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 of ⁴He 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.