Abstract
We describe the construction and operation of a closed cycle refrigerator with an operating temperature of 2.8 K. Helium gas at a pressure of 1.2 MPa is precooled to 18 K by a Stirling refrigerator (CTI 350) and expanded in a Joule-Thomson stage to ≈10 kPa. A vacuum pump circuit and compressor bring the gas back to its original pressure. The refrigerator is designed to cool the first stages of a radio astronomical receiver with a superconducting (SIS) mixer. It fits into a small radio telescope and works widely independent of orientation. After a cooldown time of 12–14 h a cooling capacity of 200–250 mW is available.
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