Theory of Weak Ion Temperature Gradient Driven Turbulence Near the Threshold of Instability.

摘要

This paper presents a theory of weak ion temperature gradient driven turbulence near the threshold of instability. The model considers kinetic ions and adiabatic electrons in a sheared slab geometry. Linear theory shows that for /eta//sub th/ < /eta//sub i/ /approx lt/ /eta//sub th/ + (1+1//tau/)L/sub n//L/sub s/ (where /eta//sub th/ = 0.95 is the instability threshold and L/sub n//L/sub s/ << 1) then gamma << omega and a weak turbulence theory applies. The nonlinear wave kinetic equation indicates that ion Compton scattering is the dominant nonlinear saturation process. The wave kinetic equation is reduced to a differential equation for the spectrum, from which it is shown that the energy scatters to the linearly stable low k/sub y/ modes. The resulting spectrum of fluctuation levels (peaked about k/sub /perpendicular// /rho//sub i/ approx. = 1) is much lower than that suggested by naive extrapolation from the strong turbulence regime. The resulting ion thermal conductivity is also extremely low, so that strong ion heating can be expected to drive the ion temperature gradient to a level where this weakly turbulent threshold regime is surpassed. 27 refs., 4 figs., 1 tab. (ERA citation 14:012399)