Nonlinear gyrokinetic equations for low-frequency electromagnetic waves in general plasma equilibria

TLDR

A nonlinear gyrokinetic formalism for low‑frequency microscopic electromagnetic perturbations in general magnetic field configurations is developed. The derived equations incorporate finite Larmor radius, plasma inhomogeneities, and magnetic geometry, and a model nonlinear equation for electrostatic drift waves is obtained for axisymmetric tokamaks. The formalism shows that diamagnetic drift effects modify nonlinear ion Landau damping of kinetic shear‑Alfvén waves, causing wave energy to cascade in wavenumber rather than frequency.

Abstract

A nonlinear gyrokinetic formalism for low-frequency (less than the cyclotron frequency) microscopic electromagnetic perturbations in general magnetic field configurations is developed. The nonlinear equations thus derived are valid in the strong-turbulence regime and contain effects due to finite Larmor radius, plasma inhomogeneities, and magnetic field geometries. The specific case of axisymmetric tokamaks is then considered and a model nonlinear equation is derived for electrostatic drift waves. Also, applying the formalism to the shear Alfvén wave heating scheme, it is found that nonlinear ion Landau damping of kinetic shear-Alfvén waves is modified, both qualitatively and quantitatively, by the diamagnetic drift effects. In particular, wave energy is found to cascade in wavenumber instead of frequency.