Nonlinear gyrokinetic simulations of reversed shear Alfven eigenmodes in DIII-D tokamak

Global gyrokinetic simulations of reversed shear Alfven eigenmodes (RSAE) excited by energetic particles (EP) in the DIII-D discharge #159,243 find that self-generated zonal flows dominate the RSAE nonlinear saturation. Effects of Coulomb collisions are negligible since the effective EP collision frequency is much smaller than the RSAE linear growth rate. Nonlinear interactions of multiple unstable RSAEs lower the saturation amplitude and resulting EP transport, and nonlinearly generate toroidal Alfven eigenmodes (TAE) and ellipticity induced Alfven eigenmodes (EAE). In both single mode and multiple modes simulations, the initial RSAE saturation amplitude and associated EP transport are an order of magnitude higher than the experimental observations, but quickly diminish within 0.1 ms after nonlinear saturation due to the formation of coherent phase space structures that flatten the EP distribution function at resonances. Results from these simulations of the RSAE only show that additional dissipation such as background micro-turbulence is needed to sustain the quasi-steady state RSAE amplitude and EP transport observed in this DIII-D experiment.