Practical Operation of Cryogen-Free Programmable Josephson Voltage Standards

Cryogen-free operation is rapidly becoming the preferred implementation of most superconducting electronics systems including programmable Josephson voltage standard (PJVS) systems. There are strong operational incentives for using the smallest possible cryocooler in order to minimize acoustic noise, system footprint, and power consumption. In addition, Nb/Nb_xSi_1-x/Nb junction technology, which operates near 4 K, offers better yield than NbN/TiN_x/NbN technology, which can operate at 8.5 K, thus making lower temperature operation near 4 K desirable. As junction density increases, however, self-heating of the junctions can create significant thermal gradients between the arrays and coldhead. Thus careful design of the overall system is required to maintain acceptable operating margins. We have developed a calorimetric measurement technique to characterize the system variables and used it to evaluate several different PJVS configurations. This technique uses the PJVS subarrays as both heat sources and temperature sensors, in conjunction with a time gated measurement technique, to characterize the thermal response of the system. A passive thermal filter incorporating a Pb thermal mass is used to reduce the temperature oscillations of the cryocooler. Our results suggest that, with appropriate system design, operation of a practical 10 V PJVS on a small (nominally 100 mW capacity at 4.2 K) cryocooler may be possible.