An Investigation Into the Heat Treatment Tolerance of WST Nb3Sn Strands Produced for Massive Fusion Coils

Multistep heat treatments are required to produce the superconducting Nb₃Sn in the International Thermonuclear Experimental Reactor toroidal field coils; however, deviations in the temperature and dwell time during heat treatment of the big conductors are unavoidable, and these could affect the performance of the Nb₃Sn strands. To investigate the influence of heat treatment tolerances, both internal-Sn- and bronze-process-type Nb₃Sn strands were heat treated with different cycles. For the internal-Sn process strands, the critical current density Jcn increases as the temperature increases from 630 °C to 650 °C and remains unchanged at 670 °C for 100 h. The Sn content in the filament increases with increasing temperature, and the grain sizes significantly increase from an average of 130-202 nm from 630 °C to 670 °C. For both the internal-Sn process strands and bronze route strands, Jcn seldom changes when the duration at 650 °C is increased from 100 to 200 h. Despite these changes, this study shows that Nb₃Sn strands are not very sensitive to small heat treatment variations at 650 °C, and a variance of ±5 °C is acceptable for both types of Nb₃Sn strands.