Design of a high-current AC down link using Bi-based superconductors

Recent processing developments in Bi/sub 1.8/Pb/sub 0.4/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/. (BSCCO-2223) bars have produced bulk BSCCO-2223 bars with properties advantageous for power applications. Cold isostatically pressed (CIP) and sinter-forged BSCCO-2223 both have low AC loss, which make them desirable for use in power devices. Thermal conductivity of the CIP bars is lower than that of the previously used sinter-forged samples by a factor of 2. CIP bars with cross-sectional areas of /spl ap/0.75 cm/sup 2/ and carrying 250 A RMS transport current have AC loss values of 30 /spl mu/J/cycle-cm at 50 Hz and 77 K. A pair of prototype downlinks were designed and built with sinter forged bars to deliver a continuous AC current of 1500 A over a temperature gradient of 77 to 4.2 K while delivering about /spl ap/200 mW of heat to the liquid-helium-cooled end. This paper discusses the design considerations and modeling of downlinks, which supply high AC currents over the 77 to 4.2 K temperature gradient with low thermal losses.