Automated Closed-Cycle Cooling to 250 mK for the Polatron
We have integrated a 4 K mechanical cryocooler with a helium sorption refrigerator to achieve closed-cycle cooling to a base temperature of 239 mK. The cryocooler consists of two Gifford-McMahon stages which pre-cool a Joule-Thomson expansion stage to below the inversion temperature of4He This in turn provides cooling to below the 5.2 K critical temperature of 4He The first two stages of the sorption cooler consist of a 4He and a 3He refrigerator, with the 4He providing cooling to below the 3.3 K critical temperature of 3He This subassembly buffers the heatload on the final 3He refrigerator. We achieve an operating temperature of 250 mK at the expected 0.5μW heat input from parasitic conduction plus the optical load. The duty cycle of the sorption cooler is 83 %. The sorption cooler is automatically cycled using digitally commanded power supplies. The system will allow for unattended cooldown from 300 K to 250 mK for cooling of the detectors on the Polatron, a ground-based receiver which will be fielded at the 5.5 metre telescope at the Owens Valley Radio Observatory.
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