Toroidal ion temperature gradient driven weak turbulence

The theory of ion temperature gradient driven turbulence ('ηi turbulence') is able to explain many qualitative features of tokamak transport, but there has not yet been accurate quantitative agreement with experimental measurements. This is perhaps because no single theory to date has combined the complexities due both to geometry (e.g., toroidicity or trapped particles) and to nonlinearity (e.g., the effects of higher eigenmodes or kinetic vs. fluid saturation). This work is part of an attempt to obtain better agreement with experiments by extending certain nonlinear studies from slab to toroidal geometry. Here, we study toroidal ηi turbulence near the stability theshold (recently explored in sheared slab geometry). The threshold regime is characterized by weak turbulence (since the growth rate, γ, is smaller than the frequency, ωr) and strong ion resonances (both nonlinear, in the form of ion Compton scattering, and linear). Although this analysis is strictly valid only for a narrow range of ηi near the threshold, the results allow assumptions about the influence of Compton scattering over a wide range of ηi. (For example, in the slab model of the instability, it can be argued that the strong nonlinear damping from Compton scattering provides a cutoff for the higher radial eigenmodes, giving a significant correction to the transport coefficient at all ηi.) (author) 8 refs