Outgas analysis of mechanical cryocoolers for long lifetime. (December 2017) - Sato, Yoichi
Highlights: Outgas characteristic of the two-stage Stirling cooler was evaluated that the CO and CO2 outgas rates rise in the operating mode and the H2 O outgas rate is independent of operating the cooler. The contribution of cold head outgas rate relative to the entire two-stage Stirling cooler is about 10%. The active balancer has little effect on outgassing of CO and CO2 . Gas sampling and analysis of the continuous running compressor of two-stage Stirling cooler for 140 days shows a decrease in the CO2 outgas rate. Abstract: Mechanical cryocoolers for space applications are required to have high reliability to achieve long-term operation in orbit. ASTRO-H (Hitomi), the 6th Japanese X-ray astronomy mission, has a major scientific instrument onboard—the Soft X-ray Spectrometer (SXS) with several 20K-class two-stage Stirling (2ST) coolers and a 4K-class Joule Thomson (JT) cooler, which must operate for 3 years to ensure the lifetime of liquid helium as a cryogen for cooling of its detectors[1, 2] . Other astronomical missions such as SPICA[3, 4], LiteBIRD[5], and Athena[6] also have top requirements for these mechanical cryocoolers, including a 1K-class JT cooler to be operated for more than 3–5 years with no cryogen system. The reliability and lifetime of mechanical cryocoolers are generally understood to depend on (1) mechanical wear of the piston seal and valve seal, and (2) He working gas contaminated by impurity outgases, mainly H2 O and CO2 released from the materials in the components of the cryocoolers. The second factor could be critical relative to causing blockage in the JT heat exchanger plumbing and the JT orifice or resulting in blockage in the Stirling regenerator and thereby degrading its performance. Thus, reducing the potential for outgassing in the cryocooler design and fabrication process, and predicting the total amount of outgases in the cryocooler are very important to ensure cryocooler lifetime and cooling performance in orbit. This paper investigates the outgas analysis of the 2ST and the 1K/4K-JT coolers for achieving a long lifetime. First, gas analysis was conducted for the materials and components of the mechanical cryocoolers, focusing on non-metallic materials as impurity gas sources. Then gas analysis of the mechanical wear effect of the piston seal materials and linear ball bearings was investigated. Finally, outgassing from a fully assembled cryocooler was measured to evaluate whether the outgas reduction process works properly to meet the requirement levels.