Table of Contents
2013/08/22
| 0040 | Increasing the temperature of the 1 K pot to 1.63 K appears to have stabilised it. However, this has led to a slightly higher mixing chamber temperature of 95.6 mK. Not sure if it has improved the stability of the mixing chamber temperature, but it may too early to tell. |
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| Still and cold plate thermometers read 4.50 kΩ and 9.77 kΩ respectively. Still has become warmer, perhaps due to higher temperature of 1 K pot. However, cold plate has become colder, perhaps due to increased circulation rate as still pressure is now 60 μbar. |
Attempting to make sense of the base temperature problems
The following is a summary of what we know
- Turning on the still heater results in a higher circulation rate.
- Turning on the still heater also results in a warmer still and mixing chamber, but colder cold plate.
- Adding more mixture to the fridge results in a colder cold plate, but warmer still and mixing chamber.
From this we can imply a few things:
- Even though the still pressure is rather low, it does not appear to be running dry. If it was running dry, then the cold plate should also warm up with the still heater on as the warm mixture passing through the still heat exchanger from the condenser would not be immersed in the liquid mixture within the still. As a result, less heat would be transferred from incoming mixture to the mixture in the still, and therefore warmer mixture would arrive at the cold plate.
- Adding more mixture warms up the still. This is not totally unexpected. However, the cold plate becomes colder with a greater amount of mixture, perhaps due to both the greater efficiency of heat transfer at the still heat exchanger due to a fuller still, and the increased circulation rate.
- The still is running warmer than it should. However, this is not a new problem. It is difficult to determine if this is due to too little or too much mixture. Warming up the still does improve the circulation rate, thus implying that the still level is fine. Having said that, still pressure is rather low. It is difficult to form a conclusion.
- The mixing chamber temperature increases with still temperature. However, the circulation rate increases with still temperature, making the cold plate colder. If anything the mixing chamber temperature should decrease with a higher circulation rate. It looks very likely that there is thermal contact between the still and mixing chamber, and the most likely culprit is the 600 mK rod.
To ensure that the phase boundary does lie within the mixing chamber, we should conduct a single shot test. Unfortunately, the actual amount of mixture in the fridge at the moment is unknown so it won't be possible to obtain an accurate figure for the proportion of He-3 within the mixture. Nevertheless, we should perform this test anyway just to be sure that the phase boundary does indeed lie in the fridge.
If there is thermal contact between the still and mixing chamber, then we should try to reduce the temperature of the still. In order to do this, we should consider using less mixture. This would affect the circulation rate and lead to a higher cold plate temperature. However, it is possible that we could get to mixing chamber temperatures of <70 mK with a poorer circulation rate.
Perhaps the first thing to do is determine the proportion of He-3 in our mixture by condensing all the mixture stored in the dump into the fridge.
| 0115 | Fill N2 cold trap full. |
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| 0205 | Mixing chamber temperature took a while to stabilise after nitrogen cold trap was filled. Perhaps due to increased circulation rate, the mixing chamber temperature started to cool at first to 94.5 mK before warming up to 95.2 mK where it remained steady. |
| Started single-shot test. Interestingly, it took around half-a-minute before mixing chamber temperature started to drop. This could possibly be an indication of too much He-3, thus mixing is occuring in the line to the still. | |
| 0330 | Mixing chamber temperature reached 65 mK before starting to warm again. Stop 1 K pot, turn on mixing chamber heater (10 mW) and open valve V5 on IGH to commence full recovery of mixture. |
| 1330 | Mixing chamber temperature 15.3 K and pressure in dump 683 mbar. Turn off mixing chamber heater. |
| 1335 | Close condenser line speedivalve and gate valve to isolate fridge. Close all valves on IGH to interrupt pumping to dump. Close nitrogen cold trap 2 speedivalve to isolate it. Fill N2 cold trap full. Start cryocleaning mixture through nitrogen cold trap 1. |
| 1345 | Open up line to IVC to check pressure with turbo pump connected. Pressure in IVC is 1.9E-6 mbar, strongly indicating no leak. Isolate IVC again. |
| 1720 | Started liquid He transfer. Main bath at 0%. |
| 1805 | Cease helium transfer. Helium liquid level 91.5%. |
| Interrupt cryocleaning and pump mixture into dump in preparation for condensation. | |
| 1900 | Move shield trolleys around cryostat. |
| Start 1 K pot. Current temperature 1.6 K. | |
| 1910 | Increase temperature of 1 K pot to 1.9 K and wait for it to calm down. |
| 1940 | Fill N2 cold trap full. |
| Temperature of 1 K pot appears to have stabilised at 1.88 K. Mixing chamber temperature 12.2 K and cooling slowly. Close all valves bar V4A on IGH. Open gate valve and condenser line speedivalve. Open nitrogen cold trap 1 speedivalve. Dump pressure 684 mbar. Commence manual condensation procedure through nitrogen cold trap 1. | |
| 2310 | Manual condensation procedure complete: valve V12A on IGH fully open. Current mixing chamber temperature 6.6 K. Wait for mixing chamber temperature to drop further. |