Review and history of sub-Kelvin 3He sorption cooling systems: From basic mechanisms to applications

Sub-Kelvin technologies are essential for applications such as space detection, quantum computing, and condensed-matter physics. Among traditional sub-Kelvin technologies, including sorption cooling, adiabatic demagnetization refrigeration, and dilution refrigeration, sorption cooling shows unique competitiveness in space environments for its vibration-free operation, compactness, and absence of electromagnetic interference. Based on the principle of pressure reduction evaporation cooling, ³He sorption coolers achieve temperatures around 300 mK, surpassing ⁴He sorption coolers (∼800 mK) due to higher vapour pressures and the absence of superfluid constraints. ³He sorption systems, initially developed in the 1960s, remain crucial for missions requiring reliability and simplicity. Advancements include modular and multi-stage designs, extending operating temperatures to 250 mK. Continuous systems using two sorption pumps mitigate intermittency, enabling stable cooling for weeks. Space applications face micro-gravity challenges, which are addressed by using porous matrices that leverage capillary forces to confine liquid ³He. Successful implementations include the Herschel satellite, which achieved 290 mK with 10 μW cooling power. Hybrid coolers, which integrate sorption coolers with ADR or DR, achieve ultra-low temperatures (∼50 mK), essential for missions like SPICA and ATHENA. Current research is focused on gravity-independent designs, mechanical precooling to eliminate liquid helium dependency, and enhancing cryochains from 300 K to 50 mK. This paper reviews ³He sorption cooler principles, technological evolution from cryostats to modular systems, space adaptations, and hybrid architectures. Key challenges related to non-equilibrium adsorption and micro-gravity fluid dynamics are discussed.