Gyrokinetic Simulations of the Nonlinear Upshift of the Critical Density Gradient for TEM Turbulence in Tokamak Fusion Plasmas

The effect of collisionality on a new nonlinear upshift of the critical density gradient for onset of Trapped Electron Mode (TEM) turbulence is investigated in detail. Both linear and nonlinear, high resolution simulations were performed on massively parallel computers using the gyrokinetic code, GS2. The TEM nonlinear upshift is analogous to the Dimits Shift for ion temperature gradient driven (ITG) turbulence, but exists in the density gradient as opposed to the temperature gradient. In the ITG case, increasing ion-ion collisions damp the zonal flows but have little effect on the linear growth rate. In contrast, electron-ion collisions strongly damp the TEM growth rate, while ion-ion collisions weakly damp zonal flows, causing an increase in the TEM upshift. Numerous simulations were run, scanning different density gradients to determine the critical density gradients for each collisionality and to examine the upshift caused by increasing collisionality. The linear critical density gradient was not significantly affected by collisionality, while both critical density gradients were determined to be larger for the nonlinear runs.