4He-circulating dilution refrigerator

Investigations are reported on a dilution refrigerator through which superfluid 4He is circulated. First, discussions are given about the effectiveness of a counterflow tube heat exchanger in which the phase-separated concentrated and dilute phases directly contact each other. Then experimental results of the cooling behavior of their 4He-circulating dilution refrigerator are presented. It is found that the cooling characteristic of the machine changes abruptly when the 4He-circulating rate exceeds a certain critical value. In the circulation rate region above this critical value, a lowest temperature of 3.4 mK has been attained. On the basis of various experimental results such as the effect of heat input, effect of tilting the cryostat, and visual observation with a machine made of glass, discussions are given of the flow state of the descending cold dilute mixture in the counterflow tube