Embedded Files
First, some notes on the mini-fridge:
The ambient temperature of the room affects the base temperature of the system. Nathan usually keeps the room at 16 °C. Airflow also impacts this: large standing displays between the cryostat and the air conditioning increase the base temperature.
The system takes a long time to warm up: turning it off on a Monday afternoon and it's Thursday afternoon before it's room temperature.
Cooldown is approximately 36 hours - 24 to 20 K, a few more to 5 K, plus settling time.
The vacuum pump runs continuously. For this reason there is no need to remove the clamp holding the vacuum can on.
The multi-layer insulation (MLI) outgasses very heavily after even just a few minutes in atmosphere. It will outgas so much that the turbopump will be unable to maintain the current required to run it. This requires closing the vacuum pump off and hard-restarting it, as once it's tripped it won't restart.
The National Instruments oscilloscope card has stopped working, so many of the functions of the software that use it are not functional.
Inside the right-hand side of the fridge's support electronics box there is a shower pump to push the cooling water through the system. It has a switch with 3 options: 0 (off), I (manual), II (auto) operation. It should only be turned to I (manual) when the cooling water is on and pumping, or it risks blowing which last time took out half the electronics in the innovation space.
The system has three stages, a 170 K stage, a 20 K stage, and a 4 K stage. The 4 K stage is mounted on a 3D-printed mesh which has broken previously and should be treated with care. The 4 K stage is operated using the external bypass valve, which when closed routes gas through a JT orifice, producing cooling. For avoidance of doubt, when the bypass valve is open, the JT is not working; when closed, the JT is working.
To open the system:
Remove the clamp on the vacuum can.
Allow gas into the chamber by breaking the vacuum at the tubing after the pirani gauge.
Remove vacuum can.
Remove the 15-layer MLI, being gentle! It usually comes off in two pieces.
Remove the screws at the base of the radiation shield - it is very thin and should be handled with care. Use a creased business card to prevent screws falling down into the base of the cryostat.
Remove 170 K radiation shield.
Remove the 7-layer MLI as before.
Remove the screws at the base of the radiation shield - may need to use a flat-head screwdriver to "lever" it off.
Remove 20 K radiation shield. Done!
To close, perform the opening instructions in reverse.
To run the system:
Run the lab logger program from the easily-memorized folder C:\Shared Data\Quantum Hub\Quantum Hub Logger v5\Builds\Run Logger\Run Logger.exe and the lab cooler program from \Run Cooler\ Run Cooler.exe
Open the valve to the vacuum pump and turn the vacuum pump on. The turbo will probably cut out, so restarting will likely be necessary. Pump down to 10^-4 mbar minimum (read pressure from the logger, not the pump's own gauge).
Follow the instructions in the simple 34-step procedure outlined in the RAL documentation. This is in paper form with the cooler. Notable exceptions:
Step 27 says "enter the following values into Run Coolers":
C1 Demand 4.3, Offset 0.0
C2 Demand 4.3, Offset 0.0
D Demand 1.2, Offset -0.5, Phase 225
This actually means set the values:
Compressor 1 and 2:
Amp 1: 4.3,
Amp 2: 4.3,
Offset 1: 0.0,
Offset 2: 0.0
Displacer and Balancer Requests:
Phase: 225,
Amp D: 1.2,
Offset D: -0.5
Step 32 similarly has:
C3 Demand 4.8, Offset 0.0
C4 Demand 6.1, Offset -0.2
which means:
Compressor 3 and 4:
Amp 3: 4.8,
Amp 4: 6.1,
Offset 3: 0.0,
Offset 4: -0.2
If there are any problems with the system, contact (ordered by likelihood of response) Matthew Hills, Martin Crook, or Tom Bradshaw.
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