Performance test results of a helium Dewar for the Resolve instrument onboard the XRISM.

• We developed a hybrid cooling system by using liquid helium, mechanical cryocoolers. • We verified that the Dewar can provide a thermal interface of less than 1.2 K. • The lifetime of the liquid helium in orbit was determined to be over 3 years in the nominal case. The X-ray imaging and spectroscopy mission (XRISM) is an X-ray astronomy satellite developed by the Japan Aerospace Exploration Agency (JAXA) and the National Aeronautics and Space Administration (NASA) to explore the evolution of the universe and its physical phenomena as the replacement for ASTRO-H. One of the primary scientific instruments of the XRISM is the Resolve, which utilizes an X-ray microcalorimeter array. This detector array is required to be cooled down to 50 mK using a complex cryogenic system with a multistage adiabatic demagnetization refrigerator (ADR) developed by NASA and a cryogenic system developed by Sumitomo Heavy Industries, Ltd. (SHI). The cryogenic system is required to cool the heat sink of the ADR to a temperature below 1.5 K in orbit for at least 3 years. To meet the specific requirements, we developed a hybrid cryogenic system consisting of a liquid helium tank, a 4K Joule–Thomson cooler, and two two-stage Stirling coolers. As a result, the specific requirements were verified by ground tests. This paper describes the design, including the major changes from ASTRO-H, fabrication, and results of the ground tests of the helium Dewar of the Resolve instrument. [ABSTRACT FROM AUTHOR]