The Simons Observatory: Magnetic Sensitivity Measurements of Microwave
  SQUID Multiplexers
  The Simons Observatory (SO) will be a cosmic microwave background (CMB)
survey experiment with three small-aperture telescopes and one large-aperture
telescope, which will observe from the Atacama Desert in Chile. In total, SO
will field $\sim$70,000 transition-edge sensor (TES) bolometers in six spectral
bands centered between 27 and 280 GHz in order to achieve the sensitivity
necessary to measure or constrain numerous cosmological quantities. The SO
Universal Focal Plane Modules (UFMs) each contain a 150 mm diameter TES
detector array, horn or lenslet optical coupling, cold readout components, and
magnetic shielding. SO will use a microwave SQUID multiplexing ($\mu$MUX)
readout at an initial multiplexing factor of $\sim$1000; the cold (100 mK)
readout components are packaged in a $\mu$MUX readout module, which is part of
the UFM, and can also be characterized independently. The 100 mK stage TES
bolometer arrays and microwave SQUIDs are sensitive to magnetic fields, and
their measured response will vary with the degree to which they are
magnetically shielded. We present measurements of the magnetic pickup of test
microwave SQUID multiplexers as a study of various shielding configurations for
the Simons Observatory. We discuss how these measurements motivated the
material choice and design of the UFM magnetic shielding.
