Pressure-gradient-induced Alfven eigenmodes: II. Kinetic excitation with ion temperature gradient

TL;DR

This paper investigates the kinetic excitation of Alfven eigenmodes driven by ion temperature gradients in tokamaks, revealing the roles of wave-particle interactions and damping mechanisms in plasma stability.

Contribution

It introduces a detailed kinetic analysis of alpha-induced toroidal Alfven eigenmodes influenced by ion temperature gradients using gyrokinetic simulations, expanding understanding of plasma instabilities.

Findings

Identification of alpha-TAEs as bound states trapped by pressure gradients

Critical temperature gradients are governed by Landau and continuum damping

The analysis applies to various magnetic shear configurations

Abstract

The kinetic excitation of ideal magnetohydrodynamic (MHD) discrete Alfven eigenmodes in the second MHD ballooning stable domain is studied in the presence of a thermal ion temperature gradient (ITG), using linear gyrokinetic particle-in-cell simulations of a local flux tube in shifted-circle tokamak geometry. The instabilities are identified as alpha-induced toroidal Alfven eigenmodes (alpha-TAE); that is, bound states trapped between pressure-gradient-induced potential barriers of the Schroedinger equation for shear Alfven waves. Using numerical tools, we examine in detail the effect of kinetic thermal ion compression on alpha-TAEs; both non-resonant coupling to ion sound waves and wave-particle resonances. It is shown that the Alfvenic ITG instability thresholds (e.g., the critical temperature gradient) are determined by two resonant absorption mechanisms: Landau damping and continuum damping. The numerical results are interpreted on the basis of a theoretical framework previously derived from a variational formulation. The present analysis of properties and structures of Alfvenic fluctuations in the presence of steep pressure gradients applies for both positive or negative magnetic shear and can serve as an interpretative framework for experimental observations in (future) high-performance fusion plasmas of reactor relevance.