Cryo-Mechanical Design of ALPACA: A Mixed-Material Radio-Frequency Transparent Vacuum Vessel Operating at 20 K

Abstract One of the most promising technologies to enable and enhance large survey capabilities for radio astronomy is the use of focal plane phased array antenna feeds, or more simply, phased array feeds (PAFs). PAFs allow for full and continuous coverage of the telescope’s field of view (FoV), and combined with cryogenic amplifiers, can result in survey speed improvements several orders of magnitude better than current multiple-feed-horn cameras. In order to locate cryogenic PAF elements and amplifiers at the telescope focal plane, a radio-frequency transparent vacuum vessel is required. Unlike typical radomes, the transmission properties must be exceptionally good when dealing with weak astronomical signals. The dome must also be sufficiently strong to carry the mechanical load on the vacuum vessel due to atmospheric pressure. Furthermore, the thermal loading on the internal cryogenic stages from the dome must be manageable for the cooling system. We have solved these problems by using a combination of welded polyethylene sheet to maintain the vacuum integrity and a closed-cell rigid foam to transfer the mechanical load to the opposite side of the vessel (a welded aluminum structure). The PAF elements and amplifiers operate at 20 K, while the foam transfers the mechanical load through an 80 K temperature stage, which also serves as a low-temperature radiation shield for the 20 K sections. The poor thermal conductivity of the foam, combined with G10-CR thermal standoffs on the opposite side, ensures the 80 K stage is sufficiently thermally isolated from room temperature conduction. The radiative loading is reduced via the usual employment of multi-layer insulation. In order to facilitate instrument maintenance and future upgrades, a modular PAF element mechanical strategy is employed. The design is such that a PAF-element-amplifier unit can be replaced without accessing the 20 K stage owing to the use of a “cryo-clamp” that uses materials with different coefficients of thermal expansion to tightly hold the units when cold. Cooling is supplied by three two-stage Gifford-McMahon cryo-coolers. This paper presents these design details for the cryostat of the ALPACA (Advanced L-Band Phased Array Camera for Arecibo), an instrument currently being designed and built for the 305 m radio telescope of the Arecibo Observatory in Puerto Rico.