First Cooldown -- Cooling from 77 to 4K

  • Push LN out of the main bath into the LN jacket:
    • Close AUX S/V and disconnect Bunsen valve
    • Connect a nitrogen gas bottle to AUX S/V, set the flow
    • Using transfer plastic hose connect liquid nitrigen transfer hose to transfer port on the jacket
    • Open AUX S/V to start the process. Observe gas coming out of a second transfer port on the jacket.
    • Wait untill all liquid is out (see no liquid running through the plastic transfer hose).
    • Wait until liquid has well and truly stopped coming out of main bath (15 minutes of gas only. LN2 remaining in bath will freeze solid and cause BIG problems.)
    • Shut the AUX S/V,
    • disconnect LN bottle,
    • disconnect plastic nitrogen hose and bung transfer ports,
    • warm up unscrew and remove nitrogen transfer tube .
  • Put helium level probe in
  • Connect the pump to AUX S/V
  • Pump Nitrogen out and flush main bath with helium from recovery.
  • On flush check that 1K pot and lamda-plate N/V are not frozen with He from main bath along the way.
  • Leave main bath with an atmosphere of He.
  • Prepare for LHe transfer
  • Insert helium transfer syphon into the helium transfer port on the cryostat.
  • Insert the other part of the syphone into He dewar slowly not to build up the pressure. Couple the syphone parts early to avoid liquid transfer. Slowly lower the syphon all the way into the dewar.
  • Pressurise dewar to blow slow cold gas and join syphon up. (Slowly lower syphon in dewar to ~15 cm above the bottom to stop garbage ending up in the main bath.)
  • Cool magnet to 4K slowly, record the magnet resistances. Pressurise He dewar with pumping baloon. Itll take ~20m3 on the gas meter to go to 4K.
  • Aim for 10L of liquid per hour (ie. 0.116m3 gas/minute or 0.010 per 5 seconds). (1L liquid -> 700L gas, 1m3=1000L)
  • During cooldown keep needle valve closed and pump the pot. @15 minutes flush the pot 1 or 2 times through the needle valve to prevent blocking. @30 mins exchange gas into IVC.
  • Fill LN2 jacket slowly after 30-45 mins (Ensure bungs are in properly to avoid air suckback into LN¬2 jacket. Blockage of these lines can blow the fridge up.)
  • When magnet is 0 Ohms (4K) raise syphon 5cm above cone to increase the transfer rate, can use He gas bottle to pressurise the dewar.
  • Monitor liquid with the level meter. Around 60% the boil-off will increase, so stop the transfer.
  • Depressurise dewar; raise lines to prevent backflow. Remove dewar syphon, bung fridge syphon and remove it.
  • Introduce dripper into He transfer port on the cryostat. First, make sure you flash the dripper with helium via the dripper S/V (observe helium coming out of the dripper tube). 2-3 minutes.
  • Slowly lower the forklift and make sure the dripper tube is right above the port and doesn't bend.
  • Once fridge is at 4K pump out IVC. Leave 1K pot vented to the main bath otherwise the 1K pot cools to 1K and the exchange gas condenses on the pot and won’t pump out. Aim for IVC pressure of mid 10e-6 range.

  • Top up LN2 jacket and check bungs to avoid suck back. Check ports are clear of ice before going home.
Topic revision: r4 - 08 Jun 2017, OlehKlochan
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