The following is a work-in-progress guide to the cryostats, made in the interest of allowing new users to operate them with minimal assistance.

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Important guidance and warnings:

• Different screws take different torsions

  • Check the sheet on the door, which gives torque ratings for screws of different metals being screwed into brass

  • Breaking screw heads off will result in serious sadness, particularly if attached to a non-removable part

• The fridge heads are attached to the base plate by very thin stainless steel walls, which are not resistant to torsion. Exercise extreme caution when screwing/unscrewing objects into them, as twisting the metal will release the (very expensive) helium stored within.

• Do not reach under the fridge heads, as there are many delicate wires which will be very difficult to repair.

• Never use a multimeter to test SQUID-connected pins

• When wiping metals, use (in the following order):

  • 1. Acetone: to clean things. Will dissolve metal, metal oxide, and organics, so make sure to get it all off.

  • 2. Isopropanol: to rinse the acetone off

  • 3. Methanol: to rinse the Isopropyl off

• Never have the LN2 (linear driver) active while the compressor is inactive

  • I.e. turn it on second when cooling, and turn it off first when warming

• Never disconnect the pulse tube from the cryostat while the system is still cold, as this will result in cold helium being trapped in the ballast tanks with no pressure relief as it warms back to room temperature

• In the event of a power failure or if for some reason the cryostat warms, it may be immediately cooled again PROVIDED that it has not warmed above the sublimation point of nitrogen

  • Once it is above this point, any ice that had formed on walls etc will vaporize, and so when re-cooled, will condense on the mainplate or fridge heads. This may damage your circuitry or the fridge’s wiring

Main operational dangers:

• Power shut-off: compressor will stop working

  • Linear driver should not be left on while compressor is off

• Water shut-off: compressor will overheat

• Compressor leak: compressor will overheat

All of the above are significantly less dangerous if there is no leak in the cryostat. However, to be safe, gingerly uncollet the relief valve, so that if the pressure spikes as the temperature rises, it has somewhere to go.

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Opening:

1. Disconnect any extraneous wires and circuitry.

2. Turn off (unplug) the linear driver, then turn off the compressor.

3. Wait for a long time.

4. Bracing the connections as shown at right, unscrew the pulse tube-cold finger connection. If it hisses, the inner valve has not closed completely; unscrew more quickly to allow it to seal.

5. Remove the pulse tube attachment/reservoir.

6. Release the vacuum: making sure the Varian valve is closed tightly, remove the covering plate next to the Granville-Phillips pressure gauge. Slowly open the Varian valve until you hear a gentle hissing. You will need to continue to slowly open the valve further and further as more and more air gets into the cryostat. At around half an atmosphere, air will woosh in and finish filling the cryostat; do not be alarmed. After the woosh, open the valve fully.

7. Attach a ground wire to some conductive part of outer casing (bolt head for example)

8. Remove the locking pin on the mounting stand

9. Using the handle, carefully rotate the cryostat 180 degrees. Keep a close eye on any extending pieces (vacuum gauge, pipes, etc) and ensure that they DO NOT STRIKE the mounting stand.

10. Put the locking pin back so the cryostat cannot rotate further.

11. Using a torque wrench set to 75 inch-pounds, loosen the bolts in the order specified on the lid. If any require more than 75 inch-pounds to loosen, make a note of which as they will need to be inspected later.

12. Remove the bolts, in the order specified. As they are removed, inspect the threads. If damaged, they must be replaced. If there is debris caught on them, it may indicate that their hole is damaged, so make a note of where they came from and inspect it carefully.

13. Remove the lid by lifting it straight up. It is VERY IMPORTANT not to scratch its underside, as this is required to keep the seal. Place the lid somewhere safe, exterior side down.

14. The two interior lids may now be removed. There is no pressure differential on these lids, so the bolts may be removed in any order. However, be VERY CAREFUL not to drop anything in between the containers, as it will be difficult to remove and will cause serious problems.

15. Once the cryostat is open, there is a risk of dust or organic materials falling into it, which will cause problems when evacuating the chambers. To minimize this risk, cover the opening with plastic film when access to the interior is not required.

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Closing:

1. Print the Fridge and Cernox pin connection sheets ( fridge: http://www.mcgillcosmology.com/twiki/pub/Internal/HardwareHowto/2014-ish-PrecisionPulseTubeDewar/fridge_beepout.pdf and cernox: http://www.mcgillcosmology.com/twiki/pub/Internal/HardwareHowto/2014-ish-PrecisionPulseTubeDewar/cernox_beepout.pdf ). Make the pin connections as indicated and fill out the resistances as measured. Make sure there is a good connection made on each pin (as shown). NEVER use a multimeter on pins connected to a SQUID. Also, check that no pins have a connection to ground (hold one lead against the casing and pass the other over the header pins).

2. Make sure there are no contacts across temperatures. I.e. that no wires from a 4K region can come into contact with wires from a sub-Kelvin stage. Also check that nothing has fallen into the bottom of the cryostat.

3. Inspect the Mylar on the undersides of the lids; replace if too worn out.

4. Put the lids into place (Mylar side up) and screw down to hand-tight, in a vague star pattern. Try to keep the Mylar on top of the screws, so that as much of the lids are covered as possible.

