One thing I don't think I asked at the time, but now am wondering given your post is how long does it take to go through the proceedure assuming it all works?
It's a good question!
Assuming everything works correctly -- which is almost certainly not the case -- here is the procedure and time estimates for going from an open cryostat at room temperature to a fully cooled cryostat at base temperature:
Day One ======= Inspect all the wiring and connections on the equipment (1/3 day) Make an indium seal and close the inner vacuum chamber (1/2 day) Start pumping out the inner vacuum chamber (rest of day, plus leave running overnight)
[Note: This week, it took me two days (Mon/Tue) to complete the "Day One" tasks]
Day Two ======= Leak test the inner vaccum chamber, which should now be evacuated to a good vacuum [0.0001 millibars] (1/5 day) Close the main bath to seal the cryostat (1/2 day) Pre-cool with liquid nitrogen (1/3 day) Let sit overnight with liquid nitrogen
Day Three ========= Pump out liquid nitrogen (1/4 day) Flush with helium gas to remove residual nitrogen gas (1/8 day) Fill with liquid helium (1/2 day) Let sit overnight with liquid helium
Day Four ======== Pump the 1K pot, start circulating the helium-3/helium-4 mixture, et cetera (1/2 day) At the end of this, if all has gone well, you should reach 5 millikelvin about halfway through the fourth day. At which point you can begin making the low temperature measurements...
Of course, to date, I have not successfully cooled down a cryostat below 400 millikelvin. In fact, our youngest grad student has been here for a year and he says that -- out of the three cryostats here in our lab at Oxford -- there has not been a cryostat cooled down to base temperature while he has been here. It's tough stuff!
Right now, three days into the week, I am stuck halfway down the list of the "Day Two" tasks. We closed the main bath, but there is a gap which has a leak. Not good. After spending a couple of hours on this, we decided to give up until tomorrow morning...
Am I right in assuming that each step closer to 5mK takes progressively longer?
Not really, actually. Once you have the liquid helium in, cooling it all down to 4K, the last steps should come pretty quickly. The more time consuming parts happen at room temperature, getting all the seals closed up properly and leak tested and whatnot. It isn't a bad assumption, mind you, but it doesn't apply here.
And, of course, in practice, the task of troubleshooting all the problems that pop up is really what takes so much gosh darn time! :)
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Date: 2006-07-19 06:58 pm (UTC)It's a good question!
Assuming everything works correctly -- which is almost certainly not the case -- here is the procedure and time estimates for going from an open cryostat at room temperature to a fully cooled cryostat at base temperature:
Day One
=======
Inspect all the wiring and connections on the equipment (1/3 day)
Make an indium seal and close the inner vacuum chamber (1/2 day)
Start pumping out the inner vacuum chamber (rest of day, plus leave running overnight)
[Note: This week, it took me two days (Mon/Tue) to complete the "Day One" tasks]
Day Two
=======
Leak test the inner vaccum chamber, which should now be evacuated to a good vacuum [0.0001 millibars] (1/5 day)
Close the main bath to seal the cryostat (1/2 day)
Pre-cool with liquid nitrogen (1/3 day)
Let sit overnight with liquid nitrogen
Day Three
=========
Pump out liquid nitrogen (1/4 day)
Flush with helium gas to remove residual nitrogen gas (1/8 day)
Fill with liquid helium (1/2 day)
Let sit overnight with liquid helium
Day Four
========
Pump the 1K pot, start circulating the helium-3/helium-4 mixture, et cetera (1/2 day)
At the end of this, if all has gone well, you should reach 5 millikelvin about halfway through the fourth day. At which point you can begin making the low temperature measurements...
Of course, to date, I have not successfully cooled down a cryostat below 400 millikelvin. In fact, our youngest grad student has been here for a year and he says that -- out of the three cryostats here in our lab at Oxford -- there has not been a cryostat cooled down to base temperature while he has been here. It's tough stuff!
Right now, three days into the week, I am stuck halfway down the list of the "Day Two" tasks. We closed the main bath, but there is a gap which has a leak. Not good. After spending a couple of hours on this, we decided to give up until tomorrow morning...
Am I right in assuming that each step closer to 5mK takes progressively longer?
Not really, actually. Once you have the liquid helium in, cooling it all down to 4K, the last steps should come pretty quickly. The more time consuming parts happen at room temperature, getting all the seals closed up properly and leak tested and whatnot. It isn't a bad assumption, mind you, but it doesn't apply here.
And, of course, in practice, the task of troubleshooting all the problems that pop up is really what takes so much gosh darn time! :)