Abstract
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 cryostat and 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 “helium mode,” the simpler of the two modes, is used while liquid helium is present and uses all five cryocoolers and 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 helium is depleted, the first stage ADR operation is unchanged, the second stage is repurposed to control the empty helium tank 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.
Highlights
The Astro-H (Hitomi) mission[1,2] hosted the Soft X-ray Spectrometer (SXS) instrument,[3,4] a high-resolution cryogenic spectrometer featuring an x-ray microcalorimeter detector array operated at 50 mK
The adiabatic demagnetization refrigerators (ADRs) and detector assembly (DA) are integrated as a single assembly called the calorimeter spectrometer insert (CSI), which is mounted to the top end of the helium tank to position the detectors at the focus of the instrument’s grazing-incidence x-ray mirror assembly
The cryogenic system is operated in the helium mode where the first two ADR stages (S1 and S2) cool and stabilize the calorimeter thermal sink (CTS) at 50 mK, recycling periodically in single-shot manner using the liquid helium as a heat sink
Summary
The Astro-H (Hitomi) mission[1,2] hosted the Soft X-ray Spectrometer (SXS) instrument,[3,4] a high-resolution cryogenic spectrometer featuring an x-ray microcalorimeter detector array operated at 50 mK. The cryogen-free mode was not operated in space, but it was qualified for flight before launch and the necessary hardware functions were all verified in orbit. The development of this hybrid cooling system with independent operational modes has yielded important lessons, which are shared in this paper to benefit future missions employing exclusively cryogen-free or hybrid cryogen/cryogen-free cooling systems. Sneiderman et al.: Cryogen-free operation of the Soft X-ray Spectrometer instrument
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