Excitation and damping mechanisms of geodesic acoustic modes in tokamaks

The tokamak is one of the most promising concepts to produce controlled thermonuclear fusion power. In tokamak configuration, deuterium-tritium fuel heated to high temperatures turns into the plasma state that has to be confined for a sufficiently long time to achieve the condition of the self-sustaining burning plasma. However, small scale instabilities driven by plasma temperature and density gradients cause turbulence, which is responsible for enhanced particle and energy losses in the system. This turbulent transport is considered as one of the main phenomena that degrade the plasma confinement. Turbulence in tokamaks generates nonlinearly large-scale ows (called zonal ows) that deplete the turbulence level and thus play a fundamental role in turbulence regulation and saturation. As part of this dynamics, an oscillatory counterpart of the zonal ows can arise because of the action of the magnetic field curvature. These oscillations, which can also couple directly to the turbulence, are called the geodesic acoustic modes (GAMs) and are the main subject of this thesis. GAMs can be driven unstable also by an anisotropic energetic particle (EP) population leading to the formation of global radial structures, called EGAMs. The EGAMs might play the role of an intermediate agent between the energetic and thermal species by redistributing the EP energy to the bulk plasma through collisionless wave-particle interaction. In such a way, the EGAMs might contribute to plasma heating. Thus, investigation of EGAM properties, especially in velocity space, is necessary for a precise understanding of the transport phenomena in tokamak plasmas. In this thesis, we numerically investigate different mechanisms of the GAM damping and excitation such as Landau damping, phase mixing and the mode formation by energetic particles and turbulence. While several aspects of the GAM dynamics can be understood in the frame of a uid model, a quantitative understanding requires a kinetic approach, which can describe the details of the GAM interaction with the plasma particles in phase space and thus capture, for instance, the effect of resonances. The global gyrokinetic particle-in-cell code ORB5 is used here as the primary numerical tool for this study. As a part of this work, additional relevant code diagnostics are developed, such as a Mode-Particle-Resonance (MPR) technique to explore wave-particle interactions in velocity space. This MPR method is employed to study EGAM dynamics in a magnetic configuration typical of the ASDEX Upgrade (AUG) tokamak, by analysing the influence of different species on the time evolution of the mode. It is shown that electrons, which are often not included in the theoretical analyses, significantly contribute to the damping of (E)GAMs in experimentally relevant AUG plasma systems. Moreover, nonlinear EGAM dynamics without considering the mode interaction with turbulence is investigated here to explore energy transfer by the mode from energetic particles to thermal species, including kinetic electron effects. It is shown that the electron dynamics decreases the EGAM saturation amplitude and consequently reduces the plasma heating by the mode, even though the mode transfers its energy to thermal ions much more than to electrons. Finally, nonlinear GAM excitation by ion-temperature-gradient driven instabilities is numerically simulated for magnetic configurations reflecting the properties of AUG and of the Tokamak of à Configuration Variable (TCV). Formation of continuum and global frequency spectra of the geodesic and GAM-like structures is demonstrated in these computations.