The nonlinear dispersion relation of geodesic acoustic modes

TL;DR

This paper investigates the nonlinear dispersion relation of geodesic acoustic modes (GAMs) in tokamak edge plasmas, revealing that turbulence can significantly enhance GAM group velocities and influence their excitation and activity patterns.

Contribution

It introduces a numerical study showing that turbulent GAM dispersion relations can differ markedly from linear predictions, especially in group velocity and excitation mechanisms.

Findings

Turbulent GAM dispersion relations are qualitatively similar to linear ones.

Turbulence can drastically increase GAM group velocities.

Asymmetry in geometry influences GAM excitation and activity.

Abstract

The energy input and frequency shift of geodesic acoustic modes (GAMs) due to turbulence in tokamak edge plasmas are investigated in numerical two-fluid turbulence studies. Surprisingly, the turbulent GAM dispersion relation is qualitatively equivalent to the linear GAM dispersion but can have drastically enhanced group velocities. In up-down asymmetric geometry the energy input due to turbulent transport may favor the excitation of GAMs with one particular sign of the radial phase velocity relative to the magnetic drifts and may lead to pulsed GAM activity.