Considerations of ion-temperature-gradient-driven turbulence

The ion-temperature-gradient-driven instability is considered in this paper. Physical pictures are presented to clarify the nature of the instability. The saturation of a single eddy is modeled by a simple nonlinear equation. It is shown that eddies that are elongated in the direction of the temperature gradient are the most unstable and have the highest saturation amplitudes. In a sheared magnetic field, such elongated eddies twist with the field lines. This structure is shown to be an alternative to the usual Fourier mode picture in which the mode is localized around the surface where k_∥ = 0. These elongated twisting eddies, which are an integral part of the "ballooning mode" structure, could survive in a torus. The elongated eddies are shown to be unstable to secondary instabilities that are driven by the large gradients in the long eddy. It is argued that the "mixing length" is affected by this nonlinear process, and is unlikely to be a linear eigenmode width.