Geodesic Acoustic Mode in Toroidal Plasma

Geodesic acoustic modes (GAMs) are low frequency modes that were first predicted theoretically to occur in magnetically confined toroidal plasmas and found later in simulations of tokamak edge plasmas. They have been observed in various toroidal devices. Thus, investigating the source of excitation of these modes has been a very active area of research. Recently, we have addressed the issue of nonlinear excitation of these modes by three-wave resonant mode coupling to electron drift waves. We have extended this work to include finite beta effects. The finite beta effects have a strong stabilizing influence on the parametric excitation process. We have also investigated the role of plasma inhomogeneity in the edge plasma on the excitation of these modes. The two space scale analysis and eigenvalue studies shows that there is a short scale structure of the GAMs determined by the resonance condition and an envelope determined predominantly by the drift-wave and ambient inhomogeneity of the edge plasmas. The continuum GAM modes with coupling to the drift waves can create discrete ''global'' unstable eigenmodes localized in the edge''pedestal'' region of the plasma. Multiple resonantly driven unstable radial eigenmodes can coexist on the edge pedestal.