A pulse tube refrigerator below 2 K

Up to now, all pulse tube refrigerators operating at the liquid helium temp erature range use He-4 as the working fluid. However, the lambda transition of He-4 is a barrier for reaching temperatures below 2 K. Theoretical anal ysis in this paper shows that, using He-3, the temperature limit is below 2 K, and the efficiency of a 4 K pulse tube refrigerator can be improved sig nificantly. A three stage pulse tube refrigerator is constructed. A compres sor with input power of 4 kW and a rotary valve are used to generate the pr essure oscillations. With He-4, a minimum average temperature of 2.19 K was reached. Replacing He-4 by He-3, at the same valve settings and operating parameters, the minimum average temperature goes down to 1.87 K and the coo ling power at 4.2 K is enhanced about 60%. After fine tuning of the valves, a minimum average temperature of 1.78 K was obtained. This is the lowest t emperature achieved by mechanical refrigerators. (C) 1999 Elsevier Science Ltd. All rights reserved.