The interaction of turbulence, magnetic islands and zonal fields in fluid plasma models with cubic non-linearities

Abstract

It is shown that magnetic islands generated by pressure-gradient-driven turbulence are common across a wide range of conditions. The interaction among turbulence, magnetic island and other large scale structures (the zonal flow and the zonal current), largely determines the dynamics of the system. Turbulence takes a background role, providing energy to the large-scale structures, without influencing their evolution directly. The growth of the zonal current is linearly related to that of the magnetic island, while the zonal flow has a strongly sheared region where the island has its maximum radial extension. The zonal current is found to slow down the formation of large-scale magnetic islands, while the zonal flow is needed to have the system move its energy to larger and larger scales. The driving instability in the system is the fluid Kinetic Ballooning Mode (KBM) instability at high beta, while the tearing mode is kept stable. The formation of magnetic-island-like structures at the spatial scale of the fluid KBM instability is observed quite early in the non-linear phase for most cases studied, and a slow coalescence process evolves the magnetic structures towards larger and larger scales. Cases that did not show this coalescence process, nor the formation of the small scale island-like structures, were seen to have narrower mode structures for comparable instability growth rates, which was achieved by varying the magnetic shear. The islands often end up exceeding the radial box size late in the non-linear phase, showing unbounded growth. The impact on the pressure profile of turbulence driven magnetic islands is not trivial, showing flattening of the pressure profile only far from the resonance, where the zonal flow is weaker, and the appearance of said flattening is slow, after the island has reached a sufficiently large size, when compared with collisional time scales.