A 4 K Gifford-McMahon Refrigerator for Astronomy

Previous abstract Next abstract

Session 51 -- Instrumentation
Display presentation, Wednesday, 9:20-6:30, Pauley Room

[51.03] A 4 K Gifford-McMahon Refrigerator for Astronomy

R.L. Plambeck, N.A. Thatte, and P.B. Sykes (UC Berkeley)

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.

Wednesday program listing