Testing magnetic interference between TES detectors and the telescope environment for future CMB satellite missions

TL;DR

This study investigates magnetic interference effects on TES detectors caused by superconductive magnetic bearings used in polarization modulators for future CMB satellite missions, finding minimal impact from DC fields and quantifying AC magnetic response.

Contribution

It provides the first assessment of magnetic interference from superconductive bearings on TES detectors in the context of CMB experiments, including response quantification.

Findings

No detectable Tc variations from DC magnetic fields.

TES shows a response to AC magnetic fields with a responsivity of ~10^5 pA/G.

Magnetic interference effects are manageable for future CMB missions.

Abstract

The two most common components of several upcoming CMB experiments are large arrays of superconductive TES (Transition-Edge Sensor) detectors and polarization modulator units, e.g. continuously-rotating Half-Wave Plates (HWP). A high detector count is necessary to increase the instrument raw sensitivity, however past experiments have shown that systematic effects are becoming one of the main limiting factors to reach the sensitivity required to detect primordial $B$-modes. Therefore, polarization modulators have become popular in recent years to mitigate several systematic effects. Polarization modulators based on HWP technologies require a rotating mechanism to spin the plate and modulate the incoming polarized signal. In order to minimize heat dissipation from the rotating mechanism, which is a stringent requirement particularly for a space mission like $LiteBIRD$, we can employ a superconductive magnetic bearing to levitate the rotor and achieve contactless rotation. A disadvantage of this technique is the associated magnetic fields generated by those systems. In this paper we investigate the effects on a TES detector prototype and find no detectable $T_c$ variations due to an applied constant (DC) magnetic field, and a non-zero TES response to varying (AC) magnetic fields. We quantify a worst-case TES responsivity to the applied AC magnetic field of $\sim10^5$ pA/G, and give a preliminary interpretation of the pick-up mechanism.