Cryo-Clean Mixture

For the system to cool down to below 100 mK, we use a mixture of liquid He3-He4 that we circulate through the fridge which provides the necessary cooling power. Before condensing and circulating the mixture, it must be clean because any contamination (air) present will freeze within the mixture as we go down to lower temperatures. This can produce a blockage in the line and prevent fridge operation. Thus, before every cooldown we clean this mixture, which can be done independently of/simultaneously with what's occurring in the MB/IVC. In order to get an effective cleaning process the mixture needs to flow. Monitoring G2 will indicate whether this is occurring or not. Mixture cleaning should go for at least an hour.

Note that V12A and V12B are leaky valves, so there are manual speedivalves in series with these valves located at the back of the IGH. They are at positions labeled “FROM COLD TRAP 1” (for V12A) and “FROM COLD TRAP 2” (for V12B). If you are reaching behind the IGH from the right, you will feel three knobs: the nearest is the V12A speedivalve, the middle is the V12B speedivalve, and the furthest is the Vent speedivalve. When either of these valves is referenced below the speedivalve is also mentioned to remind the operator that both valves must be opened/closed.

1. Before beginning, it should be the case that:
   1. one of the LN2 traps is free of contaminants. If it was recently used to cryoclean mixture, it should have been pumped to vacuum at room temperature.
   2. The mixture is in the dump. If not, it should be pumped to the dump before beginning.
   3. The Still Gate valve and Condenser Speedivalve at the top of the fridge are closed.
2. Place the LN2 trap to be used in the LN2 cold trap bath and fill the bath with LN2.
   1. Note: whether you use V12A and V13A versus V12B and V13B depends on which trap you are using. The steps below assume LN2 Trap 1, in which case you use V12A and V13A. If instead you are using LN2 Trap 2, then substitute “B” valves in each instance where an “A” valve is referred to.
3. Turn on the He-3 pump.
4. Open V4 and V12A+speedivalve. The LN2 in the cold trap will boil off a bit because warm mixture is flowing into the cold trap line. You can wait about a minute before proceeding to the next step.
5. Open V14.
   The next step will be to begin circulation. When cleaning, V6 should be open just enough to establish flow. (Never open V6 all the way while cleaning!) If it is open too much the pressure at G2 will go too high. If at any point the G2 pressure exceeds 900 mbar, close V14 to prevent further pressure increases. Then close V6 completely before opening it again to restart the cryocleaning.
6. Open V6 to 11.0%. Open V13A, which completes the circulation loop. Wait a few minutes for V6 to open up to its initial setting.
   1. Note: In the past, 12.4% was the starting value for V6 - no flow was seen at this low a setting. Later it was changed to 11.0%, because there was often flow seen as low as 11.3%. The initial setting may change again in the future.
7. Close V13A, then watch the pressure on G2. Open V6 in steps of 0.1%. When G2 begins to rise steadily (around 1 mbar/sec), V6 is open sufficiently. Open V13A
8. Cleaning has begun. Usually though after a few minutes the flow stops and V6 must be opened a little further. To check if there is sufficient flow, close V13A:
   1. If G2 increases, we have flow. Re-open V13A to re-commence cleaning.
   2. If G2 doesn't increase or only does so very slowly, then open V6 in steps of 0.1% until G2 begins to increase.
   3. If G2 increases too quickly, the cold trap line will start icing. Close V6 by a few 0.1% to correct this.
   4. This check should be performed periodically during cleaning. A typical final setting for V6 is 13.2%11.8%. The proper setting of V6 may vary.
9. During cleaning monitor the difference between G1 and G2. If the mixture is particularly contaminated G2 may rise to be significantly above G1. If this happens, end the cleaning. For more aggressive cleaning, we now open V6 one click whenever G2 settles below G1.
10. Mixture cleaning should be done for at least an hour, but sometimes we leave it to clean overnight. If you leave the mixture to clean overnight, check that the process has not stagnated and re-commence cleaning if need be.
11. To end cleaning, close V14. Then close V13A and V4, and open V9. Then open V3. This will pump the mixture out of the trap and into the dump. Open V6 another 0.1% or so to speed up pumping. When G1 is low you can open V6 further, (14% or 15% is sufficient). Continue pumping until G1 stabilizes at a minimum value (typically a few mbar) and (if you have time) until P1 is less than 0.1 mbar.
12. Close V12A+speedivalve, close V3, and close V6 completely. When V6 is fully closed (light off), close V9 and turn off the 3He pump (unless you plan to use it again soon).
13. At this point the cold trap will have absorbed contaminants which you will eventually want to pump out, after letting the trap warm up to room temperature. It is not necessary to do this immediately but it should be done before the trap is used again for cleaning or for circulation.
    1. Remove Cold Trap 1 (or Cold Trap 2 if you used it for the cleaning) from the LN2 bath and lean it against the wall.
    2. Wait for the trap to warm to room temperature. This will take several hours. (A heat gun can be used to speed the process but it usually isn't necessary).
    3. Check that all IGH valves are closed and that the mixture is in the dump. In particular make sure there is not mixture in the G1 volume.
    4. Open V12A+speedivalve to open trap to the G1 pressure gauge.
    5. Record the pressure on G1 (minus the pressure before opening the valve). This is the amount of contaminants removed by the cryocleaning.
    6. Connect the scroll pump to the 4He Rotary Pump line in the pump alley and turn on the pump.
    7. Open the manual valve at the scroll pump inlet.
    8. Open V5A, V2A, V7, and V2. This will pump the trap contents out to atmosphere.
    9. Wait until G1 stabilizes at a minimum, then close V12A+speedivalve, V2, V7, V2A, V5a, and the manual valve at the scroll pump inlet. Turn off the scroll pump.
    10. Return the cold trap to the LN2 bath.