Performance Evaluation of Suppression Structure for Superfluid Helium Film Flow at the Exit of Dilution Refrigerator Still Chamber

Superfluid helium (⁴He) exhibits special phenomena such as anomalous thermal conduction, fountain, and crawling film effects at sub-Kelvin temperatures owing to its quantum properties. Dilution refrigerators utilize ³He as the working fluid for circulation. ³He produces cooling effects as it passes through the phase interface in the mixing chamber. It is then pumped out by a room-temperature pump after evaporation in the still chamber. The superfluid helium inside the still chamber forms a liquid film on the wall and flows toward higher temperatures. This results in significant heat leakage owing to its anomalous thermal conductivity. The superfluid helium liquid film also affects the purity of ³He during the circulation process, thereby influencing the cooling capacity of the dilution refrigerator. The “knife-edge” and “small orifice” structures are the primary means for suppressing superfluid helium films. This study analyzed the impact of the purity of the working fluid ³He on the cooling capacity of the dilution refrigerator and the limitation placed on the system's ³He circulation flow rate by the size of the “small orifice”. Experimental tests were conducted to measure the mass flow rate of the superfluid helium crawling film at 0.95 K with two types of suppression structures: “small orifice” and “knife-edge with small orifice”. The use of “knife-edge with small orifice” resulted in a 35.6% improvement in the suppression effect of the superfluid helium crawling film compared with the use of “small orifice” alone.