Thermal performance of the XRS helium cryostat

The Astro-E/X-Ray Spectrometer (XRS) is required to operate for 2 years on-orbit with approximately 20 liters of superfluid helium. The total heat load on the helium must be less than 1 mW, which is significantly smaller than any previous spaceborne helium system. Consequently, the XRS cryostat incorporates several new techniques that have not been used before in spaceborne cryogenic systems, including high temperature superconducting leads, a film flow suppressor in the vent line, and kevlar suspension systems. The thermal design of the cryostat is described and results of the ground tests of both an engineering model cryostat and the flight cryostat are presented. Tests of the flight cryostat are incomplete at the writing of this paper, so only preliminary results are given.