Back to Main


Back to Research

  

 

Electronics | Mr. Freeze (dilution refrigerator) | Mrs. Astra

Principle of Operation | Cooling Down | Starting the Circulation | Warming Up|

Pumping the Transfer Tube and the Top Loading Stick | Loading and Unloading the Sample

 

COOLING DOWN MR. FREEZE TO 4 K

 

n. b. This is not a foolproof guide. It is very important that one has to know what he is doing, why he is doing it, and what’s happening in Mr. Freeze if he does it.

 

What do you need for cooling down?

  1. 100 liters of liquid helium

  2. 100 liters of liquid nitrogen

  3. a helium gas bottle with at least 50 bar

  4. one plastic tube of 1-1.5 m for pressurized gas

  5. one rubber (latex) tube of 1.5 – 2 m for liquid nitrogen transfer

  6. a heatgun or a hairdryer (an extra o-ring for the siphon entry might be useful)

The optimum timing is to start cooling down at about 9 in the morning and finish the helium transfer the same night. Then you can measure the next day after lunch.

 

Before cooling down, check the following:

  1. Has the OVC been pumped down at least 12 hours with the diffusion pump?

  2. Are the coldtraps ready to use (i.e. empty)?

 

Temperature Monitor

  1. Open the temperature monitor on the computer. If it doesn’t work, do either one of two things:

    1. Use the MEDICINE WHEN THE GPIB HAS HUNG UP.JP and click the START button several times.

    2. Pull out the plug behind the GPIB20 box and put it back in. Wait until the ready light is lit.

 

Pumping the OVC down

(If Mr. Freeze was opened to atmosphere, then the IVC needs to be pumped out as well prior to OVC pump down. In that case, the valve to be used is #25A instead of #25B.)

  1. Make sure that #27 and #25B are closed before turning on the pumps.

  2. Turn on the ROTARY A PUMP.

  3. Open completely #35, and then #30.

  4. Turn on DIFF A PUMP. Wait for 15 – 20 minutes until the diffusion pump is pumping properly.

  5. Close #30.

  6. Open #29 and #25B.

  7. When the pressure (P3) reaches 10-1 mbar, you are ready to open the diffusion pump.

  8. Close #29.

  9. Open #30 and #27.

10.      Turn on the PENNING pressure gauge.

11.      After (at least) 12 hours, the OVC should have a pressure of around 10-5 mbar.

 

If the helium bath contains air

  1. Close the OVC valve on top of the cryostat.

  2. Open the needle valves.

  3. Pump the air out of the bath with the ROTARY A PUMP through the following valves: #21-#23B-#25B-#29-#35.

  4. When the pot pressure (G6) is decreasing, open valve #22.

  5. When the pressure is down to 1 mbar, close the needle valves and stop pumping, i.e. close #21 and #23B.

  6. When the PENNING gauge shows low pressure (10-4 mbar), continue pumping the OVC.

  7. Fill the bath with nitrogen gas from a dewar using VENT 3.

 

Cooling to 77 K

  1. Put some helium from a gas bottle into the pot and the IVC.

    1. Connect the helium gas bottle to VENT 3.

    2. Make sure that #23B and #25A is closed. Open #21A, #22 and #23A.

    3. Open the valve from the gas tank to let in helium gas. Fill until the pressure indicated on G6 is 1 bar.

    4. Close #21A, #22 and #23A.

    5. Disconnect the helium gas bottle from VENT 3.

    6. Make sure that the OVC valve on top of the cryostat is closed.

  2. Mount the metal tube with a thread in the transfer entry (not the siphon entry) and screw it in. (The transfer entry is clockwise from the siphon entry.)

  3. Connect the top of the metal tube to a nitrogen dewar (use the latex tube).

  4. Remove the safety valve from the recovery entry on top of the cryostat.

  5. To save time, you may also want to fill the nitrogen bath to 30 – 50%.

  6. Fill the helium bath with liquid nitrogen (approximately 1 hour from a full 200 liter dewar).

  7. Leave the safety valve open.

  8. Wait a few hours until the temperature in the mixing chamber is 80 – 100 K (Ge resistor).

 

Cooling to 4.2 K

  1. Pump out the Helium from the IVC

    1. Turn on the ROTARY A PUMP.

    2. Make sure that #30, #25B and #23A are closed. Open #35, #29 and #25A. Wait until the pressure is about 10-1 mbar.

    3. Close #25A, #29 and #35.

  2. Connect the metal tube to the nitrogen bath (use the latex tube).

  3. Close the safety valve on top of the cryostat.

  4. Connect the helium bottle to the helium bath (use VENT 3) and press the nitrogen out from the helium bath (use around 100 mbar of overpressure).

  5. When the pressure in the bath decreases rapidly (G5), close the helium bottle, take off the rubber tube, open the bottle again and check that no nitrogen comes out of the metal tube.

  6. Unscrew the metal tube and remove it. Close the transfer entry.

  7. Pump out the helium gas out of the bath with the 4He pump.

    1. Switch on 4He ROTARY PUMP.

    2. Open #31. Read off the pressure from G5.

    3. The pressure should decrease to 1 – 2 mbar, if it stops at around 100 mbar it means that there is still some nitrogen left (proceed back to step 4). Otherwise, close #31.

  8. Empty the 1K pot from #24 without opening the needle valves.

  9. Close the 1K pot at #24 and turn off the 4He ROTARY PUMP.

10.      Fill the helium bath with helium from a helium gas bottle to 1 mbar.

11.      Make sure that the needle valves can be opened. Open one, check that the pot pressure at G6 increases, close it, then check the second valve the same way.

12.      Connect both the pot and the bath to the recovery.

    1. Open valves #31 and #24 to fill the pot with helium.

    2. Open valve #32 to open them to the recovery line.

13.      Empty three balloons of helium (from a helium dewar) into the IVC through the following valves: VENT 3, #21A, #23A. Make sure that there is no air in the tubes first.

14.      Start transferring helium from a dewar to the bath using a helium gas bottle.

15.      When the mixing chamber temperatue is 20 K, start pumping the IVC with the ROTARY A PUMP.

    1. Turn on ROTARY A PUMP.

    2. Make sure #23A and #25B is closed. Open #35, #29, #25A.

    3. Turn on the DIFF A PUMP.

    4. Open #30, #27 and #25A.

16.      Fill the helium and the nitrogen baths.

17.      Open F and C needle valves to fill pot

    1. During filling/transferring of helium, open the F needle valve to fill the 1K pot.

    2. Start pumping the 1K pot with 4He ROTARY PUMP.

    3. Close #31, open #24, turn on 4He ROTARY PUMP.

    4. The pressure at G6 should be approximately 1 mbar.

    5. When you have filled the pot, close the F needle valve and regulate flow of liquid helium into the pot using the C needle valve.

18.      Pump the IVC with the diffusion pump for about 8 hours.

 

Updated:   August 9, 2004, Ian Jasper Agulo

 
Last edited:  | Disclaimer