Design and Experimental Study of a Thermosiphon-Based Passive Heat Switch in an Integrated Adiabatic Demagnetization and Dilution Refrigerator System

This study proposes a thermal connection/isolation strategy to achieve less than 1 K pre-cooling required for Dilution Refrigerator (DR) operation in an integrated cryogenic system composed of a 4 K GM cryocooler, evaporative cooler, an Adiabatic Demagnetization Refrigerator(ADR), and a DR. We install a gravity-assisted thermosiphon-based passive heat switch (PHS) above the Gadolinium Gallium Garnet ADR stage, above the Still, and above the Mixing Chamber (MXC). An energy-balance model was used to estimate the attainable temperatures of the refrigeration chain, whereas convection and conduction models were used to design the PHS. In the OFF mode, considering conduction, the lower boundary warms from 2.2 K to 3.0 K in approximately 1.7 h, and the initial conduction heat inflow is estimated at ~ 319 µW—sufficiently low relative to the preparation time between cycles, indicating adequate thermal isolation. In the ON mode, a natural-convection correlation yields an effective heat transport of 11 mW, corresponding to a switching performance ratio about 36 times higher than in the OFF state. Experimentally, the Still pressure of 1.6 kPa was achieved, confirming the conceptual validity of the thermosiphon PHS. These results provide practical guidance for DR-interface design and robust pre-cooling in systems with small He-3 inventory, especially when combined with MXC pre-cooling pathway optimization and reduction of parasitic heat loads.