Nonlinear interplay of Alfven instabilities and energetic particles in tokamaks

TL;DR

This paper investigates the nonlinear interactions between Alfven Eigenmodes and energetic particles in tokamaks, revealing how these interactions modify mode characteristics and can be modeled quasilinearly.

Contribution

It provides the first detailed nonlinear analysis of toroidicity induced Alfven Eigenmodes using a global gyrokinetic particle-in-cell code, highlighting the wave-particle nonlinearity effects.

Findings

Frequency increases as growth rate decreases

Mode shrinks radially during nonlinear evolution

Quasilinear model accurately reproduces nonlinear effects

Abstract

The confinement of energetic particles (EP) is crucial for an efficient heating of tokamak plasmas. Plasma instabilities such as Alfven Eigenmodes (AE) can redistribute the EP population making the plasma heating less effective, and leading to additional loads on the walls. The nonlinear dynamics of toroidicity induced AE (TAE) is investigated by means of the global gyrokinetic particle-in-cell code ORB5, within the NEMORB project. The nonperturbative nonlinear interplay of TAEs and EP due to the wave-particle nonlinearity is studied. In particular, we focus on the nonlinear modification of the frequency, growth rate and radial structure of the TAE, depending on the evolution of the EP distribution in phase space. For the ITPA benchmark case, we find that the frequency increases when the growth rate decreases, and the mode shrinks radially. This nonlinear evolution is found to be correctly reproduced by means of a quasilinear model, namely a model where the linear effects of the nonlinearly modified EP distribution function are retained.