Gifford-McMahon/Joule-Thomson Refrigerator Cools to 2.5 K

A compact refrigerator designed specifically for cooling a microwave maser low noise amplifier is capable of removing heat at a continuous rate of 180 mW at a temperature of 2.5 K. This refrigerator is a combination of (1) a commercial Gifford-McMahon (GM) refrigerator nominally rated for cooling to 4 K and (2) a Joule-Thomson (J-T) circuit. The GM refrigerator pre-cools the J-T circuit, which provides the final stage of cooling. The refrigerator is compact and capable of operating in any orientation. Moreover, in comparison with a typical refrigerator heretofore used to cool a maser to 4.5 K, this refrigerator is simpler and can be built at less than half the cost.