Increase of inter-strand coupling losses in superconducting cable-in-conduit conductor under actual condition of sweep rate

Basic studies are carried out on causes of the increase of inter-strand coupling losses in cable-in-conduit conductors used for windings of large scale magnets, such as a single inner vertical (IV) coil in the Large Helical Device. In this paper, the effect of AC transverse magnetic fields periodic changing along a two-strand cable axis on loss properties of the cable is considered as a candidate of the cause. The evidence that the periodic changes produce additional inter-strand coupling losses characterized by another long coupling time constant in addition to the intrinsic one is theoretically shown. We confirmed properties by measurements on two kinds of sample wires, using inter-filament coupling in a strand instead of inter-strand coupling in cable.