Gyrokinetic investigation of the nonlinear interaction of Alfven instabilities and energetic particle-driven geodesic acoustic modes

This paper presents a study of the interaction between Alfven modes and zonal structures, considering a realistic ASDEX Upgrade equilibrium. The results of gyrokinetic simulations with the global, electromagnetic, particle-in-cell code ORB5 are presented, where the modes are driven unstable by energetic particles with a bump-on-tail equilibrium distribution function, with radial density gradient. Two regimes have been observed. At low energetic particle concentration, the zonal structure (identified as an energetic particle-driven geodesic acoustic mode) is more unstable than the Alfven mode. In the regime at high energetic particle concentration, the Alfven mode is more unstable than the zonal structure. The interplay between the modes leads to a modification of their growth rates as well as to a modification of their saturation levels. The theoretical explanation of the mode interaction is given in terms of three-wave coupling of the energetic particle-driven geodesic acoustic mode and Alfven mode, mediated by the curvature-pressure coupling term of the energetic particles. (C) 2021 Author(s).

Automated summary

This study investigates the interaction between Alfven modes and zonal structures using gyrokinetic simulations with the ORB5 code, showing different instability and mode interactions under varying energetic particle concentrations, with three-wave coupling as a possible theoretical explanation.