Performing a One Shot Test

One way to debug issues with the fridge is to cut off the helium flowing into the mixing chamber and see how the mixing chamber temperature changes. Generally the temperature will decrease, because helium is being pumped out without new warm helium flowing in to replace it. (Note that things are a bit more complicated on a cryoconcepts fridge because the pulse tubes are thermally connected to the plates via that same inflow of helium, in order to reduce vibrations. This means that there is a competing effect where the plates will want tot warm up because they are no longer thermally connected to the pulse tubes. In practice, this means that the temperature logs from the one shot test must be sent to Cryoconcepts to be interpreted).

We have performed two such one shot tests: https://elog.lns.mit.edu:8443/Despereaux/21 and https://elog.lns.mit.edu:8443/Despereaux/50

First, the still heater PID loop should be used to regulate the still temperature during the one shot test. (This is done because the still will want to decrease in temperature when the input heat load is removed, but we want to keep it warm so it boils off helium at the same rate). Generally this temperature will be kept around 500 to 700 mK, but if the fridge is in a bad state this temperature may need to be higher. (eg we used 940 mK on our last one shot test where it turned out a cable had fallen and created a thermal leak between plates).

Close V5, open V16. Continue the test until the temperature stops decreasing and starts to increase. This should take order 1 hour.

To end one shot test: Open V5, close V16, open V14 to allow mix to flow back into the system.