Detailed study of a condensation-driven dilution refrigerator reaching below 50 mK

Abstract

The condensation-driven dilution refrigerator employs a condensation pump to accomplish the circulation of ³He, offering advantages such as compactness, lightweight, and low cost. In our previous work, we constructed a prototype with a lowest temperature of around 60 mK, yet the system performance needs further improvement. In this paper, we present our latest advances. By adding a step heat exchanger made from sintered silver powder and reducing heat leakage in the system, a lowest temperature of 45 mK, a maximum cooling power of 8.4 μW @100 mK, and a corresponding thermodynamic degree of perfection of 11.28 % is achieved in the experiments. A comprehensive numerical model of the condensation-driven dilution refrigerator has been developed. The results of experiments and simulations are compared and analyzed, revealing the influence of key parameters on the system performance. Firstly, the efficiency is significantly related to the temperature of the mixing chamber, still, and condensation pump. The entropy generation analysis is conducted, and it is found that the contribution of the still to the total entropy production always remains predominant. Secondly, to circulate the ³He, the vapor pressure above the still should be greater than the saturated vapor pressure corresponding to the condensation pump temperature. This imposes a requirement on both the still and condensation pump temperatures to activate the dilution cycle with a certain mixing chamber temperature. Finally, the comparison between the experiments and simulations indicates a maximum cooling power exists at a certain high still temperature as the other conditions are kept the same. With the help of the simulation, the internal dynamic and thermodynamic characteristics during operation have been clarified, which can guide further optimizations.