The refrigeration system of the Hot Universe Baryon Surveyor (HUBS) mission intends to use the FAA paramagnetic salt adiabatic demagnetization refrigerator (ADR) to obtain temperatures below 100 mK. In order to obtain the 1 K pre-cooling temperature required by the FAA stage, one option is to use a GGG paramagnetic salt ADR, and the other option is to use a helium sorption cooler. A helium sorption cooler and the corresponding helium gas gap heat switch have been developed, and a cooling temperature of 800 mK has been obtained. This paper gives a detailed introduction to the prototype's structure, cooling performance, existing problems, and the next optimization measures.
The Hot Universe Baryon Surveyor (HUBS) mission requires a refrigeration system with temperatures below 100 mK to meet the high-resolution detection requirements of its superconducting transition edge sensor. The refrigeration scheme is to use a 4 K mechanical cryocooler as the pre-cooling stage and then use adiabatic demagnetization refrigerators (ADR) to obtain mK temperatures. One option for the pre-cooling stage is to use a pulse tube cryocooler. At present, a thermalcoupled and gas-coupled composite prototype based on helium-4 as the working gas has been successfully developed, a no-load temperature of 3.1 K, and a maximum cooling capacity of 22.0 mW at 4.2 K has been obtained, which can barely meet the demand. The calculation results show that the use of helium-3 instead of helium-4 as the working gas of the gas-coupled second and third stage is expected to further increase the cooling capacity to 53.1mW/4.2K, but 53 standard liters of helium-3 needs to be charged at room temperature. In order to reduce the amount of helium-3, a thermal-coupled three-stage pulse tube cryocooler is further designed. When the first and second compressors and their cold fingers use helium-4, while the third compressor and its cold finger use helium-3 as the working gas, the calculation results show that a cooling capacity of 57.5 mW/4.2 K can be obtained, and the amount of helium-3 that needs to be charged at room temperature is 11 standard liters, which effectively reduces the cost.
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