Measurements of the polarization of several instabilities in the DIII-D tokamak

Recently, a method to infer the polarization of modes with frequencies much less than the ion cyclotron frequency was published [X.D. Du et al., Phys. Rev. Lett. 132 (2024) 215101]. The method uses measurements of electron temperature and density fluctuations δTe and δne at the same spatial position to infer the local ratio of “acoustic polarization,” |δϕ∥|/(|δϕ∥| + |δψ|), where δϕ∥ is the effective parallel potential and δψ is related to the parallel magnetic vector potential A∥. This paper summarizes key formulas, with emphasis on their range of validity, and elaborates on the workflow required to infer the acoustic polarization from experimental data. The drift-acoustic polarization of ellipticity-induced, toroidicity-induced, and reversed shear Alfv´en eigenmodes is nearly zero, as expected for modes with predominately shear-Alfv´enic polarization. The polarization of beta-induced Alfv´en eigenmodes contains an acoustic component that increases with poloidal wave number. “Low frequency modes,” (instabilities that appear transiently when the minimum of the safety factor qmin passes through rational values) have large and highly variable acoustic polarization. In both experiment and simulation, fishbones have non-zero acoustic polarization that increases as the mode chirps down in frequency.