Excitation of geodesic acoustic modes by ion temperature gradient modes

Abstract

Geodesic acoustic modes (GAMs) can be excited by mode coupling to primary modes such as the drift waves and ion temperature gradient modes in tokamak plasmas. Recent global numerical simulations of ion temperature gradient modes show excitation of GAMs and indicate that the radial wave-number of the excited GAMs scales inversely as the ion Larmor radius. Also the modes are excited dominantly in the edge region of the device. Furthermore there is a nonlinear downshift of the GAM frequency compared with the predicted linear mode frequency. The mode-coupling analysis presented in this paper provides an explanation of the scaling of the radial wave-number, the preferential excitation of GAMs in the outer half of the plasma and the nonlinear downshift of the frequency of GAMs.