With the development of condensed matter physics, space observation, and quantum technology in recent years, the demand for ultra-low temperature refrigeration has increased. The adiabatic demagnetization refrigerator (ADR) has the advantages of being unaffected by gravity, compact structure, and relatively low cost, which can meet the needs of space and ground applications.

In this paper, a 50 mK multi-stage ADR is designed and developed which comprises a GM-type pulse tube cryocooler for precooling and three ADR stages connected in series. For easy installation and maintenance, the three ADR stages are placed on a separate cold plate which is connected to the 4 K cold plate via copper columns. The Dy₃Ga₅O₁₂ (GGG) is employed as the refrigerant in the first stage, whereas CrK(SO₄)₂ · 12H₂O (CPA) is utilized for the second stage and third stage. To control heat transfer between stages and the 4 K heat sink, active gas-gap heat switch and passive gas-gap heat switch are developed, with the latter having a switching ratio over 1000.

The 4 T, 2 T and 1 T superconducting magnets are utilized in the 1st, 2nd and 3rd stage, respectively, and a numerical model is used to optimize the design of magnetic shielding. In addition, an ADR simulation model with a proportional-integral-derivative (PID) controller is constructed to assist in tuning the controller parameters in the experiment. The lowest temperature achieved in the experiment is 38 mK, with a temperature fluctuation of 10.6 μK. The durations of different cooling power (1, 2 and 3 μW) at 100 mK are also measured. It is calculated that the no-load maintenance time is about 4.3 h, the leakage heat power is about 4.5 μW, and the total cooling capacity is about 71 mJ. This refrigerator is the first Chinese multi-stage ADR that can reach a temperature under 50 mK, which lays an important foundation for subsequent research on continuous adiabatic demagnetization refrigeration.