A new type of sub-Kelvin refrigerator, the superfluid Joule-Thomson refrigerator, has been developed and its performance has been experimentally verified. This refrigerator uses a liquid superfluid mixture of He and 4He as the working fluid and depends on the non-ideal-gas-like behavior of the 3He component in this mixture to provide cooling when the mixture is throttled. This work included the design and development of low dissipation valves and a low temperature compressor that were successfully operated at 1.2 K. Analytical and numerical models were developed to predict the performance of the refrigerator and to provide insight to improve and further develop this new technology. The refrigerator achieved an ultimate temperature of 0.68 K when rejecting heat at 1.2 K.
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