Nonlinear coupling with Alfvén eigenmodes in the DIII-D tokamak

For waves driven by wave-particle resonances, the ultimate amplitude of the wave determines the resulting particle transport. In general, the maximum amplitude is determined either by wave-particle nonlinearity (as in quasilinear flattening of the destabilizing distribution function) or by wave-wave nonlinearities (as when the instability delivers energy to damped “daughter” waves). In this study, we use calculations of bicoherence [1] and transfer entropy [2] to assess when Alfvén eigenmodes in the DIII-D tokamak have appreciable mode-mode coupling. The goal of the project is to determine empirically from an extensive database when wave-particle nonlinearity determines the saturation amplitude and when wave-wave nonlinearities predominate.

[1] G. Riggs et al, Phys.Plasmas 31 (2024) 042305.

[2] T. Schreiber, Phys.Rev.Lett. 85, 461 (2000).