In multiply-twisted superconducting cables exposed to an external AC transverse magnetic field, a component of the magnetic field parallel to the strand axis brings about an additional AC loss (longitudinal AC loss) in the strands. We have experimentally evaluated the longitudinal AC loss in two types of triply-twisted superconducting cables composed of insulated strands with different diameters, by eliminating the hysteresis loss and the transverse coupling-current loss from the total loss in the strands. The evaluated longitudinal AC losses were quantitatively explained by a theoretical expression obtained so far, under consideration of the effects of the longitudinal component of the applied field to the strand. The present results suggest that the longitudinal loss can be dominant in AC magnetic fields with a relatively large amplitude. To minimize the total loss of the strand in multiply-twisted cables, we discuss an optimum condition for the twist pitch of the strand.
