Magnetic Sensitivity of AlMn TESes and Shielding Considerations for Next-Generation CMB Surveys

TL;DR

This paper measures the magnetic sensitivity of various TES bolometers and SQUIDs used in next-generation CMB experiments, providing data crucial for designing effective magnetic shielding.

Contribution

It provides the first comparative measurements of magnetic sensitivity across different TES geometries, doping levels, and SQUID types relevant for CMB surveys.

Findings

Mo-Cu bilayer ACTPol TESes are more sensitive to magnetic fields than AlMn TESes.

Weak-link-like behavior observed in AlMn TESes at low critical currents.

Magnetic sensitivity varies with device geometry and doping, impacting shielding requirements.

Abstract

In the next decade, new ground-based Cosmic Microwave Background (CMB) experiments such as Simons Observatory (SO), CCAT-prime, and CMB-S4 will increase the number of detectors observing the CMB by an order of magnitude or more, dramatically improving our understanding of cosmology and astrophysics. These projects will deploy receivers with as many as hundreds of thousands of transition edge sensor (TES) bolometers coupled to Superconducting Quantum Interference Device (SQUID)-based readout systems. It is well known that superconducting devices such as TESes and SQUIDs are sensitive to magnetic fields. However, the effects of magnetic fields on TESes are not easily predicted due to the complex behavior of the superconducting transition, which motivates direct measurements of the magnetic sensitivity of these devices. We present comparative four-lead measurements of the critical temperature versus applied magnetic field of AlMn TESes varying in geometry, doping, and leg length, including Advanced ACT (AdvACT) and POLARBEAR-2/Simons Array bolometers. Molybdenum-copper bilayer ACTPol TESes are also tested and are found to be more sensitive to magnetic fields than the AlMn devices. We present an observation of weak-link-like behavior in AlMn TESes at low critical currents. We also compare measurements of magnetic sensitivity for time division multiplexing SQUIDs and frequency division multiplexing microwave rf-SQUIDs. We discuss the implications of our measurements on the magnetic shielding required for future experiments that aim to map the CMB to near-fundamental limits.