Performance of the SCUBA-2 dilution refrigerator

SCUBA-2 is a new wide-field submillimeter continuum instrument being commissioned on the James Clerk Maxwell Telescope on Mauna Kea in Hawaii. SCUBA-2 images simultaneously at 450 and 850 µ mu sing large- scale arrays of superconducting bolometers, with over five thousand pixels at each wavelength. The arrays are cooled to less than 100 mK by the mixing chamber of a dilution refrigerator (DR), with a radiation shield at a nominal temperature of 1 K cooled by the DR still. The DR is a "dry" system, using a pulse tube cooler for precooling of the circulating helium in place of a liquid helium bath. This paper presents key performance data for the DR. The specified performance for the SCUBA-2 DR required an unloaded mixing chamber base temperature of less than 10 mK, and providing a cooling power of 500 µW at 120 mK, and 30 µW at 65 mK (with a goal of 30 µW at 35 mK). These figures are taken from the stated performance characteristics of the conventional Leiden Cryogenics dilution refrigerator upon which the DR was to be based. The still was to run at a temperature of <900 mK, with a goal of <700 mK, while providing at least 5 mW of capacity to cool cryogenic amplifiers, filters and radiation shielding. More detailed descriptions of the cryogenic design of SCUBA-2 are available elsewhere. 5,6 The paper describes the baseline performance of the dilution refrigerator in standalone operation (outside the instrument cryostat), along with some discussion of the system performance when integrated with the full instrument. A description of the key design features of the DR is given in §2, and the behaviour of the refrigerator during cooldown is discussed in §3. The main performance characteristics are described in §4.