Losses of aluminium-stabilized superconducting conductors for Large Helical Device

The optimum design of aluminium-stabilized superconducting conductors used for the helical coil system of the Large Helical Device (LHD) project was investigated from the low loss point of view. A short sample conductor of about 50 cm long was used in the experiment. The loss measuring system 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 the analysis predicted. The peak at the lower frequency is the critical frequency of 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. For the practical condition of the LHD corresponding to 0.1 Hz, the loss can be decreased by an increase in τ_c