Optimization of Heat Treatment of Japanese $\hbox {N}{ \rm b}_{3}\hbox {S}{\rm n}$ Conductors for Toroidal Field Coils in ITER

The optimization of the heat treatment of Nb₃Sn conductors for toroidal field coils in ITER was attempted to improve the current sharing temperatures (T_cs). Using the strand, we chose the pattern at 570°C for 250 h and 650°C for 100 h as the best, which increased the critical current and maintained the residual resistivity ratio higher than 100. The behavior of the critical current of the strand vs. the magnetic field, temperature, and strain was also improved. This pattern was used on two conductors, and their performances were tested. T_cs was evaluated over 1000 electromagnetic cycles and one thermal cycle. A sharp T_cs degradation occurred at 50 cycles. Then T_cs decreased linearly. Although this tendency was similar to the conductors that were heat treated with the original pattern, the degradation rates were improved. The ac losses (Q) before cycling were approximately 10% lower than those of the original pattern. Q after cycling became almost equivalent between two patterns. The conductor was inspected after the test, which showed that the conductor under the high-magnetic-field zone had contracted by approximately 600 ppm during the test. Some clearly deformed strands were observed under the high-magnetic-field zone, which could degrade T_cs.