K400FridgeSOPRestarting

Restarting the fridge

1. Stop pumping lines and IVC

• Seal IVC at top of fridge, turn of turbo pump and disconnect.
• Stop pumping the dilution lines
  • Close 1, 2, 7 and 3
  • Close vent 2/1 S/V, turn off turbo pump and disconnect.

2. Check Helium level in the dewar via the pot

• Ideally you want to transfer helium once the fridge is running, however you can use the pot to see the liquid level in the dewar. If the He level is above the pickup tube, wait; if the He level is below the pickup tube, transfer.
  • turn on cooling fans, turn on 1K pot pump
  • Close 1a (close pot to main bath)
  • Open 4a
  • open 1k pot S/V on insert
  • Open n/v and check pressure using 'rough gauge' (if it shows >0 there is liquid flowing through the pot).
  • Close n/v
  • Close 4a
  • Open 1a (to allow pot continue to cool via main bath)

3. Clean the cold traps

• The traps can be cleaned if you choose:
  • Remove traps from trap-dewar,
  • pump using external pump via vent 2 (connected to vent 1).
  • open 7, 11a
  • warm traps with a heat gun until very hot.
  • once done, close all valves
• Fill trap dewar with LN2 if needed.
• Also fill the LN2 jacket on the dewar since you're already filling LN2.
• Put traps back into trap-dewar (if they were removed in the first place), allow to cool.
• You can cryopump fridge lines via traps if you want:
  • Open 12a, 1 and 3
  • Leave for five, ten or twenty mins depending on time available
  • Close 12a, 1 and 3

4. Cool the pot

• Close 1a (close pot to main bath)
• Open 4a to pump pot out
• Open the n/v to allow liquid to flow into the pot
• Watch the resistance of the pot increase as it cools. Ideally you are looking for around 700-740 Ohm.
• Adjust n/v to get desired resistance (usually operate fridge at around 800-830 Ohm, however the pot with a heat load will have a different operating point than without a heat load).

5. Put in the He trap

• Ensure the He trap is lowered in slowly, watch the rate meter as a guide.

6. Condense the mixture

• First check all valves through which mixture could exit the dilution unit are closed!
• Open 13a to pass mixture into the trap (assuming trap one is being used). Allow the mixutre to sit for 5 mins to cool.
• Open the still gate-valve on top of the fridge (mixture will be 'reverse' condensed via the larger still volume), place hose with slit into place, such that the valve doesn't close itself.
• Let a shots into fridge to condense by:
  • Close 13a.
  • Open 12a. See pressure on condenser gauge.
  • Let through valve 3. Throttle just fast enough to see gauge drop like the second hand of a clock.
  • The pot temperature may rise as indicated by rising resistance and the pressure should increase. Let the pot recover.
  • Condense it all in (Check condenser pressure gauge) and close 3.
  • Shut 12a.
• When done, ensure 13a is closed.
• Open the taps for the dumps (all 3 behind the gas cabinet).
• Open valve 9 to let dumps into the lower pump line (slowly if 3He pump is on). Pumping line gauge G2 should come to ~520mb.
• Open 13a to give this to the cold trap for 5-10 mins.
• Open valve 3.
• Throttle through 12a. 100mb max on condenser gauge G1. Be careful not to let this run away. Gauge has a big lag. Once line pressure G2 is below 100mb open 12 fully.
• Keep an eye on the pot. Make sure it doesn't run out (may need to run slow-continuous fill).
• When pressure gets down to ~50mb it's time to circulate the mixture.

7. Circulate mixture

• When pressure gets down to ~50mb it's time to circulate the mixture.
  • Shut 13.
  • Turn 3He pump on if it isn't already.
  • Valve 3 shut.
  • Valve 1 open.
  • Condenser valve open.
  • Close valve 9.
  • Open valve 13 to equalise.
  • Slowly open 6 to ~200mb and let settle a bit. (keep G1 and G2 < 200mB)
  • Watch cooling on still and RuOx. Keep an eye on the pot.
  • Watch top gauge and open 6 fully as it comes down.
  • Ensure pressures still keep dropping. (~keeping G1 and G2 < 200mB.)
  • Once pressures are low, open 10 to drain dumps.
  • Wait until line pressures drop right down again, Shut 10.
  • Check 1,6,12a and 13a are properly open.
  • Roots pump on (may need to plug it in at the wall).
• Needle valve set for continuous fill. Looking for 10mb (unless the RF amp is running, in which case aim for 20mbar).
• Once line pressures are settled and low (about one hour), set still heater to 3.5mW.
• Do a helium transfer (standard end transfer: 20-25cm above IVC or 90-95%).

Hint: If the pot crashes during condense step, you'll see the pot pressure drop (it is empty), in this case, open n/v fully until the pot is filled. Then close the n/v fully until G3 shows zero (P2 can show pressure) and finally crack the n/v open about 1/8th of a turn. You want P2 to display pressure and G3 to be zero.