Transition from resistive ballooning to ηi driven turbulence in tokamaks

The mechanisms of turbulent transport in the collisional tokamak edge plasma are investigated linearly and nonlinearly, focusing specifically on the transition from resistive modes to ion temperature gradient driven (ηi) modes. Linear eigenvalue calculations demonstrate that resistive ballooning and toroidal ηi modes can exist as separate roots with similar growth rates but with differing structure along the magnetic field. While the typical transverse scale length of the resistive modes depends strongly on collisionality, the transverse scales of the ηi mode are essentially independent of the collisionality, even in the absence of any assumption on the adiabaticity of the electrons. Three-dimensional nonlinear simulations quantitatively describe the transition between the resistivity dominated outermost edge and regions with moderately higher temperature where resistive modes are stabilized and the collisionless ηi mode dominates. A significant result is that the ηi modes continue to drive significant particle transport even in regimes where the linear stability analysis indicates the electrons are dominantly adiabatic. The particle transport is driven by the nonadiabatic, long-parallel-wavelength component of the wave spectrum. As in earlier calculations of ηi turbulence, the self-generated poloidal flows are relatively strong, much stronger than in cold ion simulations, and strongly influence the saturation levels and associated transport rates. The implications of the results for understanding edge transport and density and temperature profiles are discussed.