Design of a Miniature Adiabatic Demagnetization Refrigerator

Abstract

In order to provide continuous cooling at 50 mK for space or laboratory applications, we are designing a miniature adiabatic demagnetization refrigerator (MADR) anchored at a reservoir at 5 K. Continuous cooling is obtained by the use of several paramagnetic pills placed in series with heat switches. We aim at a fast cycling process (≈ 500 s) in order to reduce the size of the pills. For that purpose, we developed and tested magnetoresistive heat switches based on single crystals of tungsten. They provide good thermal performance for a very short switching time, depending only on the ramping speed of the magnet used. Magnets for cycling the pills and the heat switches are made from high current density NbTi wires. High permeability materials are used to shield these magnets, which is crucial to minimize stray magnetic fields. The field in the heat switch magnets is maximized using a holmium core. The magnets have been modelled using a numerical simulation. Small magnets(≈ 100 cm 3 ) with an operating field of 3 T have been designed and tested with a ramping time of less than 30 s. We have tested a single-stage MADR with a small pill, magnetoresistive heat switch and fast cycling time. These tests confirmed the feasibility of our design.