We are having problems with our JT cryostat. We have cooled down our system three or four times and it easily reached the base temperature (~ 1.8 K). However we cannot cool down the system below 30 K anymore (LakeShore temp. controller displays 30 K (sample position) and ~9 K (cold finger). We have already recharged the He pressure in the Thermal Switch, cleaned all the system using N2 gas and pumped out several times to remove any moisture or dirt, baked out the trap, and purged the whole line several times. In addition the pressure inside the compressor is fine. Nevertheless, we cannot reach the base temperature again. Any idea? Thank you!
Sounds like you have a great leakage into your cryostat. Because $/Delta$ of 21 (30-9) K is too large. Check and tight up the flanges. Better check the system by He leak-seeker. Note, the vacuum itself could be nice, if your pump is powerful!
9K on the cold finger of the closed cycle sounds a bit high. Have you purged the lines and cold head? The usual procedure is to cool it down as far as it will go, then disconnect the pressure lines from the compressor, let everything warm up, and then flush clean He gas several times. Make sure that the compressor is charged to the correct pressure after the operation. This is a common problem of closed cycle refrigerators if you disconnect the pressure lines often.
I'd agree with Andrey, considering that you already flushed the system. You might have a leak from your He recirculating system to the vacuum can, causing a large heat leak to the cold stage. If your system was contaminated, you would notice unusual pressures on either the high side or the low side. Have you been keeping track of them? If so, compare to previous recorded values.
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