Development of the Second-Generation Double-Stage Stirling Cryocooler for Innovative Space Missions

Mechanical cryocooler for space application is efficient way to cool down the optical detector, telescope and thermal shields to cryogenic temperature below about 100 K in the aspect of mass and size. The 20 K-class double-stage Stirling cycle cryocooler with cooling power of 200 mW at 20 K and lifetime of 1.5 years was originally developed for a cooling component of the Japanese IR telescope satellite AKARI launched in 2006. Based on this AKARI cryocooler, improvements with higher cooling performance and reliability with 1) the optimized 8-mm diameter displacer at second stage, 2) the flexure bearings for displacer supporting and 3) selection of low-outgassing materials and optimal baking process were investigated to develop the second-generation double-stage Stirling cryocooler for application to the next innovative astronomy mission such as ASTRO-H/SXS (2015) and SPICA (2022). The verification tests by using the EM (Engineering Model) were performed and maximum cooling power of 17.6 K with 200 mW at the 2nd cold stage and 96.1 K with 1000 mW at the 1st cold stage was obtained with margin. Mechanical performance test was also carried out and proved tolerability for mechanical environment of qualification level of ASTRO-H/SXS. Continuous running to verify specified lifetime of over 3 years is still under testing and 13545 hours (~ 560 days) in total has just achieved as of August 2012.