Nonlinear interaction of radial eigenmodes in ITG turbulence

Analytical and numerical calculations indicate there exists a spectrum of unstable radial ion temperature gradient driven (ηi) modes. Also studies reveal that background electric fields provide a mechanism for coupling among different radial modes. However, such radial nonlinear interaction has not been properly treated, leading to deficiencies in past work. For example, in the conventional approach of treating the renormalized turbulent diffusivities as the eigenvalues of the nonlinear eigenmode equation, there emerges the dubious artifact of turbulent diffusivities corresponding to different radial eigenmodes. By treating radial modes at different helicities as a turbulent background, and considering the interaction of different radial modes on the same rational surface, the authors calculate the necessary l-mode coupling spectral flow in l space, allowing saturation of all radial modes. The saturation level is then calculated, and compared with the mixing length theory estimates. 3 refs