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³He and ⁴He Sorption Refrigerators are subkelvin cryogenic refrigerators that use the reversible adsorption and desorption of helium 3 or helium 4 by a porous sorbent, commonly activated charcoal, to generate a thermally driven pumping action. During regeneration, the sorption pump is heated to release helium gas. The gas is precooled, condensed, and collected as liquid helium in an evaporator. When the pump is subsequently cooled, the charcoal adsorbs the returning helium vapor, reduces the vapor pressure above the liquid, and drives evaporative cooling of the cold stage. A ⁴He sorption stage generally provides cooling in the region below 1 K and may act as the precooling stage for a ³He circuit. A ³He sorption stage can reach approximately 0.2 K to 0.3 K under suitable precooling and heat load conditions. Systems may use a single ⁴He stage, a single ³He stage, or a multistage ⁴He and ³He configuration. Single shot systems require periodic regeneration, while continuous systems alternate multiple sorption modules to maintain uninterrupted cooling. The category includes the helium sorption refrigerator itself and integrated cryogenic systems containing a clearly identifiable ³He or ⁴He sorption stage. Dilution refrigerators, pulse tube cryocoolers, Gifford McMahon cryocoolers, and Joule Thomson coolers fall outside this category when no helium sorption refrigeration stage is present.
