Simulation study on foil thermal bus in the salt pill of ultra-low temperature adiabatic demagnetization refrigerator

The adiabatic demagnetization refrigerator holds significant application value in deep space exploration and advanced physical research. However, hundreds or thousands of metal wires are used in the traditional hydrated paramagnetic salt pill, leading to high labor costs and manufacturing complexity. In this paper, an innovative foil structure is proposed to replace the wire thermal bus structure, and a simulation study is conducted to evaluate its performance. The cooling performance of the salt pill with wire and foil structures is compared within the volume fraction of CPA f _CPA range of 84%–98%. The holding time of the salt pill with the foil structure is 6%, 16%, and 36% shorter than that of the salt pill with the wire structure at f _CPA of 84%, 92%, and 98%, respectively. The foil structure is then subjected to perforation treatment to further optimize its performance. With the same f _CPA of 92%, the holding time of the salt pill with perforated foils reaches its maximum and is only 4.8% shorter than that of the salt pill with wires. This corresponds to 14 perforated foils (a spacing of 0.8 mm and a ratio of perforation width to spacing of 0.7) and 325 wires, respectively. Finally, the effect of the demagnetization rate on the cooling performance of the salt pill with different structures is investigated within the range from 4 mT/s to 40 mT/s. With the demagnetization rate of 4 mT/s, the temperatures of the salt pill with the foil, perforated foil and wire structures are 94.5 mK, 81 mK and 75.4 mK, respectively. The results indicate that the perforated foil structure exhibits a much closer performance to the wire structure.