AbstractA low ac loss (Nb, Ti)3Sn bundle-type forced-flow-cooled multifilamentary superconductor was developed for application as a toroidal field coil (dc operation) and a poloidal field coil (pulsed operation) of the fusion reactor. The diameter of the filament was reduced to 1.42 μm (the finest possible in the present technical stage) and filaments in the bronze matrix and the stabilizing copper were segmented by the high resistive CuNi layer and Nb barriers for the reduction of ac losses. The ac losses of a superconducting strand measured by an improved boil-off method are lower than for an ac superconducting (NbTi/Cu/CuNi) wire.A small forced-flow-cooled superconducting coil manufactured with this conductor (rated current Id = 2.5 kA, Bmax = 10 T) showed a high stability margin in dc operation. Repeated pulse operation to the rated current with a high sweeping rate of 13 kA/s (0.85 T/s) was possible without quench in a backup field of 10 T.
Research and development of superconductors and superconducting coils for the Large Helical Device