Thermal design of the SCUBA-2 instrument detector stage and enclosure

The SCUBA-2 instrument is a new wide field submillimeter imager currently being designed for the James Clerk Maxwell telescope on Mauna Kea in Hawaii. The instrument will observe simultaneously in the 450 and 850 mum bands and has a field of view of approximately 50 square arcminutes. To meet the performance requirements the detectors require a heat sink at a temperature of 50 mK or lower, and must be surrounded by an enclosure at a temperature of 1.1 K or below. Cooling is provided by the mixing chamber and still of a cryogen-free dilution refrigerator (DR). via thermal links of the order of a metre in length. A challenging set of requirements result from the need for a small temperature drop between the detectors and the refrigerator insert despite the large distance between them. the need to provide flexibility in the links to allow for movement during thermal contraction, and the need to allow for the detectors to be removed from the cryostat. Further. the arrays require a mounting structure which provides rigid mechanical support from the 1-K stage yet causes a very small heat input to millikelvin stage. This paper describes the design which has been evolved to meet these difficult (and often conflicting) requirements.