Global gyrokinetic simulations of the impact of magnetic island on ion temperature gradient driven turbulence*

The effect of island width on the multi-scale interactions between magnetic island (MI) and ion temperature gradient (ITG) turbulence has been investigated based on the global gyrokinetic approach[1]. It is found that the coupling between the island and turbulence is enhanced when the MI width (w) becomes larger[2,3]. A vortex flow whose amplitude depends sensitively on the island width, can be excited, which will finally lead to a strong  shear flow thus a decrease of the turbulence transport. The shearing rate induced by the vortex flow is minimum at the O-point while it is maximum at both of the two reconnection points of the island, i.e., the X-points, regardless of the island width. There exists a nonmonotonic relationship between zonal flow (ZF) amplitude and island width, showing that the ZF is partially suppressed by medium-sized MIs whereas enhanced in the case of large island. A larger MI can tremendously damage the ITG mode structure, resulting in higher turbulent transport at the X-point whereas a lower one at the O-point, respectively. Such phenomenon will be less distinct at very small island widths below w/a ~ 8% (a is the minor radius), where it shows that turbulence near the X-point is hardly affected although it is still suppressed inside the island. Furthermore, the influence of different island sizes on turbulence transport level is also discussed. [4]