Influence of Thermal Cycles on Current Sharing Temperature in ITER Toroidal Field Insert Coil Tests

The ITER toroidal field (TF) conductors are cable-in-conduit conductors having Nb₃Sn strands. A TF insert coil (TFI) was fabricated, and its current sharing temperature (T_cs) was measured by using a uniform magnetic field and also applying hoop stress to simulate the actual operating conditions (68 kA in 11.8 T) of the ITER TF coils. The current sharing temperature (T_cs) of the TFI was degraded by warm-up and cool-down (WUCD) cycles that were performed after an initial 1000 electromagnetic loading cycles in its first test campaign. Current sharing temperature (T_cs) continued to degrade with subsequent WUCD cycles, which raised concerns that T_cs had decreased below the ITER requirement of 5.7 K. To further investigate this T_cs degradation by WUCD cycles, additional WUCD cycles were applied to the TFI (nine in total) and its T_cs was remeasured. Consequently, T_cs degradation rate reduced from −106 mK to −5 mK per WUCD cycle after the sixth WUCD cycle, and after the ninth WUCD cycle, T_cs was nearly equal to 5.7 K.