MNTES: Modeling Nonlinearity of TES detectors for Enhanced Cosmic Microwave Background measurements with LiteBIRD

Abstract

Traditional methods of converting electronic readout counts to optical power incident on Transition Edge Sensors (TES) for Cosmic Microwave Background (CMB) observations involve a linear approximation. For the upcoming LiteBIRD CMB satellite, strict nonlinearity requirements must be met to prevent contamination of the science band at 4f by the 2f signal, which arises from differential transmission or emissivity related to the half-wave plate's rotation rate fHWP. These constraints cannot be met using hardware solutions alone and therefore require a form of nonlinearity correction. We present MNTES, a novel physics-based, nonlinear calibration technique. This method leverages our physical understanding of the TES power balance equation, accounts for imperfect voltage bias by casting the bias network as its Th\'evenin equivalent, and can incorporate external information such as time-varying magnetic fields and focal plane temperature variations. The detector-specific parameters of MNTES will be measured during the ground calibration campaign prior to the LiteBIRD launch, yielding conversion functions that can take raw time-ordered data and output the reconstructed incident optical power. MNTES will allow us to achieve LiteBIRD's goal of measuring the primordial tensor fluctuation spectrum to {\delta}r < 0.001.