With larger leaks bringing in air from
the room atmosphere this may not be so effective. When He gas
is the reason for a poor vacuum, cold pumping will certainly help.
As an alternative to periodic pumping with a high throughput turbopump
A large diameter five foot copper tube connected the pump
to the magnet Dewar via a high vacuum valve. The current
in the vac-ion pump shows the vacuum at any point in time. The
only disadvantage is that He is not too well absorbed by the electrodes
of the Vac-ion pump. A diffusion pump had to be used about every
three to four months to bake out the Vac-ion pump overnight to
Consider -- the much higher partial
He pressure in the NMR lab as compared to the Natural abundance
of He in air (about 5 10-4 torr) and how much faster
will a leak in a Dewar lead to excessive He boil-off as a result?
When the LHe boil-off rate doubled and tripled on one
Oxford magnet, there was moderate success in cold-pumping the
magnet with the proviso that the turbo pump must be no closer
than the 50-gauss line to avoid eddy current forces from overloading
it. To reduce the flow resistance, if possible make the stainless
line from magnet to pump much larger in diameter than the magnet
vacuum valve. In the case of a moderately slow leak, this procedure
can easily extend the life of the Dewar to a time more convenient
and economical for you to bring down the field and warm it up
for proper repair. The suggestion to pump a failing magnet
while cold, remember 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 manufacturers of high vacuum equipment
like Alcatel, Leybold, and Varian. Cold-pumping a magnet is usually
not very effective. A magnet 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). Since the presence of traces of helium gas in the
cryostat (due 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 does not present a great hazard
and it is not a very expensive procedure, but there is not much
benefit from cold pumping our magnets. In particular, pumping
one magnet had no benefit at all in restoring its liquid helium
hold time to specifications, and pumping One magnet
provided only a few days of reduced boil off. Ultimately, that
magnet continued to degrade, and a complete overhaul was necessary.
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