Development programs focusing on Nb3Sn strands for the International Thermonuclear Experimental Reactor (ITER) are being performed by each party of the ITER group. The object of development is not only to achieve high superconductor performance such as high current density and low hysteresis loss but also enable reliable low-cost mass production, one way of which is to reduce strand breaking during the manufacturing process, and long-length strand fabrication. In Japan, the Japan Atomic Energy Research Institute (JAERI) is involved in this program backed by strong collaboration with many domestic industries. Totally, 11.1 tons (2, 400km-length) of strands that meet ITER specifications have been fabricated successfully. Through this development work, the Nb3Sn strand mass-production technique in Japan has been substantially improved.
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 Jc 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.
Development work to produce a high-performance Nb3Sn superconducting wire for the center solenoid coil of the ITER was carried out. The effects of concurrently adding Ti and Ta to bronze-processed Nb3Sn wires were examined. In addition, a high-Sn-concentration bronze matrix was applied. Then, the relation between hysteresis loss and filament diameter was examined. Moreover, the cause of wire breakage during processing was elucidated. As a result, a reliable manufacturing process for high-performance Nb3Sn superconducting wire was established.
For the actualization of the ITER, it is necessary to develop Nb3Sn strands having not only excellent superconducting properties of high critical-current density (high Jc) and low hysteresis loss (low Wh) but also good productivity with low cost. We have been developing Nb3Sn strands by the internal tin diffusion process, because it provides better controllability of the tin concentration and better cold workability than other processes. Strands meeting the specifications of superconducting properties were developed by optimizing the diameter and spacing of the filaments and the diameter of the tin core inside the strand. In this project, mass-production strands meeting the ITER specifications were developed by arranging the filaments for a concentric circle and using a Cu/Ta barrier. As the result, we successfully completed manufacturing 600km (2.75 tons) of strands having excellent properties: 627A/mm2 for Jc, 134kJ/m3 for Wh, 175 for RRR and 3, 411m for piece length. In this development, Jc was found to be improved by increasing the diameter of the tin core. These strands are expected to be applied for not only poroidal field coils but also toroidal field coils.