Development of Adiabatic Demagnetization Refrigerator (ADR) using Passive Heat Switch

Abstract

Magnetocaloric effect (MCE) is a phenomenon that can change the temperature or the entropy of materials by the alternating magnetic field. An adiabatic demagnetization refrigerator (ADR) is one way of the magnetic refrigeration system using MCE. The ADR is composed of a magnetic material, a magnet that produces alternating field on the magnetic material, and heat switches. The heat switch allows heat flow between the magnetic material and the heat sink or the cooling target by changing state (on/off) of the heat switch. Generally, active control of heat switch in ADR is needed and, therefore, the complexity of the control system is increased as a result. To solve this problem, this study investigates a novel adiabatic demagnetization refrigerator with passive heat switches: a superconductor whose thermal conductivity changes with the magnetic field and a thermosiphon whose heat transfer capability changes with temperature of the condenser. Pure lead wires are used as superconducting heat switch, and helium is used as a working fluid of the thermosiphon. A prediction model is developed by one dimensional numerical analysis, and feasibility of the designed refrigerator is confirmed. Experimental apparatus is constructed and the experiments are conducted based on the result of the prediction model. Warm end temperature of the refrigerator is maintained as 6 K by a two stage G-M (Gifford-McMahon) refrigerator. And the HTS (High Temperature Superconducting) magnet used in the experiments provides 0~3 T alternating magnetic field on the magnetic material. Experimental results show that the temperature of the thermosiphon is maintained at 4.4 K, which is lower than that of the warm end, during some part of the cycle period. This study can simplify the refrigeration system by operating all the heat switches of ADR passively. In addition, if the temperature of the warm end can be further lowered to 4 K, cold end temperature can be maintained at 2 K, so that the superfluid helium can be obtained.