| 0145 | Fill N2 cold trap full. |
| 0250 | Mixing chamber temperature has been rising for the last half-hour. Quicken pace of helium transfer. |
| 0540 | IVC pressure is rather worrying. It reached 3E-6 mbar during the transfer, which is a tad high, but can possibly be explained due to the short amount of time the line to the turbo pump had been opened. Additionally, such a pressure shouldn't pose too much of problem. However, the IVC pressure has increased to 1.4E-5 mbar and the temperature has been decreasing for the past couple of hours. There are two possibilities: we have a leak; or something inside the fridge is acting as a cryopump. |
| | Usually the 1 K pot is the likely culprit for the cryopump possibility during a transfer as helium can leak into it through the needle valve. However, its temperature is above 7 K. The mixing chamber temperature is 9.5 K, so it could still be a possibility. |
| | It is difficult to believe that there is a leak as there was no sign of one until we started to fill the main bath with liquid helium. Having said that, this cannot be ruled out, and unfortunately is the most likely cause. |
| | Assuming there is a cryopump, we should definitely try to condense and circulate the mixture. The question is when we should attempt this. After some helium boils off, the 1 K pot may heat up a little and this may result in a lower pressure and a rising temperature. It may be worth waiting to see what happens. If the mixing chamber reaches a temperature of 4.2 K, then this would clearly indicate a leak. We know the turbo pump is good enough to pump the IVC out at these temperatures as long as nothing inside is acting as a cryopump. Let's continue to monitor the situation. |
| 1535 | Fill N2 cold trap full. |
| | Mixing chamber temperature 7.9 K and IVC pressure 4.6E-6 mbar. The rate of descent of mixing chamber temperature has slowed to a crawl. However, it is still descending. If possible, it would be best to wait until it starts to rise before attempting to condense and circulate the mixture. |
| 2000 | Mixing chamber temperature has started to rise within the last hour, although pace is currently that of a snail. IVC pressure is 4.6E-6 mbar, and has been for the last four hours. Helium liquid level is 75.7%, leaving us with around 30 hours of helium. Potentially, we could wait until tomorrow in order to condense and circulate the mixture. The mixing chamber temperature is currently 7.9 K. It's very tempting to wait, and I do not believe we would gain very much by rushing it now. Condensing the mixture in 14 hours' time still leaves us with enough mixture to start the 1 K pot and condense & circulate the mixture before another transfer is required. The detector can be flashed during the transfer and for a couple of hours afterwards so little time is wasted. Waiting would also give more time for any trapped helium to be pumped out of the IVC. The disadvantage is if we fail to reach a suitable base temperature, it would mean at least a day's delay. |
| | I think it is probably worth waiting until the morning. Continue to cryoclean mixture overnight. |