Saturation of energetic-particle-driven geodesic acoustic modes due to wave-particle nonlinearity

TL;DR

This paper investigates the nonlinear saturation mechanisms of energetic-particle-driven geodesic acoustic modes using gyrokinetic simulations and analytical theory, revealing wave-particle interactions and scaling laws relevant for plasma stability.

Contribution

It provides a combined numerical and analytical study of EGAM saturation, highlighting wave-particle nonlinearity and the transition from adiabatic to non-adiabatic dynamics.

Findings

Saturated electric field scales quadratically with linear growth rate.

EP bounce frequency is independent of bulk plasma temperature.

Transition from adiabatic to non-adiabatic dynamics near saturation.

Abstract

The nonlinear dynamics of energetic-particle (EP) driven geodesic acoustic modes (EGAM) is investigated here. A numerical analysis with the global gyrokinetic particle-in-cell code ORB5 is performed, and the results are interpreted with the analytical theory, in close comparison with the theory of the beam-plasma instability. Only axisymmetric modes are considered, with a nonlinear dynamics determined by wave-particle interaction. Quadratic scalings of the saturated electric field with respect to the linear growth rate are found for the case of interest. The EP bounce frequency is calculated as a function of the EGAM frequency, and shown not to depend on the value of the bulk temperature. Near the saturation, we observe a transition from adiabatic to non-adiabatic dynamics, i.e., the frequency chirping rate becomes comparable to the resonant EP bounce frequency. The numerical analysis is performed here with electrostatic simulations with circular flux surfaces, and kinetic effects of the electrons are neglected.