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 [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 δϕ∥ (the effective parallel potential) and δψ (proportional to the parallel magnetic vector potential) and hence the “acoustic polarization.” This poster 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 reversedshear Alfvén eigenmodes is nearly zero, as expected for modes with predominately shear-Alfvénic polarization. In some cases, however, the acoustic polarization increases, a process thought to occur when Alfvén eigenmodes couple linearly or nonlinearly to zonal flows or other modes with electrostatic polarization. The polarization of beta-induced Alfvén eigenmodes contains an acoustic component that increases with poloidal wave number. In both experiment and simulation, fishbones have non-zero acoustic polarization that increases as the mode chirps down in frequency.