Development of the sorption-integrated adiabatic demagnetization refrigerator (ADR) without active heat switches

Abstract

This study reports on the test results of a proof of concept sorption-integrated adiabatic demagnetization refrigerator (ADR) for sub-Kelvin cooling. The configuration of the cooler is primarily simplified by eliminating heat switches which require active control. The whole refrigerator apparatus is composed of the high temperature superconducting (HTS) magnet to magnetize the single crystal Gadolinium Gallium Garnet (GGG), and the 4He sorption cooler. Both the sorption cooler and the magnet are cooled by a 4 K two-stage Gifford-McMahon (G–M) cooler. The sorption cooler consists of a sorption pump filled with activated carbon of 13.9 g and a gravitiy-assisted-4He thermosiphon with volume of 5.0 cm3. The pressure inside the thermosiphon is controlled by changing the adsorption capacity of the sorption pump. The performance of the developed cooler is preliminarily evaluated by a single-shot test. The sorption cooler precools the 3 T-magnetized GGG from 4.40 K to 1.94 K by evaporating the liquid helium contained in the thermosiphon. When helium inside the thermosiphon is evacuated by the sorption pump, the magnetized GGG is thermally disconnected from the 4 K heat sink. Subsequently, the GGG is adiabatically demagnetized by discharging the HTS magnet and reaches temperature of 0.55 K. However, after demagnetization, the gas helium inside the cooling system is condensed by the GGG, consequently reducing its cooling capacity. Detailed experimental results are presented and discussed in the remainder of the paper.