Thermal design of the XRS helium cryostat

The required lifetime for the Astro-E X-Ray Spectrometer (XRS) is 2 years, with a goal of 2.5 years. To meet this requirement, significant advances in state-of-the-art long-life cryogenic systems are required. The XRS system is a hybrid neon/helium system with a final stage of cooling provided by an adiabatic demagnetization refrigerator. The thermal design of the helium cryostat is described in this paper. To achieve a lifetime of 2.5 years with a helium volume of approximately 20 litres, the heat load on the helium must be of the order of 800 mu W or less. The expected lifetime and sensitivity of the lifetime to changes in the design or external heat loads is modelled. Results of preliminary thermal conductivity measurements are presented and future tests are identified. A study of heat loads that were small enough to be neglected in previous designs of long-life cryogenic systems was undertaken. A summary of the findings is presented. (C) 1996 Elsevier Science Limited