Development and Mass Production of Bronze-Processed Nb₃Sn Strand with Low AC Loss and High Critical-Current Density for the ITER

Abstract

The developmental work and mass production of high-performance bronze-processed Nb₃Sn superconducting strands for the ITER pulse coils were performed. Using fine hexagonal single-core rods, the undesirable deformation of filament shape was drastically improved and uniform filament-spacing was obtained. As the result, Nb₃Sn strand having a bronze/Nb ratio of 2.3 and filament diameter of 3.0μm showed a very high non-Cu J_c of 667A/mm² at 12T and 4.2K without external strain. Sufficient low hysteresis loss of 88.2mJ/cm³ at ±3T was also observed. These results fully satisfy ITER specifications. The maximum piece length reached 18.9km, while average piece length was 8.5km, thus proving excellent workability. By applying the developed technology, mass production of 1, 280km in total was successfully completed. This means that the mass-production technology for the high-performance Nb₃Sn superconducting strand of the ITER was established; thus clearing the way for construction of the reactor.