Superconductors as Permanent Magnets

Abstract

A unique property of a superconductor is the possibility of lossless current flow at temperatures below the critical temperature of the superconductor. Hence it is possible to make permanent magnets since fields will exist because of these persistent currents. The potentialities of superconductors in this respect may be distinguished by considering the supercurrent paths in different topological configurations. Experiments on solid and hollow cylinders of lead are reported and compared to theoretical predictions. While the singly connected soft superconductor is uninteresting as a permanent magnet owing to the Meissner effect of flux exclusion, the multiply connected sample has at least some theoretical interest in this application. Considerable improvement may be expected when using the hard or high-field superconductor, since several theories of its behavior imply a connectivity of extremely high multiplicity. It is shown that an energy product of over twenty-five million gauss-oersteds is obtained from Nb3Sn at 4.2°K. One possible approach to understanding this behavior derives from the Mendelssohn filamentary mesh model. A macroscopic theory of this model postulating that a critical current density as a function of field is the basic property of high-field superconductors is shown to be capable of accounting for the observations. It can be predicted on this basis that other materials or other experimental conditions will make possible much higher values of energy product than reported here.