Review of superconducting magnetometers and cryogenic refrigeration techniques

Abstract

Superconductors are characterized by many unique properties which are all manifestations of the macroscopic quantum nature of the superconducting state. Several of these properties have direct application to devices for shielding, stabilizing, measuring and producing magnetic fields. The zero resistance property of superconductors has been used to construct magnets capable of producing magnetic fields up to 140 kgauss. This same property also provides for excellent magnetic shielding, i.e., the net magnetic flux linking a closed superconducting path must remain constant as long as the path remains superconducting. The somewhat more subtle property of quantized magnetic flux has been used in the construction of sensitive magnetometers with field resolution as good as 10-7 gauss. Tunneling of superconducting electron pairs through a non-superconducting barrier was predicted by Josephson in 1962 and verified experimentally the following year by Rowell. Various extensions of the Josephson tunneling properties of superconductors have remarkable device applications. In particular the tunneling characteristics are changed appreciably by external magnetic fields. Magnetometers using this property have been used to resolve field changes as small as 10-10 gauss. The application of these properties of superconductivity, especially to sensitive magnetometry, will be reviewed to lay the groundwork for the more detailed papers to follow in this session. All of the devices noted above require temperatures near 4 °K in order to function. This is a particularly significant requirement for space instruments and for many earth-based geophysical measurements as well. Liquid helium refill systems are the simplest cooling techniques, although serious logistic and handling problems may be encountered in field applications. Closed cycle miniature refrigerators could make superconducting magnetometers, shields, etc., practical field instruments. The present state of the art in all of these cooling techniques will be reviewed