The effects of spatially inhomogeneous equilibrium electric fields on a fully kinetic model of the ion temperature gradient (ITG) mode in a sheared slab magnetic field is investigated. It is found that a gradient in the electric field of either sign, or a positive second derivative, reduces both the growth rate and the transport rates for heat and particles. The electric field structure in the region of a few centimetres inside the separatrix of a DIII-D plasma exhibits a well which is deepened during the transition to the H-mode. This deepening of the electric field well is found to be strongly stabilizing for ITG modes in the experimental range of parameters. This is consistent with the observation of reduced fluctuations in this region.
Self-sustained turbulence and H-mode confinement in toroidal plasmas