Development of adiabatic demagnetization refrigerator for HUBS

The Hot Universe Baryon Surveyor (HUBS) is a satellite concept proposed in China to address the so-called “missing baryon problem”, which has serious implications on the formation and evolution of galaxies. At the heart of HUBS there is a high-resolution soft X Ray spectrometer based on transition-edge sensors operating below 100 mK. A cooling system is needed to provide such a low-temperature environment. It necessarily consists of a precooling stage and a cold stage. The cold stage could be enabled by an adiabatic demagnetization refrigerator (ADR). Because an ADR could be operated without gravity’s assist and has no refrigerant consumption, it is a good candidate for satellite mission like HUBS. For HUBS, the ADR will be employed to cool the detector down to below 100 mK from the precooling stage (at about 4K), which is enabled by mechanical pulse tube refrigerators. ADR for HUBS is now under development in our lab. The key technologies of building an ADR including growth of paramagnetic salt pill crystals, gas-gap heat switch are under development. A preliminary design of the ADR is completed and the design parameters are optimized. In this paper, we report on the status of the development.