Development and Testing of a High Performance Cryogenic Thermal Switch

Abstract

This paper presents development details and performance test results of a high performance cryogenic thermal switch (CTSW) for coupling redundant cryocoolers to cryogenic components with minimal off-cooler parasitics. Because gas-gap, hydride-pumped CTSW designs have not reliably met performance goals of an "on" resistance less than 2 K/W and an "off" resistance greater than 1000 K/W, a simpler, more reliable device was sought. The device that was developed is based on the reversible and highly reliable physical process of differential thermal contraction/expansion of stainless steel relative to beryllium. The 250 gram Swales differential coefficient of thermal expansion (CTE) CTSW (or SDCC) has just 3 machined parts: two cylindrical beryllium discs (same diameter, different lengths) and a thin-walled stainless steel tube. In ground testing, the SDCC demonstrated an "off" resistance of 1400 K/W and an "on" resistance of 1.2-2.0 K/W over a cold end temperature range of 25-50 K and a warm end temperature range of 230-300 K. Other issues addressed in the paper include alternative gas-gap (non hydride-pumped) designs, the option of "cross-strapping" and CTSW reliability. Finally, an advanced SDCC design is also briefly described that can reduce CTSW mass to just 50 grams with virtually no (surface area-induced) parasitic heat input into the cryogenic system.