Abstract
We report on the design and test of a three-stage refrigerator, consisting of a liquid nitrogen bath with heat exchangers and two subsequent pulse tube stages. The liquid nitrogen bath serves for precooling the oscillating 4He-gas flow. The PTR stages are operated in orifice and double-inlet mode with the orifice valves, reservoirs and second-inlet valves located at ambient temperature. Er 3Ni shot is used for the regenerator matrix in the coldest stage. A minimum temperature of 2.13(1) K has been achieved, which, at the average operating pressure of 19.5 bar, is only 0.19 K above the λ-line for the superfluid phase transition of 4He.
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