Miniature refrigerators for cryogenic sensors and cold electronics

Abstract

A surprisingly wide diversity of sensors, instruments or devices used for military, civil, scientific and medical purposes require cooling to cryogenic temperatures. Some work better or faster at low temperatures, are more sensitive or more effective. For others cryogenic cooling is essential to utilize phenomena which exist at very low temperatures, sometimes a narrow or precise temperature range. The cooling capacities required range from fractions of a watt to as much as 10W at temperatures extending from a minimum of 1 K or less to those well above the limit of the cryogenic range (120 K). At present there is a concentration of interest for fractional watt cryocoolers at near liquid-nitrogen temperatures (80 K). These needs are currently filled by immersion in liquid or solid cryogens, miniature Linde-Hampson cryocoolers with Joule-Thomson expansion energized by compressed gas from storage bottles and miniature mechanical refrigerators. Following a long period of intensive development and evaluation of many different mechanical systems miniature Stirling engines appear to emerge as the system of choice. They are smaller, weigh less, are more efficient and can be made at a reasonable price. Early disappointment with limited life and high maintenance requirements have been alleviated as the technology matures, designs are stabilized and manufacturing and test techniques develop. The present large scale application for miniature cryocoolers in infrared night vision and missile guidance systems will probably remain and expand as ‘smart’ projectile and beam guided weapons are widely introduced. Other large scale applications are in prospect with the development of semi-conductor ‘cold electronics’ systems and the new high temperature superconducting devices.