Preliminary identification and characterization of nonlinear wave-wave, wave-beam, and wave-particle interactions in beam-driven tokamak plasma

We present the analysis of 18 DIII-D shots heated by neutral-beam injection (NBI). [Typical plasma parameters are n~2x10¹³ cm^-3, T~1keV.] As energetic ions seeded by NBI resonate at the frequencies of various Alfven eigenmodes (AEs), we observe rich toroidal Alfven eigenmode (TAE) activity as these weakly-damped modes are driven. Both steady and modulated beam power have been investigated. Statistical techniques were used to benignly filter out the polluting effect of edge-localized modes (ELMs) in the observed fluctuation spectra. Guided by recent simulations [Spong et al 2021; Nucl. Fusion 61, 116061] which identify nonlinear coupling between TAEs and zonal flows, we seek to correlate the observed nonlinear interaction between ensembles of AEs and lower-frequency MHD modes with coincident perturbations in the fast-ion distribution function. Evidence for energy exchange due to this coupling is given by higher-order spectral techniques. In particular, we report on the evolution of consistent bicoherent features in magnetic fluctuation data.