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Dilution Refrigerator
Most of my experiments have been done with a dilution refrigerator. This cryogenic cooler
employs the liquid state of the two isotopes of helium, He3 and He4.
The cooling of the system takes advantage of a phase boundary that separates the isotopes
below 870 millikelvin. Analogous to evaporative cooling, maintaining this phase barrier
requires energy which comes from the surrounding environment thereby cooling it. Using this cooling
process my dilution refrigerator can achieve a base temperature of 15 millikelvin.
Early in my PhD I noticed problems with the temperature stability of the dilution refrigerator.
I traced the problem to a faulty valve in the pump that circulates the helium mixture in the
refrigerator. The oil that lubricated the pump had migrated though the pumping lines
as far as the mixing chamber, the coldest part of the aparatus. I took the system
apart, cleaned it, and put it back together. After the repair the fridge had comparable cooling
power and a base temperature of 17 mK indicating successful operation. My experience repairing
my dilution refrigerator is well-known and I get requests for help and advice from other
users.
