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

TL;DR

This study uses global gyrokinetic simulations to explore how magnetic island width influences turbulence and transport in plasma, revealing complex interactions including vortex flows, shear flows, and varying zonal flow responses.

Contribution

It provides new insights into the multi-scale interactions between magnetic islands and ITG turbulence, highlighting the effects of island size on turbulence suppression and transport.

Findings

Enhanced coupling between island and turbulence with larger island width

Vortex flow triggers shear flows that reduce turbulent transport

Nonmonotonic zonal flow response to island size

Abstract

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. It is found that the coupling between the island and turbulence is enhanced when the MI width (w) becomes larger. A vortex flow that is highly sensitive to the width of the magnetic island can be triggered, ultimately resulting in a potent shear flow and a consequent reduction in turbulent 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.