Refrigeration of separate, user-supplied payloads with Normal–Insulator–Superconductor tunnel junctions

Abstract

Normal metal–Insulator–Superconductor (NIS) tunnel junctions can be used to selectively remove the hottest electrons in the normal metal, thereby causing it to cool. NIS tunnel junctions have already been used to cool lithographically integrated payloads [1], but this requires integration of two disparate fabrication processes. To increase the flexibility of NIS refrigerators, we have designed a stage cooler based on NIS tunnel junctions that will be able to cool arbitrary, user-supplied payloads from 300mK to 100mK. This stage cooler can be backed by a helium-3 refrigerator to provide a lightweight and simple means of reaching 100mK in space applications. In this paper, we describe the design of our stage cooler and present calculations of the cooling power and time required to reach 100mK.