Turbulent excitation of plasma oscillations in the acoustic frequency range

Abstract

The impact of geodesic curvature on flux-driven electrostatic ion temperature gradient turbulence in the core of tokamak plasmas is studied by means of three-dimensional fluid global numerical simulations. The emphasis is on the dynamics of the axisymmetric fluctuations. The simulations evolve the equilibrium and the perturbed fields as a whole. The coupling of poloidal harmonics induced by the curvature results, on the one hand, in the presence of neoclassical transport, besides the turbulent one, and on the other, in the generation of oscillations in the acoustic frequency range. The neoclassical thermal conductivity is evaluated for the considered isotropic model, and scales as the plateau conductivity. The computed conductivity is shown to agree perfectly with that theoretical estimate. Geodesic acoustic modes (GAMs) are only observed transiently in the simulations. The GAM oscillations are strongly reduced in the final turbulent stationary state. The main peak in the poloidal velocity spectra is observed at a lower frequency. Detailed analysis of the simulations in the turbulent stationary state, in particular by means of a singular value decomposition of the space-time data, shows that a second linear branch of axisymmetric modes, having a frequency somewhat lower than the acoustic one, is more effectively excited by the turbulence. The result is a quasicoherent mode with a radial wavelength somewhat larger than the ion Larmor radius.