Advanced High-Field Coil Designs: 20 Tesla

Abstract

INTRODUCTIONRecent advanced mirror fusion reactor studies indicate a trend toward using very high-field barrier solenoid coils. Indeed, the MARS and FPD studies call for solenoid magnet peak fields of 24-and 20-T, respectively. During these studies, most of the investigators and magnet, designers felt that only a portion of these fields could be obtained by using superconducting magnets, with the remainder being supplied by inner, copper, resistive coils. Such coils pose many problems in a high-radiation environment. It is therefore desirable to generate as much of the field as possible with superconductors, thus minimizing or eliminating the resistive coil problems by making the coils smaller. The data base on the material properties of superconductors, compiled through 1981, shows that superconducting magnet technology also has problems at high fields. The main factors limiting conceptual superconducting coil designs to peak field values of ipproximately 15 T are:1. Low critical current densities in Nb^Sn at fields > 15 T.2. The increased stabilizer resistivity, due to higher magnetoresistance, requires the use of more stabilizer, thus lowering the overall current density.3. Lar?e magnets and high fields produce enormous electromag netic forces, which create prohibitive strains on an unreinforced superconductor.