With the development of space observation, quantum technology and other frontier scientific research fields, the demand for ultra-low temperature refrigeration in sub-Kelvin region is increasing. Compared with dilution refrigeration and sorption refrigeration, adiabatic demagnetization refrigeration (ADR) has the outstanding advantages of high efficiency, compact, gravity independence and accessibility of working materials, which make ADR a promising technology for sub-Kelvin cooling.

A single-stage ultro-low temperature adiabatic demagnetization refrigerator is designed and developed. The thermodynamic principle and quantitative analysis are presented, from the macroscopic and microcosmic view, and operating results show good agreement with the theoretical value.

This refrigerator is precooled to 3 K by a GM-type refrigerator, with 252 g gadolinium gallium garnet (monocrystalline) used as a working medium. The maximum magnetic field of 4 T is provided by a superconducting coil. Flexible heat connection is used between the pre-cooler and ADR, so heat generated by vibration decreases. From (3 K, 4 T), the lowest temperature can reach 0.47 K by adiabatic demagnetization, which is consistent with the result drawn from the entropy data. In a constant-temperature-control mode, the demagnetization rate is controlled by a feedback loop, so the temperature can be held in the presence of a load. A cooling capacity of 2.7 J is provided at 1 K, with temperature fluctuation being lower than 0.5 mK, and the second thermodynamic efficiency of adiabatic demagnetization refrigeration is 57%. at 0.8 K, the cooling capacity is 1.2 J.

Future work on improving the performance includes the improving of the on-off ratio of the heat switch, so, the irreversible loss caused by the heat transfer temperature difference in conduction state can be reduced. Improving the heat transfer performance of the salt pill, the heat can be ejected in a shorter period.

This refrigerating machine is the first Chinese adiabatic demagnetization refrigeration system that can be operated in circulation, which is expected to be the 1^st stage of a three-stage adiabatic demagnetization refrigeration system in a 50 mK temperature zone. This study lays a foundation for further developing continuous multistage adiabatic demagnetization refrigeration at ultra-low temperature.