Heat Treatment Trials for ITER Toroidal Field Coils

Cable-in-conduit (CIC) conductors using Nb 3 Sn strands are used in ITER toroidal field (TF) coils. Heat treatment generates thermal strain in CIC conductors because of the difference in thermal expansion between the Nb3Sn strands and the stainless-steel jacket. The elongation/shrinkage of the TF conductor may make it impossible to insert a wound TF conductor into the groove of a radial plate. In addition, it is expected that the deformation of the winding due to heat treatment-based release of the residual force in the jacket may also make it impossible to insert the winding in the groove, and that correcting the winding geometry to allow insertion of the winding may influence the superconducting performance of the TF conductor. The authors performed several trials using heat treatment as the part of activities in Phase II of TF coil procurement aiming to resolve the above-mentioned technical issues, and evaluated the elongations of 0.064, 0.074 and 0.072% for the straight and curved conductors and 1/3-scale double-pancake (DP) winding, respectively. It was confirmed that correction of the deformed winding did not influence the superconducting performance of the conductor.