Maintenance for Gas Flow Cryostat (Miss Piggy)

During the summer of 2018 MissPiggy developed a superfluid helium leak into the vacuum space. Because of this leak a jump in the base temperature was observed every 10 to 15 minutes. When cooling the cryostat below 2 K the leak opens-up and slowly spoils the vacuum, which eventually leads to the loss of liquid in the phase separator and increase in the cryostat temperature. At higher temperature the leak closes, the vacuum recovers and the cryostat cools down again. We were able to run well with a small constant current of the diffuser heater to keep the diffuser temperature at 2.3 K and the sample at 2.1 K. Because of this we think that the leak is in the diffuser close to the heater.

The diffuser was designed at TRIUMF (DWG NO TEX0093B), it has 2 silver solder joints, one soft solder joint and an indium seal. To narrow down the source of the leak, during the mini-shutdown we opened the cryostat and brushed vac-seal on the soft solder joint. After testing the cryostat, we will decide if all the joints need to be redone or just the soft solder and indium?

Also, I used this opportunity to replace the backup uncalibrated diffuser temperature sensor with a calibrated one. Now the cryostat has two calibrated GaAlAs diodes on the diffuser.