Geodesic acoustic modes excited by finite beta drift waves

Presented in this paper is a mode-coupling analysis for the nonlinear excitation of the geodesic acoustic modes (GAMs) in tokamak plasmas by finite beta drift waves. The finite beta effects give rise to a strong stabilizing influence on the parametric excitation process. The dominant finite beta effect is the combination of the Maxwell stress, which has a tendency to cancel the primary drive from the Reynolds stress, and the finite beta modification of the drift waves.

The zonal magnetic field is also excited at the GAM frequency. However, it does not contribute to the overall stability of the three-wave process for parameters of relevance to the edge region of tokamaks.