High Frequency Geodesic Acoustic Modes in Electron Scale Turbulence

TL;DR

This paper investigates the electron-scale geodesic acoustic modes driven by electron temperature gradient turbulence, deriving a dispersion relation, comparing growth rates with gyrokinetic simulations, and identifying a new saturation mechanism affecting turbulence levels.

Contribution

It introduces a fluid model for el-GAMs driven by ETG modes, derives their dispersion relation, and reveals a new turbulence saturation mechanism involving el-GAM interactions.

Findings

Good agreement between fluid model and gyrokinetic simulations for ETG growth rates.

Identification of a new saturation mechanism for ETG turbulence via el-GAM interactions.

El-GAMs can be stabilized by increased finite β and non-adiabaticity, with Maxwell stress reducing GAM growth rates.

Abstract

In this work the finite $\beta$-effects of an electron branch of the geodesic acoustic mode (el-GAM) driven by electron temperature gradient (ETG) modes is presented. The work is based on a fluid description of the ETG mode retaining non-adiabatic ions and the dispersion relation for el-GAMs driven non-linearly by ETG modes is derived. The ETG growth rate from the fluid model is compared to the results found from gyrokinetic simulations with good agreement. A new saturation mechanism for ETG turbulence through the interaction with el-GAMs is found, resulting in a significantly enhanced ETG turbulence saturation level compared to the mixing length estimate. It is shown that the el-GAM may be stabilized by an increase in finite $\beta$ as well as by increasing non-adiabaticity. The decreased GAM growth rates is due to the inclusion of the Maxwell stress.