Turbulent viscosity bifurcation and the H-mode

The quasi-linear perpendicular viscous stress due to kinetic ion temperature gradient (ITG) modes in a sheared slab is shown to possess a maximum at a critical value for the gradient of the equilibrium electric field. This feature is present for a wide range of plasma parameters. As previously demonstrated, the ITG mode is stabilized for sufficiently large electric field gradients. It is shown how the existence of a maximum in the turbulent viscosity of ITG modes can play a role in a bifurcation theory of the H-mode. Such a model has the novel feature that the ITG mode is responsible for both the observed bifurcation in the electric field gradient and the imposed transport as it becomes stabilized.