Experimental investigation on a sub-Kelvin adiabatic demagnetization refrigerator (ADR) integrated with a 4He sorption cooler

This paper experimentally investigates a sub-Kelvin adiabatic demagnetization refrigerator (ADR) with an integrated sorption pump. The developed ADR consists of a high-temperature superconducting (HTS) magnet, in which a single crystalline gadolinium gallium garnet (GGG) with a volume of 10.4 cc is installed, and the integrated ⁴He sorption cooler. Both the magnet and the sorption cooler reject heat to a commercial 4 K two-stage Gifford-McMahon (GM) cooler. The sorption cooler provides the cooling effect through evaporation of liquid helium, which occurs inside a helium pot and is driven by a sorption pump filled with activated charcoal. In order to integrate the sorption cooler with the ADR, the GGG serves as an evaporator of the helium pot. The cooling performance of the ADR is evaluated through a continuous cyclic operation. During the regeneration process, the GGG is magnetized, and the helium pot is filled with helium desorbed from the sorption pump. Once the 4.1  T-magnetized GGG and the liquid helium in the helium pot are sufficiently cooled by the GM cooler, the sorption cooling process is initiated to precool the GGG below 2  K. Finally, the GGG is demagnetized by discharging the magnet through an external dump resistor, achieving the lowest temperature of 0.29 K. The maximum cooling capacity of the ADR at 0.8 K is estimated to be 1.35 J/cycle. Detailed results and findings are discussed in the remainder of the paper.