Monolithic Regenerator Technology for Low Temperature (4 K) Gifford-McMahon Cryocoolers

Abstract

A two-stage Gifford-McMahon (GM) cryocooler has been selected to produce and maintain the low temperatures required by the superconducting magnet system in an active magnetic regenerative liquefier (AMRL). The operation of practical AMRLs requires relatively !arge magnetic fields (e.g., 8 T). Currently, these fields can only be produced via Iow-temperature superconducting magnets that typically operate at liquid helium temperatures (4.2 K). Until a few years ago, obtaining non-zero cooling powers below 10 K was impossible with regenerative cryocoolers. This is mainly because the volumetric heat capacity of pressurized helium (the working fluid) is much higher than the corresponding value for Iead (the material commonly used in low temperature regenerators). Several teams have successfully modified GM cryocoolers to reach temperatures < 4 K with specially manufactured second-stage regenerators. The authors have successfully modified a commercial two-stage GM cryogenic refrigerator to reduce its minimum, no-load temperature from 6.1 ± 0.1 K to 3.42 ± 0.05 Kat a nominal operating frequency of 1.2 Hz. This was accomplished by using a monolithic regenerator that is relatively simple to manufacture. The cooling power at 4.2 ± 0.1 K was measured to be 0.43 W with zero thermal Ioad at the first stage. The superconducting magnets in our AMRL have been designed to operate at 4.5 K. The refrigeration power available at this temperature was measured tobe 0.50 ± 0.01 W with a simultaneous first-stage Ioad of approximately 20 W at 42.8 ± 0.5 K.