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SIS mixers on the BIMA array are cooled to 4 K with novel closed cycle refrigerators. These refrigerators utilize the Gifford-McMahon cycle, which is used in most commercially available cryopumps. In this cycle helium is expanded from 20 to 6 atm after it is precooled in a heat exchanger matrix, or regenerator. Conventional GM refrigerators bottom out at temperatures of 8 to 10 K because their regenerators have inadequate heat capacity at lower temperatures. By adding a third stage to a standard commercial refrigerator, and by using spheres of Er$_3$Ni as the third stage regenerator, we reach much lower temperatures.
Our refrigerator operates at 2.2 K with no heat load applied to the third stage. With 10 W on the first stage, 1 W on the second stage, and 50 mW on the third stage, the third stage temperature can be maintained at 3.5 K. By comparison with hybrid Joule-Thomson/GM refrigerators traditionally used to reach 4~K, our refrigerator is far more compact, cools more quickly (to 4~K in a little over an hour), requires only a single helium compressor, and is less prone to clogging by contamination. The principal challenge in building such a refrigerator is to fabricate a helium-tight reciprocating seal which operates reliably at 7~K on the refrigerator's third stage.
This work was supported by NSF Grant AST91-00307.
