Progress on a 4 K to 10 K Continuously Operating Adiabatic Demagnetization Refrigerator

An adiabatic demagnetization refrigerator (ADR) is under development for use in cooling relatively large loads (10–100 mW) at 4 K and rejecting that heat to a cryocooler operating at 10 K. ADRs can operate in this temperature range with an efficiency of 75% of Carnot, saving as much as 2/3 of the required overall input power. In addition this ADR can provide cooling down to 0.4 K. The ADR magnet consists of 8 short coils wired in series and arranged in a toroid to provide self‐shielding of its magnetic field. Eliminating passive or active shields saves 30% of the mass of the system. The average field is 3 Tesla using 5 amps. In the first model the coils are wound with ordinary NbTi superconducting wire and operated at 4 K. A second version will then use fine Nb₃Sn wire to provide complete 10 K operation. As a refrigerant for this temperature range we are using readily available gadolinium gallium garnet (GGG) crystals, which provide suitable performance. In the future we will switch to either GdLiF₄ or GdF₃, which have 13% or 30%, respectively, more cooling power per volume than GGG. We use gas gap heat switches to alternately connect the toroid to the cold load and the warm heat sink. A small continuous stage maintains the cold end at 4 K while the main toroid is recycled. Data on toroid , magnet performance with respect to central field vs. current and fringing field vs. current are given. Data on the transient heating due to hysteresis losses within the superconductor are also presented.