Up to now, all pulse tube refrigerators operating atthe liquid helium temperature range use ~4He as the working fluid.However, the lambda transition of ~4He is a barrier for reachingtemperatures below 2 K. Theoretical analysis in this paper showsthat, using ~3He, the temperature limit is below 2 K, and theefficiency of a 4 K pulse tube refrigerator can be improvedsignificantly. A three-stage pulse tube refrigerator is constructed.A compressor with input power of 4 kW and a rotary valve areused to generate the pressure oscillations. With ~4He, a minimumaverage temperature of 2.19 K was reached. Replacing ~4He by~3He, at the same valve settings and operating parameters, theminimum average temperature goes down to 1.87 K and thecooling power at 4.2 K is enhanced about 60%. After fine tuningof the valves, a minimum average temperature of 1.78 K wasobtained. This is the lowest temperature achieved by mechanical refrigerators.
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