The Simons Observatory: Magnetic Sensitivity Measurements of Microwave SQUID Multiplexers

A summary of E. M. Vavagiakis et al., “The Simons Observatory: Magnetic Sensitivity Measurements of Microwave SQUID Multiplexers,” IEEE Trans. Appl. Supercond. 2020 (in review)

Cosmic microwave background (CMB) experiment cameras rely on superconducting detectors and readout systems which are sensitive to magnetic fields. Experiments like the Simons Observatory (SO) use these devices for precision measurements of the microwave sky. CMB map artifacts introduced from devices improperly shielded from telescope scan-synchronous pickup of Earth’s magnetic field, magnetic components inside of the camera cryostats, and other sources could be difficult to remove and jeopardize science goals. The magnetic shielding design for SO’s readout components is underway, and this work is helping to define that design.

SO’s detectors (which make up the camera pixels) will be read out using uMUX (Microwave SQUID Multiplexing) SQUIDs (Superconducting Quantum Interference Devices), which are developed by NIST (National Institute of Standards and Technology). The behavior of these SQUIDs under the effect of magnetic fields is not yet well understood. Laboratory measurements of superconducting device magnetic field sensitivity provide valuable data on device response and magnetic shielding requirements, as well as checks on models and simulations (see my previous work on this topic.)

We present measurements of the magnetic pickup of test microwave SQUID multiplexers as a study of various magnetic shielding configurations. To measure the magnetic pickup, measurements of SQUID response are made while we vary externally applied magnetic fields via a set of DC Helmholtz coils. We test a variety of shielding material configurations, including single layers of superconductors (aluminum and niobium), double layers of superconductors, and annealed A4K from Amuneal, a high-permeability magnetic shielding alloy.

Experimental setup for the SQUID test package (“512 box”) magnetic field testing. The package was mounted to the 100 mK plate of a Bluefors dilution refrigerator. A set of Helmholtz coils apply DC fields perpendicular to the uMUX chips, as shown installed around the DR on the left. On the right, a schematic of the DR mixing chamber plate and mounted package is shown along with the coils.

Our measurements of unshielded SQUID pickup agree with our previous results. Layers of superconductors near the SQUID chips provided various degrees of magnetic shielding. The best shielding performance was observed for the sandwiches of superconducting materials. The presence of a single sheet of A4K near the chips degraded the shielding performance of an aluminum sandwich by a factor of ~4, and increased the sensitivity of the uMUX SQUIDs when compared to no shielding at all. The presence of A4K also introduced a large field polarity asymmetry. We discuss why the geometry of A4K is important, as well as why its performance may be difficult to simulate. Our results are guiding ongoing tests as we work towards finalizing the design of SO’s detector and readout package magnetic shielding. Read the full paper here, and stay tuned!

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