Gyrokinetic modelling of anisotropic energetic particle driven instabilities in tokamak plasmas

TL;DR

This paper investigates how the anisotropic distribution of energetic particles influences the excitation of geodesic acoustic modes in tokamaks using gyrokinetic simulations, revealing sensitivity to distribution shape and injection angle.

Contribution

It introduces a gyrokinetic simulation study of anisotropic energetic particle effects on instabilities in tokamaks, incorporating realistic distributions from RABBIT and comparing with experiments.

Findings

Growth rate depends on the phase-space shape of the distribution.

Injection angle affects the instability growth rate.

Simulated growth rates are lower than experimental measurements.

Abstract

Energetic particles produced by neutral beams are observed to excite energetic-particle-driven geodesic acoustic modes (EGAMs) in tokamaks. We study the effects of anisotropy of distribution function of the energetic particles on the excitation of such instabilities with ORB5, a gyrokinetic particle-in-cell code. Numerical results are shown for linear electrostatic simulations with ORB5. The growth rate is found to be sensitively dependent on the phase-space shape of the distribution function. The behavior of the instability is qualitatively compared to the theoretical analysis of dispersion relations. Realistic neutral beam energetic particle anisotropic distributions are obtained from the heating solver RABBIT and are introduced into ORB5 as input distribution function. Results show a dependence of the growth rate on the injection angle. A qualitative comparison to experimental measurements is presented and few disagreements between them are found, being the growth rate in the simulations much lower than that from experiments. An explanation for the difference is advanced.