Optimization research of salt pill in an ultra-low temperature adiabatic demagnetization refrigerator

Adiabatic demagnetization refrigeration is a crucial technology for cooling bolometric infrared detectors and cryogenic X-ray detectors in the sub-Kelvin temperature range (below 1 K). To improve the cooling performance and reduce the total mass of a four-stage continuous adiabatic demagnetization refrigerator (ADR) (operating between 4 K and 50 mK) for future space missions, a two-dimensional transient ADR model has been developed. Through this model, the effects of salt pill's aspect ratio on the cooling performance (holding-time) and total mass of the ADR have been studied. As the salt pill's aspect ratio increased, the holding-time of ADR gradually decreased, while the total mass of the ADR firstly increased and then decreased. The optimal aspect ratios of stage 1 ∼ stage 4 ADRs have been 2.4, 2.2, 1.3 and 1.4, with efficiencies of 56.20 %, 82.78 %, 75.45 %, and 88.36 % respectively. Additionally, the effects of the thermal bus diameter and the volume ratio of magnetocaloric material in the salt pill have also been investigated. With a thermal bus diameter of 0.2 mm, the maximum holding-times achievable for stage 2, stage 3, and stage 4 ADRs have been 1711 s, 1832 s, and 4050 s, and the corresponding optimal number of thermal buses have been 459, 1957, and 505 respectively.