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 J(cn) increases as the temperature increases from 630 degrees C to 650 degrees C and remains unchanged at 670 degrees 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 degrees C to 670 degrees C. For both the internal- Sn process strands and bronze route strands, Jcn seldom changes when the duration at 650 degrees 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 degrees C, and a variance of +/-5 degrees C is acceptable for both types of Nb3Sn strands.