Electric field inhomogeneities and ion temperature gradient modes in a sheared slab

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. The electric field structure in the region a few centimeters inside the magnetic separatrix of a DIII-D plasma exhibits a well which is deepened during the transition to H-mode. Both first and second derivatives of the electric field are needed in order to model the electric well structure throughout the region of suppressed fluctuations. It is found that a gradient of 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 combined effect of a deepened electric well is found to be strongly stabilizing for ITG modes in the experimental range of parameters for H-mode. The stabilizing effect is primarily attributed to changes in the Landau damping by the ion kinetic response function. The kinetic electron response has a slightly destabilizing influence for small electric field gradients. These slab model calculations demonstrate that inhomogeneous electric fields can reduce the growth rates obtained from linear stability theory for ITG modes, and add support to the conjecture that electric field changes at the L/H transition suppress turbulent transport in a narrow zone near the plasma edge. The mechanism of turbulence suppression is at the level of linear stability rather than the saturated turbulence decorrelation of Biglari, Diamond, and Terry. 2 figs