Electromagnetic drift wave instability in tokamak plasmas with strong pedestal gradient

Abstract

The linear eigenmode characterizations and the nonlinear turbulence energy spreading of the drift waves in a tokamak plasma with strong pedestal gradient are numerically investigated based on an electromagnetic Landau fluid model. By the linear eigenmode analysis, it is found that the dominant instability in the low $\beta$ regime is the ion-temperature-gradient (ITG$^c$) mode and the electron drift wave instability (eDWI$^p$) in the core and edge region with strong density gradient, respectively. Multiple eigenstates of the eDWI$^p$ with different peak locations in the poloidal direction can be obtained by the eigenvalue problem solver. The dominant one is the high order eDWI$^p$ corresponding to the unconventional ballooning mode structure with multiple peaks in the poloidal position, in contrast to the conventional modes that peak at the outboard mid-plane, and has been verified through initial value simulation. In the high $\beta$ regime, the dominant eigenmodes in the core and edge region are the conventional and unconventional kinetic ballooning modes respectively. In the nonlinear simulation, an inward turbulence spreading phenomenon during the quasi-saturation phase of the edge turbulence is clearly observed. The inward speed of the turbulence energy front in the high $\beta$ regime is much faster than that in the low $\beta$ regime. It is interestingly found that the speed of the turbulence energy front increases with the increase of the plasma $\beta$ in the low $\beta$ regime, while it is almost unchanged in the high $\beta$ regime. It is identified that the turbulence spreading in the low and high $\beta$ regimes are determined by the nonlinear dynamics and the linear toroidal coupling respectively.

Effect of resonant magnetic perturbation on edge–core turbulence spreading in a tokamak plasma

“…The zero Dirichlet condition is imposed for all perturbations on the outer boundary. The equilibrium profiles of safety factor, density and ion temperature profiles that were used are shown in figure 1, where n eq and T eq are analytical expressions combining the quadratic or exponential function at the core region and the hyperbolic tangent function at the edge, and q is a simple quadratic function [43]. Steep gradients exist in the edge density temperature profiles.…”

“…In the absence of a RMP field, the dominant instabilities are drift wave type instabilities driven by the steep density or temperature profiles, increasing first at the edge region and saturating before the subdominant instabilities increase in the inner region. A detailed analysis of the instabilities is provided in figures 2 and 3 in [43], which shows the dominant electron drift wave type instabilities at the edge as well as the subdominant ion temperature gradient instabilities and kinetic ballooning mode at the core. Figures 2(a) and (b) show the temporal evolution of the kinematic energies and heat flux of the perturbations, respectively.…”

“…In this work, the effect of RMP on the inward spreading of turbulence during pedestal collapse is investigated based on the global electromagnetic Landau-fluid simulation [42], which has been used to simulate the effect of plasma beta on turbulence spreading, as shown in [43]. Unlike electrostatic simulations with an embedded magnetic island or radial electric field, the electromagnetic perturbations are calculated consistently.…”