Characteristics of trapped electron transport, zonal flow staircase, turbulence fluctuation spectra in elongated tokamak plasmas

Abstract

The effects of plasma elongation on trapped electron mode (TEM) driven turbulence and transport are investigated with detailed analyses of fluctuation, zonal flow and zonal pressure. For the study, global nonlinear TEM simulations with different elongations (kappa) are performed by employing a bounce-averaged gyrokinetic code gKPSP. From the simulations, kappa-scalings of the electron heat conductivity (chi(e)), ion heat conductivity (chi(i)), and particle diffusivity (D) are found as chi(e) proportional to kappa(-0.8), chi(i) proportional to kappa(-1.4) and D proportional to kappa(-1.4), respectively. Higher plasma elongation is found to enhance zonal flows and their shearing rate at short radial scales, which in turn reduce the radial correlation length of turbulence and limit avalanche-like large scale transport events. Radial corrugations of density and temperature profiles also emerge due to quasi-stationary zonal flows at short spatial scales. In addition, it is found that the Doppler shift caused by the quasi-stationary zonal flows can modify the phase velocity of fluctuation in the laboratory frame and result in turbulence spectra consisting of components propagating in both electron and ion diamagnetic directions, while the transport is mainly governed by TEM.