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