Mass Production of Nb₃Sn Strands by Internal Tin Diffusion Process for the ITER Project

Abstract

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 J_c) and low hysteresis loss (low W_h) 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 J_c, 134kJ/m³ for W_h, 175 for RRR and 3, 411m for piece length. In this development, J_c 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.