A Robust Cooling Platform for NIS Junction Refrigeration and sub-Kelvin Cryogenic Systems

Recent advances in Normal metal–insulator-superconductor (NIS) tunnel junctions (Clark et al. Appl Phys Lett 86: 173508, 2005 , Appl Phys Lett 84: 4, 2004 ) have proven these devices to be a viable technology for sub-Kelvin refrigeration. NIS junction coolers, coupled to a separate cold stage, provide a flexible platform for cooling a wide range of user-supplied payloads. Recently, a stage was cooled from 290 to 256 mK (Lowell et al. Appl Phys Lett 102: 082601 2013 ), but further mechanical and electrical improvements are necessary for the stage to reach its full potential. We have designed and built a new Kevlar suspended cooling platform for NIS junction refrigeration that is both lightweight and well thermally isolated; the calculated parasitic loading is < 300 pW from 300 to 100 mK. The platform is structurally rigid with a measured deflection of 25 μ m under a 2.5 kg load and has an integrated mechanical heat switch driven by a superconducting stepper motor with thermal conductivity G = 4.5 × 10 - 7 W/K at 300 mK. An integrated radiation shield limits thermal loading and a modular platform accommodates enough junctions to provide nanowatts of continuous cooling power. The compact stage size of 7.6 cm × 8.6 cm × 4.8 cm and overall radiation shield size of 8.9 cm × 10.0 cm × 7.0 cm along with minimal electrical power requirements allow easy integration into a range of cryostats. We present the design, construction, and performance of this cooling platform as well as projections for coupling to arrays of NIS junctions and other future applications.