Experimental analysis of condensation-pump dilution refrigerators

Abstract

<sec>Subkelvin refrigeration is necessary for many frontier research fields such as condensed-matter physics, astronomical observation, and quantum computing. Dilution refrigeration utilizes the entropy increase of ³He atoms when they flow from the concentrated phase to dilute phase to provide cooling, and has the advantages of continuous operation, large cooling power and no electromagnetic interference. It is the most widely used method among subkelvin refrigeration at present. For research scenarios requiring small cooling powers, the condensation-pump dilution refrigerator utilizes a condensation pump to achieve cold cycle of ³He, with no need of complex ambient pump systems or gas circuits, and has become a new research topic because of its compact structure, convenient operation and low cost.</sec><sec>A condensation-pump dilution refrigerator is built and investigated in this work. The refrigerator consists of a mixing chamber, a continuous tube-in-tube heat exchanger, a still, and a condensation pump, and it is precooled by a GM-type pulse tube cryocooler below 4 K and an adsorption refrigerator below 400 mK. The ³He evaporates from the still, condenses in the condensation pump and provides cooling in the mixing chamber after being precooled in the heat exchanger.</sec><sec>By means of the multi-temperature measuring system, the cold cycle of the dilution refrigerator can be summarized as three stages: precooling by the adsorption refrigerator, cycle start-up, and continuous dilution cooling. The operating characteristics of the system are analyzed. The experiments showed that the lowest no-load temperature reached 108 mK when the condenser temperature was 378 mK. Meanwhile, the temperature oscillation appeared, and the possible reasons are analyzed. In the future, the system performance will be improved by 1) adjusting the spiral mode and position of the continuous tube-in-tube heat exchanger and 2) increasing the heat transfer area between the cold plate and the fluid in the mixing chamber to reduce the fluid-solid temperature difference.</sec><sec>The refrigerator introduced in this work can easily expand many existing cryogenic platforms working at higher temperatures, and effectively support developments of high-end equipment.</sec>