The performance of GGG salt pill with heat transfer via 3He

Abstract

For the GGG salt pill in an adiabatic demagnetization refrigerator, its role is to absorb load heat in a single-stage ADR or magnetization heat from subsequent stages in a multi-stage ADR. Conventional GGG salt pill designs relying solely on pure thermal conduction cannot accommodate high heat flux. This paper presents a GGG salt pill utilizing ³He as the heat transfer medium, designed to enhance heat flux capacity and minimize eddy current heating. The design incorporates simulations of eddy current heating and calculations of ³He filling pressure. An ADR simulation model and experimental setup based on this salt pill are developed. The adiabatic demagnetization experiments achieve a minimum temperature of 693 mK under initial conditions of 4 T and 4.2 K. Simulation results reveal a non-uniform magnetic field distribution, where the magnetic flux density at the edge of the GGG axis is only 57 % of that at the centre. This renders approximately 10 % of the available entropy change unusable. Cooling capacity under two distinct heat load ranges (400–1000 μW and 60–120 μW) is evaluated through experiments, numerical simulations, and formula calculations using the GGG entropy. Furthermore, simulated temperature distributions during isothermal demagnetization demonstrate slight temperature gradients, confirming the superior heat transfer effect of ³He. This work provides theoretical and experimental foundations for the design and optimization of high-performance salt pills in ADR.