A sort of workingman’s de†nition of cryogenic temperatures is temperatures below around 123 K, which equals –150°C or –238°F. In this temperature range and below, a number of physical phenomena begin to change rapidly from room-temperature behaviors, and new phenomena achieve greatly increased importance. Thus, study at cryogenic temperatures typically involves a whole set of new temperature-speci†c discipline skills, operational constraints, and testing methodologies. One of these special attributes of cryogenics is the science and engineering of achieving cryogenic temperatures, both in the laboratory as well as in a sustained “production” environment. The latter can extend from a hospital magnetic resonance imaging (MRI) machine, to a long-wave instrument on a space telescope, to a night-vision scope on a military battle†eld. A number of technologies can provide the cooling required for these and other applications; the choice generally depends on the desired temperature level, the amount of heat to be removed, the required operating life, and a number of operational interface issues such as ease of resupply, sensitivity to noise and vibration, available power, and so on. This chapter provides an overview of the common means of achieving cryogenic tem-

peratures for useful exploitation, including both passive systems involving the use of liquid and frozen cryogens, as well as active cryorefrigeration systems-commonly referred to as cryocoolers. Separate subsections articulate the basic operating principles and engineering aspects of the leading cryocooler types: Stirling, pulse-tube, Gifford-McMahon (GM), Joule-Thomson (JT), and Brayton. Because this †eld is very extensive, the goal of the chapter is to provide an introductory description of the available technologies and summarize the key decision factors and engineering considerations in the acquisition and use of cryogenic cooling systems. After summarizing the details of the refrigeration systems themselves, the remaining

40% of the chapter is devoted to reviewing the critical aspects of cryogenic cooling system design and sizing-including load estimation and margin management-and cryocooler application and integration considerations. Key integration topics include thermal interfaces and heat sinking, structural support and mounting, vibration and electromagnetic interference (EMI) suppression, and interface issues with electrical power supplies. The †nal subsection touches on techniques for measuring the performance of cryogenic refrigeration systems.