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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]

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]

NIST and HYPRES, Inc. have been collaborating to develop a 1 Volt DC programmable Josephson voltage standard (PJVS) that operates on a closed-cycle refrigerator. The goal of this work is to construct a platform that will allow the chip to work at 4 K without liquid helium, thereby making the system more convenient and eliminating the need for users to handle liquid cryogens. In our existing PJVS systems, the Josephson chip temperature is the only parameter that is not computer controlled. The addition of a cryocooler will allow automated warming and cooling of the Josephson device and enable an intrinsic voltage standard system in which every control function is automated, and the only required user input is the desired output voltage. The cryocooler package is designed to allow PJVS chips to be easily interchanged between the cryocooler and liquid helium cryoprobes. [ABSTRACT FROM AUTHOR]

HYPRES is developing a prototype digital system comprising a Nb RSFQ analog-to-digital converter (ADC) and SiGe amplifiers on a commercial two-stage cryocooler. This involves the detailed thermal, electrical, and mechanical design of the ADC chip mount, input/output (I/O) cables, and electromagnetic shielding. Our objective is to minimize the heat load on the second (4 K) stage of the cryocooler, in order to ensure stable ADC operation. The design incorporates thermal radiation shields and magnetic shielding for the RSFQ circuit. For the I/O cables, the thermal design must be balanced against the acceptable attenuation of rf lines and resistance of dc bias lines. SiGe heterojunction bipolar transistor (HBT) signal conditioning circuits, placed on the first (60 K) stage of the cryocooler, will amplify the mV-level ADC outputs to V-level (e.g., ECL) outputs for seamless transition to room-temperature electronics. Cooling these HBT circuits lowers noise and improves their high-frequency performance. Demonstration of this prototype should lead the way to commercialization of high-speed digital superconducting systems, for such applications as wireless communication, radars, and switching networks. [ABSTRACT FROM AUTHOR]