Temperature Stability of SNIS Josephson Arrays Between 4.2 K and Critical Temperature in Cryocooler

Cryogen-free operation is essential to expand applications of superconductivity, is unavoidable in some special cases and, in perspective, provides a solution to the expected shortages of liquid helium. In electrical metrology applications, high-temperature operation to reduce the refrigerator size and complexity is not yet possible since arrays of Josephson junctions for voltage standard applications made with high-temperature superconductors are not presently available. The superconductor-normal metal-insulator-superconductor (SNIS) technology developed at INRIM uses low-temperature superconductors but allows operation at temperatures close to 6 K. Thus, it is interesting for the cryocooler operation of a programmable voltage standard. We report on measurements of SNIS devices cooled with a closed-cycle refrigerator and in liquid helium to test the electrical behavior and its dependence on specific fabrication parameters that can be used to optimize the temperature stability.