Extra AC losses for a CICC coil due to the non-uniform current distribution in the cable

Extra AC losses were observed during the Experiments on a Single Inner Vertical coil (EXISV). The Inner Vertical (IV) coils are the smallest poloidal coils for the Large Helical Device (LHD) and their inner and outer diameters are 3.2 m and 4.2 m, respectively. The coil consists of 16 pancake coils wound with cable-in-conduit conductor (CICC) whose strands are NbTi/Cu without any surface coating. Many causes for the extra AC losses were considered, such as the decrease of a contact resistance between strands due to the large electromagnetic force in the conductor or due to the stress during the coil winding process, etc. and possibilities were investigated from the experimental data. Finally, we found that a coupling current with a very long time constant of 124 s caused the AC loss increase. The coupling current with such a long time constant cannot be explained from the symmetric twisting configuration of the CICC but can be explained as a local loop current corresponding to a cyclic change of the non-uniform current distributions in the cable. The non-uniform current distribution could be induced by an asymmetry of the strand transposition in the cable. To verify the above reasoning, we did fundamental experiments on a two-strands-cable, which has an intended asymmetry in the cable twisting. Extra AC losses were also observed for an asymmetric two-strands-cable, and it was demonstrated that the non-uniform current distribution causes an increase of AC losses.