Linear and Nonlinear Studies of Trapped Electron Mode Turbulence

Linear stability diagrams are presented to clarify the onset of toroidal drift modes, including ITG, and resonant and non-resonant TEMs, as a function of density and temperature gradients. Several hundred linear gyrokinetic stability analysis were performed with the GS2 code\footnote{W. D. Dorland \textit{et al.}, Phys. Rev. Lett. 85 (2000) 5579.} to generate a stability diagram, varying density and temperature gradients around the ``Cyclone Base Case.'' Two separate studies have previously found that zonal flows play very different roles in TEM turbulence. The first,\footnote{D. R. Ernst \textit{et al.} Phys. Plasmas 11(5) (2004) 2637. Also IAEA-CN-149/TH/1-3 (2006).} found that zonal flows play a strong role near threshold, where they produce a nonlinear upshift. The second,\footnote{T. Dannert \textit{et al.} Phys. Plasmas 12 (2005) 072309.} for a case well above threshold, found that zonal flows have little effect on the turbulent saturation level. To better understand this behavior, we are performing a series of nonlinear gyrokinetic simulations to analyze the anisotropy of the turbulent eddies, and the role of zonal flows, as a function of drive.