Nonlinear zonal dynamics of drift and drift Alfven turbulences in tokamak plasmas

The present work addresses the issue of identifying the major nonlinear physics processes which may regulate drift and drift-Alfven turbulence using a weak turbulence approach. Within this framework, based upon the nonlinear gyrokinetic equation for both electrons and ions, we present an analytic theory for nonlinear zonal dynamics described in terms of two axisymmetric potentials, δφz and δAparallelz, which spatially depend only on a (magnetic) flux coordinate. Spontaneous excitation of zonal flows by electrostatic drift microinstabilities is demonstrated both analytically and by direct 3D gyrokinetic simulations. Direct comparisons indicate good agreement between analytic expressions of the zonal flow growth rate and numerical simulation results for Ion Temperature Gradient (ITG) driven modes. Analogously, we show that zonal flows may be spontaneously excited by drift-Alfven turbulence, in the form of modulational instability of the radial envelope of the mode as well, whereas, in general, excitations of zonal currents are possible but they have little feedback on the turbulence itself. (author)