Nonlinear Toroidal Mode Coupling: a New Paradigm for Drift Wave Turbulent Spectra in Toroidal Plasmas

Global gyrokinetic particle simulations and nonlinear gyrokinetic theory find that electron temperature gradient (ETG) instability saturates via nonlinear toroidal coupling. In such nonlinear toroidal coupling. In such nonlinear interactions, wave energy at the unstable high toroidal-mode number domain cascades toward the damped lower toroidal-mode number domain via scattering off the driven low-mode number quasi modes. The saturated electrostatic ETC turbulence is dominated by these nonlinearly generated radial streamers and the length of the streamers scales with the device size. Both the fluctuation intensity and transport level are, however, independent of the streamer length, and are smaller than the mixing-length estimates. the nonlinear toroidal coupling mechanisms strictly requires the toroidal geometry and, as such,,, represents a new paradigm for the spectral cascade of plasma turbulence in toroidal systems. (Author)