This chapter reviews various methods commonly used for achieving the cryogenic temperatures required of superconducting materials. The problems associated with cryogenic refrigerators (cryocoolers), such as poor reliability, low efficiency, electromagnetic noise, and high cost, have hindered the marketability of superconductors. Recent advances in cryocoolers are beginning to alleviate some of these problems. This chapter discusses the operating principles, advantages, and disadvantages of the six common types of cryocoolers. The types of cryocoolers covered here are the Joule-Thomson, Brayton, Stirling, Gifford-McMahon, and pulse tube systems. Recent advances in all types are described. The use of flexure or gas bearings has led to lifetimes of 10 years or more for space applications. Small pulse tube cryocoolers have achieved efficiencies as high as 24% of Carnot at 80 K (higher than any other cryocooler), yet maintain the advantage of no moving parts at the cold end. Temperatures down to 2 K can now be easily achieved with some of these cryocoolers. Improved shielding and integration methods, along with careful selection of materials, have allowed SQUIDs to be cooled with cryocoolers for use in magnetocardiography, nondestructive evaluation, and SQUID microscopes. Some of these integration issues are discussed. Power applications of superconductors are now requiring significantly increased refrigeration powers of 1 kW or more at 70 K. Cryocooler costs are still a serious problem for many applications, but they could be significantly reduced when manufactured in large quantities.
Citation: Handbook of Superconducting Materials
Publisher Info: Taylor & Francis, London,
Pub Type: Books
brayton refrigerator, cryocoolers, cryogenics, gifford-mcmahon refrigerator, Joule-Thomson refrigerator, pulse tubes, refrigeraotrs, review, stirling refrigerator