Sweep-rate dependences of losses in aluminum-stabilized superconducting conductors for the Large Helical Device

We measured the frequency characteristics of ac losses in the aluminum-stabilized superconducting conductor for the helical coil system of the Large Helical Device (LHD) project, in order to clarify the dependence of the loss on the sweep rate of the poloidal field. In our experiment, a straight and short conductor of about 50 cm long was used as a sample. The sample was subjected to the ac ripple field superposed on the bias field. The data of the ac loss observed in the frequency range from 0.1 to 100 Hz have two peaks just as our analysis predicted. The peak at the lower frequency is the critical frequency related to the inter-strand coupling time constant τ_c. The other peak at the higher frequency is the skin frequency of the normal metal around the strand bundle in the conductor. The lower frequency peak mainly shifts according to the cabling condition of the strand bundle. Under the operating condition of the LHD corresponding to 0.1 Hz, the loss can be decreased as τ_c increases. It is very different from the common design concept. This result suggests that it is necessary to grasp the sweep-rate dependence of the loss in the conductor in order to optimize its structure from the low-loss point of view.