APPROACHING THE HE-4-LAMBDA LINE WITH A LIQUID-NITROGEN PRECOOLED 2-STAGE PULSE TUBE REFRIGERATOR

We report on the design and test of a three-stage refrigerator, consis ting of a liquid nitrogen bath with heat exchangers and two subsequent pulse tube stages. The liquid nitrogen bath serves for precooling the oscillating He-4-gas flow. The PTR stages are operated in orifice and double-inlet mode with the orifice valves, reservoirs and second-inle t valves located at ambient temperature. Er3Ni shot is used for the re generator 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 lambda-line for the superfluid phase tr ansition of He-4. (C) 1996 Elsevier Science Limited