Alfvén Eigenmode (AE) interactions in Tokamaks: DIII-D Frontier Science Experiments connecting the physics of nonlinear waves and processes in space plasmas

Measuring nonlinear AE interactions in tokamaks and linear devices, relevant to solar wind, fusion, and magnetospheric physics, may reveal processes that saturate the amplitude of such fluctuations, and thus may explain additional nonlinear physics responsible for particle energization. DIII-D data is being analyzed to identify the nonlinear energy transfer among toroidal AE occurring via three-wave interactions, usually mediated by a much lower frequency mode, such as a zonal flow, an energetic particle mode, or a lower frequency toroidal AE. To understand the influence of such interactions on the saturated amplitudes of driven AE and thereby identify the process that sets the amplitude of kinetic AE in the magnetosphere, we aim to determine the differences in AE amplitudes in the presence or absence of nonlinear interactions in DIII-D plasma. Their linear damping rates, and the rate of energization of the ions, suggest AE would rapidly damp without continual energy injection, so these kinetic AE must be driven by the dynamics of the geomagnetic storm. The nonlinear interaction between counterpropagating AE, i.e., Alfvén wave collision, is believed to be a fundamental building block of Alfvénic turbulence in space and astrophysical plasma environments.