Cryo working again!

So during three months after coming back to the university I was not able to perform any measurement on Scheer II because of a leak from the liquid helium bath to the inner vacuum chamber of my cryostat. This makes that all stages from the cryo go quickly to a 4K temperature, making it impossible to condense the He3/He4 mixture and therefore to reach the 30 mK base temperature.

In a red circle, the welded part that had a leak.

It was a cold leak that at the beginning just appeared at very low temperatures. With the time and several cooldowns, the leak became progressively bigger and we could localise it at 'high' temperatures (around 240k). It was in one of the weldings of feedthrough for additional wiring to the IVC. It was necessary to weld it two times and to perform further tests, but finally, the leak closed.

In the meantime, we needed to figure out different 'devices' to cool down the IVC and be able to reach with the helium gun the different points where we suspect the leak to appear. Also, we had some problems, being the worst the quick evaporation of most of the He from inside the dewar because the leak suddenly opened while cooling down without exchange gas.

Fast evaporation of all the LHe inside the dewar (you can tell because of the gas released from the overpressure valve and because the impedance for the recovery system is completely frozen)

After all, I could reach the base temperature and measure during August. I'll show this data on our next online meeting. See you there! :)


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