Analytical study on magnetic component of geodesic acoustic mode

Abstract

Abstract To understand the role of geodesic acoustic modes (GAMs) in regulating the transport by interacting with background electromagnetic turbulence, a thorough theoretical understanding of the magnetic components of the mode is required. In this study, the magnetic components of 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 Ampère’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 β 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. The various magnetic components derived in this study may provide some guidance for experimental measurements.