Operation of YBCO Current Leads as Bias Lines to Cryocooler-mounted 4K Superconducting Electronics

We report on the construction and operation of flexible, 15-line YBCO lead assemblies as low heat-leak bias lines to a complex Rapid Single Flux Quantum (RSFQ) circuit on a 2-stage cryocooler. The leads are used to supply DC bias currents of about 30mA per line to the circuitry operating at 4K. They span the temperature difference between the 1st (∼50K) and 2nd (∼4K) stages of a Gifford-McMahon cryocooler, substantially cutting the heat load to the 2nd stage when compared to optimized normal metal leads. No measurable change in the temperature of the 4K stage associated in Joule heating in the superconducting leads or end joints was observed when the full set of 24 bias lines was taken to operational current, and no degradation was observed after the cryocooler maintained temperature for six months. The leads are made by patterning 12 mm-wide 2G tape, the heat load of which is dominated by conduction through the cable's Hastelloy substrate. We have measured the thermal and electrical transport properties of the Hastelloy® substrate and believe that they are significantly changed by processing history.