Gyrokinetic simulation of magnetic-island-induced electric potential vortex mode

Ion temperature gradient (ITG)-driven turbulence with embedded static magnetic islands is simulated by utilizing a gyrokinetic theory-based global turbulence transport code (GKNET) in this work. Different from the traditional equilibrium circular magnetic-surface average (EMSA) method, an advanced algorithm that calculates the perturbed magnetic-surface average (PMSA) of the electric potential has been developed to precisely deal with the zonal flow component in a non-circular magnetic surface perturbed by magnetic islands. Simulations show that the electric potential vortex structure inside islands induced by the magnetic islands is usually of odd parity when using the EMSA method. It is found that the odd symmetry vortex can transfer into an even one after a steep zonal flow gradient, i.e. the flow shear has been built in the vicinity of the magnetic islands by adopting the PMSA algorithm. The phase of the potential vortex in the poloidal cross section is coupled with the zonal flow shear. Such an electric potential vortex mode may be of essential importance in wide topics, such as the turbulence spreading across magnetic islands, neoclassical tearing mode physics, and also the interaction dynamics between the micro-turbulence and MHD activities.