Pulse Tube Head Realignment

The fridge is designed such that the pulse tube (PT) head should be electrically isolated from the two frames. If there is an electrical touch between the PT head and the frames, then this likely indicates that there is a physical touch between the PT head and the tubing that goes down to the plates. This will increase the vibrations transmitted from the PT to the detectors, and the PT head should be aligned to remove this touch.

(Note that the two frames are not isolated from one another, and we should expect a 0.5 Ohm resistance between, for example, a bolt in one frame and a bolt in the other frame).

See the procedure_MIT_PT_realign.docx file in the main repo for pictures and instructions from Olivier.

This procedure is most easily done when the vacuum on the dilution lines is broken. When Olivier visited, we were in the middle of cooling down (around 100 K), and he broke the vacuum by pausing the cooldown and manually letting helium from the dump tank into the lines until they were around 1 atm pressure.

After breaking vacuum, Olivier fully lowered all of the bolts so the flange was sitting on the stops. At this point the touch was removed (ohmeter read OL). He then raised each bolt just enough to prevent the flange from touching the stop (he checked this by inserting a piece of paper in between the flange and the stop). He again confirmed that the resistance measured OL.

He pumped all helium back into the dump tank so the lines were under vacuum and confirmed the resistance was OL. He then repeated the process of putting Helium back in the lines to 1 atm and pumping out, measuring the resistance at each step, in order to make sure that the touch wouldn't come back in future cooldowns.