FABRICATION OF SILVER-SHEATHED BI(2/2/1/2) SUPERCONDUCTING MAGNET BY MEANS OF PARTIAL MELT AND SLOW COOLING PROCESS

Fabrication of silver-sheathed Bi(2:2:1:2) superconducting coil has been attempted by means of the partial melt and slow cooling process. In order to establish the fabrication process of long length Bi(2:2:1:2)/Ag tape with high J(c), the nominal compositions and heat-treating conditions have been optimized. d.c. and a.c. current operation around 20 K has been carried out using GM-cryo-cooler, a.c. loss characteristics have been discussed. The large scale magnet (81/185 mm in inner/outer diameter and 218 mm long) succeeded in generating magnetic field of 0.68 T at the center part of room temperature bore of 45 mm in diameter at 20 K. In order to obtain the guide for material selection between these materials for the cryogen-free application, a comparative study of J(c)-B characteristics for Bi(2:2:1:2) and Bi(2:2:2:3) tapes has been done at 4.2 and 27 K. Detailed measurements of V-1 characteristics have been performed at 27 and 64 K as a function of applied magnetic field for Bi(2:2:1:2)/Ag and Bi(2:2:2:3)/Ag. The E-J curves are well described by an expression, E approximately exp[-(J0/J)mu] below their transition magnetic fields H(g), rather than the commonly used power law, i.e. E approximately J(n) where mu, J0 and n are constants. For realizing the cryogen-free medium field magnet (e.g. H approximately 10 T), Bi(2:2:1:2)/Ag will be useful only below 20 K and Bi(2:2:2:3)/Ag will be feasible at temperature range from 20 to 30 K.