Cryogen-free operation of the Soft X-ray Spectrometer instrument

The Soft X-ray Spectrometer (SXS) is the first space-based instrument to implement operational redundancy of a sub-Kelvin cooling system. Its cooling system includes a superfluid helium cryostatand five cryocoolers, provided by Japan Aerospace Exploration Agency, and three adiabatic demagnetization refrigerators (ADRs) with four active heat switches, provided by NASA. These elements are configured in one of two ways to control the heat sink of the x-ray microcalorimeter detectors at 50 mK. The “ heliummode,” the simpler of the two modes, is used while liquid heliumis present and uses all five cryocoolersand two ADRs. The first two ADR stages operate together and reject their heat directly to the liquid at ∼1.1  K. In the “cryogen-free mode,” for operation after the heliumis depleted, the first stage ADR operation is unchanged, the second stage is repurposedto control the empty heliumtank at ∼1.5  K, and the third stage transfers heat from the 1.5-K stage to the 4.5-K interface of the Joule–Thomson cooler. The development and verification details of this capability are presented within this paper and offer valuable insights into the challenges, successes, and lessons that can benefit other missions, particularly those employing cryogen-free or hybrid cooling systems.