Current Leads and Optimized Thermal Packaging for Superconducting Systems on Multistage Cryocoolers

Packaging of a superconducting electronic system on a compact multistage cryocooler requires careful management of thermal loads from input and output leads, in order not to exceed the heat lift capacity of the various stages of the cooler. In particular, RSFQ systems typically require a large total bias current or greater. A general analysis of resistive wires shows that the tradeoff between heat flow and Joule heating yields a minimum heat load from optimized bias leads on a low- stage, given by , where is the thermalization temperature of the leads on the previous (hotter) stage. This is independent of the material, number, and geometry of the leads, as long as the total lead resistance is optimized. A similar tradeoff between heat flow and signal attenuation can be applied to the optimization of high-frequency input/output lines. Superconducting leads are not subject to these limitations, and can result in further reduction in heat load. Design examples are presented for an RSFQ-based radio receiver on either a two-stage or a four-stage cooler.