Operation of SNIS arrays in a cryocooler

Avoiding liquid helium is now a worldwide issue, thus cryocooler operation is becoming essential for a wider use of superconductive electronics. Higher temperature operation of devices is advisable to reduce the refrigerator size and complexity, but arrays of Josephson junctions made with high temperature superconductors for voltage standard applications are not to date available. The SNIS (Superconductor-Normal metal-Insulator-Superconductor) junction technology developed at INRIM, based on low temperature superconductors, but capable of operation well above liquid helium temperature, allow operation in a compact cryocooled standard, providing a good compromise between device and refrigerator requirements. In this work, the behavior of SNIS devices cooled with a closed-cycle refrigerator has been investigated, both in DC and under RF irradiation. Issues related to thermal design of the apparatus to solve specific problems not faced with liquid coolants, like reduced cooling power and minimization of thermal gradients for uniform operation of the chip are discussed in detail.