Kinetic theory of geodesic acoustic modes in toroidal plasmas: a brief review

TL;DR

This review comprehensively discusses the kinetic theory of geodesic acoustic modes (GAM) in toroidal plasmas, emphasizing linear and nonlinear aspects, resonant wave-particle interactions, and applications to experimental observations.

Contribution

It provides a unified theoretical framework for GAM and energetic particle-induced GAM (EGAM), integrating kinetic treatment, system nonuniformity, and realistic magnetic geometry.

Findings

Resonant wave-particle interactions are key to GAM dynamics.

Theories of EGAM are applied to realistic fusion devices.

Nonlinear GAM excitation by turbulence is systematically reviewed.

Abstract

Geodesic acoustic modes (GAM) are oscillating zonal structures unique to toroidal plasmas, and have been extensively studied in the past decades due to their potential capabilities of regulating microscopic turbulences and associated anomalous transport. This article reviews linear and nonlinear theories of GAM; with emphases on kinetic treatment, system nonuniformity and realistic magnetic geometry, in order to reflect the realistic experimental conditions. Specifically, in the linear physics, the resonant wave-particle interactions are discussed, with the application to resonant excitation by energetic particles (EPs). The theory of EP-induced GAM (EGAM) is applied to realistic devices for the interpretation of experimental observations, and global effects due to coupling to GAM continuum are also discussed. Meanwhile, in the nonlinear physics, the spontaneous GAM excitation by microscale turbulences is reviewed, including the effects of various system nonuniformities. A unified theoretical framework of GAM/EGAM is then constructed based on our present understandings. The first-principle-based GAM/EGAM theories reviewed here, thus, provide the tools needed for the understanding and interpretation of experimental/numerical results.