Development of drift-resistive-inertial ballooning transport model for tokamak edge plasmas

Abstract

A new model is developed for transport driven by drift-resistive-inertial ballooning modes (DRIBMs) in axisymmetric tokamak plasmas. The model is derived using two-fluid reduced Braginskii equations in a generalized ŝ−α geometry. The unified theory includes diamagnetic effects, parallel electron and ion dynamics, electron inertia, magnetic perturbations, transverse particle diffusion, gyroviscous stress terms, electron and ion equilibrium temperature gradients, and temperature perturbations. A mixing length approximation is used to compute electron and ion thermal transport as well as particle fluxes from eigenvalues and eigenvectors of the linearized equations. The prediction for the saturation level is obtained by balancing the DRIBM growth rate against the nonlinear E×B convection. The parametric dependence of DRIBMs is investigated in systematic scans over density gradient, electron and ion temperature gradients, magnetic-q, collision frequency, magnetic shear, and Larmor radius. The DRIBM threshold is determined as a function of the collision frequency and the density and temperature gradients. The transition from DRIBM to the toroidal ion temperature gradient (ITG) mode is observed for small density gradients. For steep density gradients, the effect of ITG on DRIBM is found to be stabilizing. The transport seen in the DRIBM is of a scale that is consistent with the observed experimental anomalous transport, suggesting that the DRIBM mode could be the principle agent responsible for edge transport in Ohmic and L-mode discharges.