LOSSES OF ALUMINUM-STABILIZED SUPERCONDUCTING CONDUCTORS FOR LARGE HELICAL DEVICE

We investigate an optimum design of aluminum-stabilized superconducting conductors used for the helical coil system of the Large Helical Device (LHD) project from the low loss point of view. A short sample conductor of about 50 cm long is used in our experiment. The loss measuring system we originally fabricated is mainly composed of a superconducting split magnet of a race-track type and a superconducting transformer. The data of the ac loss observed in the frequency range from 0.1 Hz to 120 Hz show the existence of two peaks just as our analysis predicted. The peak at the lower frequency is the critical frequency of the inter-strand coupling time constant tau(c): The other peak at the higher frequency is the skin frequency of the normal metal around the strand bundle in the conductor. At the practical condition of the LHD corresponding to 0.1 Hz, the loss can be decreased by the increase in tau(c).