Helium fills
Graham, Lachlan (Lachlan.Graham@dbce.csiro.au)
Mon, 29 Jul 96 17:46:00 PDT
Dear Colleagues,
Recently we filled our horizontal 24/40 magnet with cryogens and energised
it. As it was a slightly tense exercise I have prepared this outline for
others' benefit. I wish to acknowledge that several people from this group
were generous in helping us with information.
I assume we are starting from a warm magnet
1. Evacuate the OVC as far as it will go. Our manual specified 5x10^-5
mbar. This will require a diffusion pump with rotary backing pump or a
turbomolecular pump similarly backed. Use the biggest pump you can get your
hands on. Leave the pump running after you get to the specified pressure.
2. Fill the nitrogen tank with liquid nitrogen. We turned off the vacuum
pump at this point.
3. Fill the helium tank with liquid nitrogen. Leave overnight (or in our
case over the weekend). I purged and sucked on the helium tank three times
using high purity (better than 99.995%) nitrogen before the liquid nitrogen
fill. I also filled it with nitrogen gas to about 1 atm to prevent air
entry.
4. Use about 1-1.5 psi of pressure from cylinder of high purity nitrogen to
force out as much liquid as possible from the helium tank. This requires a
tube which fits into the transfer syphon of the magnet. The liquid nitrogen
was piped using silicone tube into the nitrogen tank which needed a top-up
by this point. The gas pressure was applied at the magnet manifold.
5. When no more liquid came out a rotary vacuum pump was connected to the
magnet manifold together with a supply of high purity helium gas (better
than 99.995%). This was set up with valves etc such that no air could get
into the magnet. The helium tank was then sucked down to about 15 mbar (our
gauge was not too accurate here) then refilled with helium gas. This was
repeated four times. The tank was then filled with He gas to about 1 atm.
6. Liquid helium fill was commenced and for our case required about 360
litres for a full magnet.
The Bruker WWW sites also have information on helium and nitrogen filling.
I wish to thank those colleagues who supplied us with information and help.
This is the best use of the Internet.
L.J.W. Graham