The Simons Observatory Microwave SQUID Multiplexing Detector Module Design

Abstract

Advances in cosmic microwave background (CMB) science depend on increasing the number of sensitive detectors observing the sky. New instruments deploy large arrays of superconducting transition-edge sensor (TES) bolometers tiled densely into ever larger focal planes. High multiplexing factors reduce the thermal loading on the cryogenic receivers and simplify their design. We present the design of focal-plane modules with an order of magnitude higher multiplexing factor than has previously been achieved with TES bolometers. We focus on the novel cold readout component, which employs microwave SQUID multiplexing (μmux). Simons Observatory will use 49 modules containing 70,000 bolometers to make exquisitely sensitive measurements of the CMB. We validate the focal-plane module design, presenting measurements of the readout component with and without a prototype detector array of 1728 polarization-sensitive bolometers coupled to feedhorns. The readout component achieves a 95% yield and a 910 multiplexing factor. The median white noise of each readout channel is 65 pA / Hz . This impacts the projected SO mapping speed by <8%, which is less than is assumed in the sensitivity projections. The results validate the full functionality of the module. We discuss the measured performance in the context of SO science requirements, which are exceeded.

The Simons Observatory: Design and Measured Performance of a Carbon Fiber Strut for a Cryogenic Truss

“…The truss consists of two aluminum 6061 rings separated by 24 carbon fiber struts. The larger-diameter ring is thermally connected to the second stage of a Cryomech 1 PT420 pulse tube cooler (PTC), henceforth referred to as the 4 K stage of the instrument, and the smaller diameter ring is thermally connected to the still of a BlueFors 2 1 Cryomech Inc, 6682 Moore Rd., Syracuse, NY 13211 USA 2 Bluefors Oy, Arinatie 10, 00370 Helsinki, Finland SD400 dilution refrigerator (DR), henceforth referred to as the 1 K stage. The dimensions of the truss rings are given in Table 1.…”

“…The truss must be mechanically strong enough to support all of the instrument components mounted to the 1 K stage of the instrument. This includes both the telescope optics, composed of silicon lenses [6,8,16], and the detector system, composed of seven universal focal-plane modules [2]. The total mass of these components is 215 kg.…”

“…Instruments for measuring the cosmic microwave background (CMB) have seen steady improvement in sensitivity in recent decades [1]. These improvements have primarily been achieved by using larger detector arrays with more array elements and optical systems that are cooled to cryogenic temperatures to minimize unwanted thermal emission from the instrument [2,3,4,5,6,7,8,9]. These two instrument features have created a need for cryomechanical systems that are capable of supporting the steadily increasing size and mass of these instrument components.…”

The 90 and 150 GHz universal focal-plane modules for the Simons Observatory

“…We have previously published the overall design and architecture of the UFM and demonstrated its full functionality, including achieving 1000x multiplexing of TES bolometers at the noise required for the SO forecasts [5]. In this paper, we describe the specifications and implementations particular to the MF UFM.…”

“…The spline-shaped feedhorn (gold) feeds a silicon detector stack (grey) consisting of the photonic choke, detector array, and backshort. Above the detector stack lies the cold multiplexer (blue), termed the universal µmux module (UMM) [5]. An outer mechanical and magnetic shield (yellow) protects the inner pieces of the UFM.…”