Gyrokinetic investigation of the nonlinear interaction of Alfv\'en instabilities and energetic-particle driven geodesic acoustic modes

TL;DR

This study uses gyrokinetic simulations to explore how Alfvén modes interact with energetic-particle driven geodesic acoustic modes in a realistic fusion plasma, revealing different regimes based on energetic particle concentration.

Contribution

It provides the first detailed gyrokinetic simulation analysis of the nonlinear interaction between Alfvén modes and energetic-particle driven geodesic acoustic modes in a realistic tokamak equilibrium.

Findings

At low energetic particle concentration, zonal structures saturate at higher levels.

The zonal structure is more unstable than the Alfvén mode in the low concentration regime.

A 3-wave coupling mechanism explains the interaction between modes.

Abstract

This paper presents a study of the interaction between Alfv\'en 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 particles concentration, the Alfv\'en mode saturates at much higher level in presence of zonal structures; on the other hand at high energetic particles concentration the difference is less pronounced. The former regime is characterized by the zonal structure (identified as an energetic particle driven geodesic acoustic mode), being more unstable than the Alfv\'en mode. In the latter regime the Alfv\'en mode is more unstable than the zonal structure. The theoretical explanation is given in terms of a 3-wave coupling of the energetic particle driven geodesic acoustic mode and Alfv\'en mode, mediated by the curvature-pressure coupling term of the energetic particles.