Development status of the mechanical cryocoolers for the Soft X-ray Spectrometer on board Astro-H

Abstract

Astro-H is the Japanese X-ray astronomy satellite to be launched in 2015. The Soft X-ray Spectrometer (SXS) on board Astro-H is a high energy resolution spectrometer utilizing an X-ray micro-calorimeter array, which is operated at 50mK by the ADR with the 30liter superfluid liquid helium. The mechanical cryocoolers, 4K-class Joule Thomson (JT) cooler and 20K-class double-staged Stirling (2ST) cooler, are key components of the SXS cooling system to extend the lifetime of LHe cryogen beyond 3years as required. Higher reliability was therefore investigated with higher cooling capability based on the heritage of existing cryocoolers. As the task of assessing further reliability dealt with the pipe-choking phenomena by contaminant solidification of the on-orbit SMILES JT cryocooler, outgassing from materials and component parts used in the cryocoolers was measured quantitatively to verify the suppression of carbon dioxide gas by their storage process and predict the total accumulated carbon dioxide for long-term operation. A continuous running test to verify lifetime using the engineering model (EM) of the 4K-JT cooler is underway, having operated for a total of 720days as of June 2013 and showing no remarkable change in cooling performance. During the current development phase, prototype models (PM) of the cryocoolers were installed to the test SXS dewar (EM) to verify the overall cooling performance from room temperature to 50mK. During the EM dewar test, the requirement to reduce the transmitted vibration from the 2ST cooler compressor was recognized as mitigating the thermal instability of the SXS microcalorimeter at 50mK.