On-orbit performance of the adiabatic demagnetization refrigerator on XRISM

The X-ray Imaging and Spectroscopy Mission (XRISM) observatory was launched on Sep 7, 2023 from Tanegashima Space Center in Japan. Resolve, one its two instruments, performs high-resolution spectroscopy in the soft x-ray band (0.2-13 keV) using a 6x6 microcalorimeter array. The array is cooled to 50 mK by a 3-stage ADR, which is linked to both a liquid helium dewar (at <1.2 K) and a Joule-Thomson (JT) cryocooler operating at <4.5 K. While liquid helium is present, two of the ADR stages provide detector cooling at 50 mK and auxiliary cooling to 0.5 K to intercept some parasitic heat loads, while rejecting waste heat to the helium. Once the helium is exhausted, Resolve enters a cryogen-free mode in which all three ADR stages are operated to provide detector cooling at 50 mK and to continuously cool the helium tank to 1.4 K. In this mode, waste heat is rejected to the JT cryocooler. Cryogen-free operation can be sustained as long as the JT and other cryocoolers remain fully operational. At launch, the helium tank contained approximately 35.4 liters of liquid. Within a few days, the helium cooled below 1.15 K, with an estimated 35.0 liters remaining. With an expected time average heat load of 0.68 mW, the helium lifetime was projected to exceed 4 years, but measurements of He volume on orbit suggest a significantly longer lifetime. Details of the ADR’s design and performance are presented.