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

•A 2D transient ADR model in the temperature range of sub-Kelvin has been developed.•The influences of salt pill on the cooling performance and mass of ADR were studied.•The thermal bus diameter and the ratio of magnetocaloric material were optimized.•The optimal aspect ratios of stages S1 ∼ S4 have been 2.4, 2.2, 1.3 and 1.4. 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.