Evolution of contact resistance and coupling loss in prototype ITER PF NbTi conductors under transverse cyclic load

Cyclic energizing of a magnet coil with Cable in Conduit Conductors (CICC), as for fusion applications, results in an anomalous change of the interstrand contact resistance (R_c) and coupling loss (nτ) due to the alternating transverse forces. Previously, three Nb₃Sn ITER conductors have been tested in a cryogenic press, up to 40 cycles. Now, for the first time, the behavior of NbTi conductors under cyclic load is investigated and results are presented for three full-size prototype ITER Poloidal Field (PF) Coil conductors. One conductor has bare copper strands and no petal wrapping while the others have a Cr and solder strand surface coating. The press can transmit a maximum transverse force of 800 kN/m directly to a cable section of 400 mm length at 4.2 K. Each conductor is tested up to 220 kN/m and 40,000 full loading cycles. The magnetization of the conductors and the R_c between combinations of strands and strand bundles is measured at various number of cycles. It appears that the R_c can vary for up to orders of magnitude during cyclic loading.