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A 4 K pulse tube cryocooler for the HUBS mission

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.