K400FridgeSOPCryopump

Procedure for removing mash from the 3He circuit with a cryopump -- for maintenance on cabinet valves, 3He pump, etc. This procedure assumes that the fridge has been shut down by the normal procedure and all valves on the top half of the cabinet are sealed. G1 should read zero, and G2 should read approximately the dumps pressure -- normally ~500-600 mbar.

Preparation

We want to verify that the cryopump we will be using is leak-tight and does not contain mixture. Anything with mixture in it should be labeled, but you never know...

• Make sure you have a full or nearly-full dewar available to cool the cryopump -- they're not terribly long, and they do need to be well-immersed in liquid. Connect it to recovery.
• Connect a tee to Vent 1 on the back of the cabinet. One port should go directly to the cryopump's line, the other to the leak detector via a Speedivalve.
• Pump out the line up through the top valve of the cryopump and check it for leaks -- welds and Swage fittings in particular
• If the line is good, isolate the leak detector, open V7 and V2, then open the top of the cryopump. If pressure appears at G1, there may be mixture inside -- you'll probably want to just cool and seal the cryopump again and get a different one.
• Assuming no pressure at G1, let the leak detector pump on the entire cryopump assembly and check it for leaks. Again, welds and Swage fittings are particularly good candidates to start leaking.
• If the cryopump tests OK, we're ready to empty the gas cabinet. Isolate the leak detector and seal the cryopump at its top valve.

Evacuating the cabinet

• Open V9 to equalize pressure between dumps and back of 3He pump.
• Record dumps pressure and room temperature, as well as the gas meter reading on recovery
• Seal V9.
• Close taps at top of dumps
• Slowly lower the cryopump into the liquid. Some pressure will build -- this is normal. Try to keep your exhaust rate under about 2 L per second.
• Open the cryopump top valve, then 7, 2, 5, 10, and 9 on the cabinet in that order. G1 and G2 should equalize quickly, then fall to zero over the course of about 30 seconds as the mixture condenses into the cryopump.
• Once G1 and G2 both read zero, seal valves 9, 10, and 5.
• Next we need to push this mixture into the dumps through V4. Open the dumps tap behind V4 to give a path for the gas, but don't open V4 yet.
• Slowly raise the cryopump above the liquid. As soon as you see pressure on G1, open V4 to let the mixture retreat to the dumps. This can happen very quickly, so be aware.
• Record pressure on G1 -- this is your new dumps pressure.
• Seal V4 and dumps tap.
• Lower cryopump back into the liquid. Watch as G1 falls to zero, then seal V2, V7, and the cryopump top valve.
• Warm the cryopump, record the gas meter reading, and check that all valves at top of cabinet and all dumps taps are sealed.
• LABEL THE CRYOPUMP -- it will have mixture in it at a few atmospheres of pressure! Taping over the valve handle is not a bad idea.
• Tape over the V4 valve handle -- the line behind it contains mixture, and you don't want that to get back into the cabinet, or worse, into the room.

-- ZacharyKeane - 23 Jul 2010

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