Analytical study on magnetic component of geodesic acoustic mode

TL;DR

This study analytically investigates the magnetic components of geodesic acoustic modes within a gyrokinetic framework, revealing their dependence on plasma parameters and harmonic coupling, with implications for plasma stability analysis.

Contribution

It provides an analytical derivation of magnetic component amplitudes for m=1 and m=2 harmonics considering plasma parameters and harmonic coupling effects.

Findings

Both m=1 and m=2 magnetic components exist and are dominated by cosine and sine functions.

Magnetic component amplitudes increase with plasma pressure to magnetic pressure ratio and safety factor.

The m=1 magnetic component can be significantly enhanced due to harmonic coupling, becoming comparable to m=2.

Abstract

The magnetic components of geodesic acoustic mode (GAM) are analytically investigated under the gyrokinetic framework with both the m=1 and m=2 harmonics are considered, where m is the poloidal mode number. With the quasi-neutrality condition and Ampere's law, the amplitudes of various poloidal magnetic components are derived. It is shown that both m=1 and m=2 magnetic components exist and are dominated by the cosine and sine components, respectively. In addition, it is found that the amplitudes of all magnetic components increase with respect to the ratio of plasma pressure to magnetic pressure \b{eta} and safety factor q. Most importantly, the amplitude of m=1 magnetic component is significantly enhanced due to the coupling of magnetic drift frequency with the first and second harmonics of the distribution functions, thus it can be comparable to that of m=2 magnetic component under certain conditions.