ABSTRACT
This chapter introduces regenerative cryocoolers of Gifford McMahon (GM) and pulse tube types. Both cryocoolers consists of a constant flow compressor package, a set of SS flexible lines and a coldhead expander. The pulse tube cryocooler is a new generation cryocooler eliminating the cryogenic moving parts used in the GM cryocooler. Thermodynamic Cycles and engineering aspects of the GM and pulse tube cryocoolers are discussed and compared.
Traditional regenerator materials have very low specific heat below 10 K and lead to a minimum attainable temperature of ~6 K for two-stage cryocoolers. Owning to the discovery of rare earth regenerative materials, the GM and pulse tube cryocooler could be developed and commercialized to operate below 4 K. The rear earth materials exhibit large heat capacities at the temperatures of 4–20 K due to their magnetic phase transition. Antiferromagnetic materials of HoCu2 and Gd2O2S were chosen to use in the latest commercial 4 K cryocoolers. Currently, the commercial GM cryocoolers provides cooling capacities from 0.1 to 1.5 W at 4.2 K and the pulse tube cryocoolers from 0.25 to 2.0 W at 4.2 K.
