Nonlinear dynamics of energetic-particle driven geodesic acoustic modes in ASDEX Upgrade

TL;DR

This study investigates the nonlinear behavior of energetic-particle driven geodesic acoustic modes (EGAMs) in tokamaks using gyrokinetic simulations, revealing how EP parameters influence plasma heating and mode saturation.

Contribution

It introduces a Mode-Particle-Resonance diagnostic in the ORB5 code to analyze EGAM nonlinear dynamics in a realistic ASDEX Upgrade scenario.

Findings

EGAMs can significantly transfer energy from energetic particles to thermal plasma.

Electron dynamics reduce EGAM saturation amplitude and plasma heating.

Varying EP parameters can enhance plasma heating by EGAMs.

Abstract

Turbulence in tokamaks generates radially sheared zonal flows. Their oscillatory counterparts, geodesic acoustic modes (GAMs), appear due to the action of the magnetic field curvature. The GAMs can be driven unstable by an anisotropic energetic particle (EP) population leading to the formation of global radial structures, called EGAMs. The EGAMs can redistribute EP energy to the bulk plasma through collisionless wave-particle interaction. In such a way, the EGAMs might contribute to the plasma heating. Thus, investigation of EGAM properties, especially in the velocity space, is necessary for precise understanding of the transport phenomena in tokamak plasmas. In this work, the nonlinear dynamics of EGAMs without considering the mode interaction with the turbulence is investigated with the help of a Mode-Particle-Resonance (MPR) diagnostic implemented in the global gyrokinetic particle-in-cell code ORB5. An ASDEX Upgrade discharge is chosen as a reference case for this investigation due to its rich EP nonlinear dynamics. An experimentally relevant magnetic field configuration, thermal species profiles and an EP density profile are taken for EGAM chirping modelling and its comparison with available empirical data. The same magnetic configuration is used to explore energy transfer by the mode from the energetic particles to the thermal plasma including kinetic electron effects. For a given EGAM level the plasma heating by the mode can be significantly enhanced by varying the EP parameters. Electron dynamics decreases the EGAM saturation amplitude and consequently reduces the plasma heating, even though the mode transfers its energy to thermal ions much more than to electrons.