Finite-element modelling of magnetic shielding for SCUBA-2

摘要

SCUBA-2 is a new wide-field sub-millimeter camera under construction for the James Clerk Maxwell Telescope on Mauna Kea in Hawaii. SCUBA-2 images simultaneously at 450 and 850 /irai using large-scale arrays of superconducting bolometers, with over five thousand pixels at each wavelength. Time division multiplexed readouts and cryogenic: amplifiers, both based on superconducting quantum interference devices (SQUIDs), are also used in the design. The SCUBA-2 detector arrays must be well shielded against magnetic fields, since the performance of the bolometers can be seriously affected by the presence of a strong field, and the SQUIDs are themselves sensitive magnetometers. This shielding is to be provided by a combination of high-permeability and superconducting layers on both the ambient temperature and cryogenic stages of the instrument. To optimise and demonstrate the effectiveness of the shielding design, a finite-element modelling method was employed, using the Ansoft? Maxwell 3D? package. Although a number of approximations had to be made in the modelling, the finite-element results allow a good estimation of the effectiveness of the shielding at attenuating external magnetic fields to be made. This paper describes the modelling process, outlines the key results and summarises the final shielding design.