Operation of an ADR using helium exchange gas as a substitute for a failed heat switch

• A 3-stage ADR was built for the Soft X-ray Spectrometer instrument on Astro-H, scheduled for launch in late 2015.
• The failure of one of the ADR’s four heat switches prevented the ADR from being recycled and cooling the detectors to 50 mK.
• Testing of the instrument was critical to assess compatibilities between the ADR, detectors and cryocoolers.
• A process for using exchange gas in the guard vacuum to allow the ADR to be cycled was implemented.
• The process was successful, allowing the ADR and detectors to be cooled for periods of 10–12 h at 50–60 mK.

The Soft X-ray Spectrometer (SXS) is one of four instruments on the Japanese Astro-H mission, which is currently planned for launch in late 2015. The SXS will perform imaging spectroscopy in the soft X-ray band (0.3–12 keV) using a 6 × 6 pixel array of microcalorimeters cooled to 50 mK. The detectors are cooled by a 3-stage adiabatic demagnetization refrigerator (ADR) that rejects heat to either a superfluid helium tank (at 1.2 K) or to a 4.5 K Joule–Thomson (JT) cryocooler. Four gas-gap heat switches are used in the assembly to manage heat flow between the ADR stages and the heat sinks. The engineering model (EM) ADR was assembled and performance tested at NASA/GSFC in November 2011, and subsequently installed in the EM dewar at Sumitomo Heavy Industries, Japan. During the first cooldown in July 2012, a failure of the heat switch that linked the two colder stages of the ADR to the helium tank was observed. Operation of the ADR requires some mechanism for thermally linking the salt pills to the heat sink, and then thermally isolating them. With the failed heat switch unable to perform this function, an alternate plan was devised which used carefully controlled amounts of exchange gas in the dewar’s guard vacuum to facilitate heat exchange. The process was successfully demonstrated in November 2012, allowing the ADR to cool the detectors to 50 mK for hold times in excess of 10 h. This paper describes the exchange-gas-assisted recycling process, and the strategies used to avoid helium contamination of the detectors at low temperature.