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Abstract: 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. [Copyright &y& Elsevier]

We report on High Temperature Superconductor (HTS) DC current leads developed for the specific purpose of delivering small currents (<100 mA) to cryocooled electronic devices operating at 4 K, with significantly reduced heat leak. Multi-stage cryocoolers can provide a suitable platform for niobium-based superconducting electronics at 4 K; the necessary multiple parallel biasing of the circuitry can result in total currents of several amps, which can produce substantial thermal loading of the cryocooler when conventional resistive leads are used. Our approach has been to adapt the comparatively mature technology of high-current second-generation (2G) YBCO-coated conductor tape to low current needs by splitting the tape into electrically isolated narrow lines by ion milling. Performance issues discussed are: obtained critical currents, thermal conductance of the composite conductors, line-to-line electrical isolation, resistance of the joints and robustness. Operation as a current lead between the first and second stage of a Gifford-McMahon cryocooler is reported. [ABSTRACT FROM AUTHOR]

HYPRES is developing a class of digital receivers featuring direct digitization at radio frequency. The complete system, consisting of a cryopackaged Nb superconductor All-Digital Receiver (ADR) chip followed by room-temperature interface electronics and a field-programmable gate array (FPGA) based post-processing module, has been developed. Depending on the targeted application the ADR chip comprised either a low-pass delta with phase modulation-demodulation architecture or X-band band-pass sigma-delta modulators together with digital in-phase and quadrature mixer and a pair of digital decimation filters. The chips were fabricated using a 4.5-kA/cm2 HYPRES process and were cryopackaged using a commercial-off-the-shelf cryocooler. Recently, with significant improvements in chip cryopackage, room-temperature electronics and FPGA programming we were able to achieve stable operation of a low-pass ADR at 28.16 GHz and X-band ADR at 30.72 GHz clock frequencies. Experimental results are presented and discussed. [ABSTRACT FROM AUTHOR]

A custom-designed laboratory prototype of a four-stage Stirling-type pulse tube cryocooler was recently developed by Lockheed Martin for niobium integrated circuits (ICs) operating close to 4 K. Basic system performance has been verified by integration with a Nb IC test chip, with cells that include a high-speed rapid single flux quantum (RSFQ) binary counter. For 650 W total compressor power, extended stable operation of the counter at T = 4.5 K was demonstrated with a clock frequency up to 46 GHz, with 25 mW of excess cooling capacity on the coldest stage. The thermodynamic, electromagnetic, and mechanical performance are promising for the development of an improved compact cryocooler for practical superconducting electronic applications in fields such as wireless communications. [ABSTRACT FROM AUTHOR]

We report on the performance of a 1-V dc programmable Josephson voltage standard (PJVS) operating on a commercially available Gifford-McMahon cryocooler at 4 K. The NIST-fabricated niobium superconductor-normal metal-superconductor junction chip has been shown to operate successfully as integrated with this cryopackage, generating 1.229 V at the maximum operating frequency of 19.364 GHz with Shapiro steps at constant voltage that are flat to within less than 100 nV. Because of the large operating margin (greater than 1 mA), these steps were highly stable and immune from noise. Differences in behavior of the PJVS in liquid helium and on the cryocooler are discussed. This demonstration paves the way towards automated warming and cooling of the Josephson device and a fully automated voltage standard system. [ABSTRACT FROM AUTHOR]

Packaging of a superconducting electronic system on a compact multistage cryocooler requires careful management of thermal loads from input and output leads, in order not to exceed the heat lift capacity of the various stages of the cooler. In particular, RSFQ systems typically require a large total bias current Ib ∼ 1 A or greater. A general analysis of resistive wires shows that the tradeoff between heat flow and Joule heating yields a minimum heat load Qmin from optimized bias leads on a Iow-T stage, given by Qmin [0.3 mW/(A - K)]ThotIb, where Thot is the thermalization temperature of the leads on the previous (hotter) stage. This is independent of the material, number, and geometry of the leads, as long as the total lead resistance is optimized. A similar tradeoff between heat flow and signal attenuation can be applied to the optimization of high-frequency input/output lines. Superconducting leads are not subject to these limitations, and can result in further reduction in heat load. Design examples are presented for an RSFQ-based radio receiver on either a two-stage or a four-stage cooler. [ABSTRACT FROM AUTHOR]