Superconducting Materials and High Magnetic Fields

Abstract

The recent discovery that Nb₃Sn remains superconducting in magnetic fields exceeding 88 kgauss while carrying current densities in excess of 100 000 amp/cm² has stimulated widespread activity directed toward the construction of superconducting magnets and the understanding of ``hard'' superconductivity. Some of the relevant events that have transpired since the discovery of superconductivity by Onnes in 1911 are reviewed. The critical current versus magnetic field (I_c vs H) characteristics of several materials are discussed and the justification of using such measurements on small samples as a basis for magnet design is presented. Nb‐Zr alloys appear useful for magnets capable of fields as large as 80–100 kgauss while Nb₃Sn appears to be useful for fields of 200 kgauss. Superconducting magnets of Nb₃Sn and of Nb‐Zr have been constructed and tested by several laboratories. Fields as large as about 70 kgauss have been generated with Nb₃Sn magnets and 60 kgauss with Nb‐Zr magnets. Fields exceeding 100 kgauss have been attained by augmenting the field produced by a superconducting Nb₃Sn magnet with the field from a conventional Bitter solenoid. Superconducting magnets capable of fields of at least 100 kgauss are almost a certainty and it is quite likely that magnets will eventually be constructed to produce fields of 200 kgauss. Fields of several hundred kgauss are an intriguing possibility.