Electromagnetic drift wave instability in tokamak plasmas with strong pedestal gradient

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.