Design and performance of a closed-cycle liquid helium precooling system for developing a large cooling power dilution refrigerator

Along with the rapid development of quantum computing, the demand for high cooling capacity dilution refrigerators has been increasing significantly. The large and cumbersome precooling system for the dilution stage, which dominates the system volume, the refrigeration efficiency, and the vibration, has become one of the main challenges of developing high-capacity dilution refrigerators. This paper presents a novel precooling method utilizing a closed-cycle liquid helium loop coupled with a group of 4 K cryocoolers. This closed-cycle liquid helium precooling system not only allows for split configuration, but also can significantly reduce the mechanical vibrations transmitted to the cryostat，thus relax the vibration requirements for cryocoolers. It incorporates discrete precooling stages at 77 K, 4.2 K, and 2 K temperatures sustained by liquid nitrogen (LN2), liquid helium (LHe), and superfluid helium (SHe), respectively, with zero helium consumption during operation, just like a conventional dry fridge. In the experiments carried out to prove the concept, four 1.5W@4 K Gifford-McMahon (GM) cryocoolers are positioned externally to the cryostat to reliquefy the 4He gas evaporated from the 4.2K and 2K stages during the precooling process. Subsequently, the condensed helium is recirculated into the cryostat. Experiments demonstrated a helium liquefaction capacity of 80 L/day, with transfer losses below 20 L/day. The cooling power of each stage can be adjusted independently, providing the flexibility needed to accommodate the complex installation constraints of quantum computing devices. Successful operation of a 500 μW@100 mK dilution unit installed in the cryostat proves the feasibility of the precooling system. Since the cryocoolers and the dilution unit could be facilely enlarged or increased by number, this precooling system is expected to provide a practical and flexible method for developing high-capacity dilution refrigerators.