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

Abstract

In the next decade, new ground-based cosmic microwave background (CMB) experiments such as Simons Observatory, 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 and POLARBEAR-2/Simons Array bolometers. MoCu 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 (µMUX) 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.

Receiver development for BICEP Array, a next-generation CMB polarimeter at the South Pole

“…The SQUID/TES readout system is known to be susceptible to external magnetic fields 39,40 and Bicep Array's magnetic shielding architecture is designed to minimize the level of spurious signals generated as the telescope moves along the Earth's magnetic field lines. We studied multiple configurations using COMSOL Multi-Physics software, 41 which allowed us to simulate the Meissner behavior of a superconducting material (see Figure 7).…”

Design and Performance of the First BICEP Array Receiver

“…In the design of these PCBs and the architecture of the cables, we paid particular attention to pairing signal and return in order to minimize cross-talk among channels. Moreover, SQUIDs and TES detectors must be carefully shielded from sources of magnetic fields [7] [8], including the Earth's field. We studied multiple configurations using COMSOL Multi-Physics software (see figure 4) [9].…”

A cryogenic continuously rotating half-wave plate mechanism for the POLARBEAR-2b cosmic microwave background receiver

“…Measurements of the PB-2b AlMn TESs give dT c /dB = 0.3 mK/G 58 and dP sat /dT c = 0.1 pW/mK, 59 while simulations of the PB-2b optics 48 give dT CMB /dP opt = 12 µK CMB /aW. We again require that NET HWP CMB NET arr CMB such that…”