5. For the outerlid:

   1. Remove the o-ring and set aside somewhere clean

   2. Using many Kimwipes, wipe the lip of the outer wall.

   3. Clean the o-ring with isopropanol ONLY, and grease it evenly with Silicone Vacuum Grease (LDS 91-S)

   4. Put it back in the groove

   5. Wipe the inside of the outer lid with acetone, isopropanol, and then methanol.

6. Align the outer lid with the screw holes and carefully lower it. Try to have the alignment good enough that there is no need to shift it around once it is placed

7. Drop all the screws until threads, then turn screws until they bottom out (then loosen them slightly), then go back and hand tighten them all, then torque wrench them to specification

8. Rotate the cryostat 180deg so that the lid you have just closed is facing downward.

9. Redo the pin connection checks as in step one to ensure that nothing has changed. Also check the ground contacts.

10. Move the cryostat into the RF room. Park it so that its cart frame aligns with the red tape on the floor.

11. Connect the pulse tube to the cryostat

12. Connect the vacuum pump to the cryostat (make sure to wash the o ring and connector, and grease the o ring with vacuum lube)

13. Close the Varian valve. Verify that the relief valve on the Varian turbo pump is shut tightly. Turn on the vacuum pump. Slowly (!) open the Varian valve to keep the pump’s TC1 reading around 1e+1, until the valve is open all the way and the turbo pump can take over efficiently. Check that pressure curve looks okay. Allow pump to run overnight. DO NOT MOVE the vacuum pump while in operation, as this may cause the turbo pump to destabilize and self-destruct.

14. If you are using a new cryoboard (or have made a new SD card for the cryoboard), you must first calibrate the DAC/ADC settings, using the crython program calibrate.py and a calibration dongle (essentially a DB37 with a collection of 10k resistors attached between the pins).

15. Check that the cryoboard has its drive and gain resistors in place, in accordance with your configuration file.

16. Connect the Fridge and Cernox headers to the cryoboard; power on the fan tray, backplane, and cryoboard (the cryoboard requires an external power source which will output -5V, +25V, and +5V, supplying power to the heaters)

17. Update the configuration files for the cryostat according to the current set up

    1. ssh daquser@northpost.physics.mcgill.ca -p 2666

    2. cd crython, cd mcgill

    3. Update the configuration and calibration files

       1. cd crython

       2. python2 configure.py mcgill/configurations/<your config file here>

       3. If you haven’t changed any of the configurations of the sensors, you do not need to reconfigure the board. If you have, use the argument --upload to reconfigure. This takes a couple minutes and will prompt you to reboot the board.

18. Use CryoMonitoring programs to observe pressure, temperature, etc

    1. 192.168.1.139

    2. Cryoboard logging for temp

       1. Same procedure as for pressure

    3. Pressure logging for pressure

       1. Use Screen to start a shell pressure logging process in the background and detach to leave it running

       2. Check it by navigating to its containing folder and: tail -f log_<date/name>pressure.dat

1. To verify that pressure is dropping correctly, check log_pressure_reference.dat

   1. For example of with a leak, check leak reference .dat file

2. Go until pressure ideally gets below 10^-5 (though lately it’s only been getting to about 10^-4). Then, firmly close the main valve (plastic, on top). Turn off the vacuum pump (press the power button). The turbo pump will spin down; if it hits a strong resonance at lower frequencies, you may gently open the relief valve, but do so only incrementally.

3. Once the vacuum pump is completely off, check that the main valve is closed, then unhook it from the cryostat. Close the hole with a covering plate.

4. Open the water circulation system.

5. Fill out the compressor and cryostat logs. For instance, the compressor logs can be found in a labelled drawer in the Lista cabinets, or here: CryostatLog.pdf
   

6. Starting the compressor and the LN2 linear driver:

   1. Verify that the compressor’s high and low pressure are the same, about 220psi

   2. Compressor must be turned on first, followed within a few seconds by the LN2. Find a buddy and synchronize your watches to ensure that this happens.

   3. To turn on the compressor, press the green button. It will make a loud noise; do not be afraid.

   4. To turn on the LN2, plug it in.

   5. Remember to fill out the compressor’s log book.

7. The pulse tube is now operational. Cooling will take many hours.

8. Once the mainplate cools to about 4K, to expedite the cooling process, form a thermal connection from the mainplate to the heads. To do this, heat the pumps to about 35K (enough that the helium within them can evaporate), which will help cool the otherwise isolated fridge heads. You will also need to keep the heat switches open from the main plate to the pumps as you do this, as a connection there will prevent the pumps from warming.

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General usefulness:

• Remote log in to Warwick’s data acquisition account (daquser):

  • From a remote network:

    • “ssh -X daquser@northpost.physics.mcgill.ca -p 2666”

  • From Winterland:

    • “ssh daquser@warwick.local”

* CryostatLog.pdf: log page for compressors and cryostats

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This topic: CryoElectronics > DigitalFMux > CryoElectronics > CryostatHowTo Topic revision: r4 - 2020-03-02 - MacleanRouble

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