Design of cryocoolers for microwatt superconducting devices

The primary applications of the cryocoolers are for cooling various Josephson devices such as SQUID magnetometers and amplifiers, voltage standards, and microwave mixers and detectors. The common feature of these devices is their extremely low inherent bias power requirement, of the order of 10/1 W per junction. This provides the possibility of designing compact, low-power cryocoolers for these applications. Several concepts were explored and a number of laboratory model cryocoolers were built. These include low-power nonmagnetic regenerative machines of the Stirling or Gifford-McMahon type, three or four-stage Joule-Thomson machines, liquid-helium dewars with integral small cryocoolers to reduce the evaporation rate, and liquid-helium dewars with integral continuously or intermittently operated small helium liquefiers to permit operation of cryogenic devices for indefinite time periods.