Linear kinetic effects of core plasma on low frequency Alfven and acoustic eigenmodes in tokamaks

TL;DR

This paper investigates how core plasma properties influence the excitation and coupling of low frequency Alfvén and acoustic eigenmodes in tokamaks, using a generalized dispersion relation framework.

Contribution

It introduces a comprehensive formalism to describe low frequency mode behavior, highlighting the significant role of core plasma effects over energetic particles.

Findings

Core plasma properties strongly affect mode excitation.

Diamagnetic frequency influences mode coupling and polarization.

Energetic particles provide non-resonant drive to some modes.

Abstract

The resonant and non-resonant effects of core plasma on the excitation of low frequency modes with $\omega < \omega_{BAE}$, such as Beta-induced Alfv\'en Acoustic Eigenmodes (BAAEs) and Kinetic Ballooning Modes (KBMs) are examined in the framework of the generalized fishbone-like dispersion relation. The formalism of the fishbone-like equation contains all the necessary ingredients to describe the features of these low frequency fluctuations, and explain experimental findings. Core plasma properties (diamagnetic frequency and precession resonance with trapped ions) strongly affect the excitation of the modes, and in the case of BAAEs more effectively than the energetic particles. The diamagnetic frequency of the core plasma also contributes to the coupling of the BAAEs with the KBMs, thus affecting the excitation and polarization of both modes. Energetic particles can still provide a non-resonant drive to some of the low frequency modes.