Multiscale interactions between the tearing mode (TM) and ion temperature gradient (ITG) turbulence are studied numerically using a self-consistent gyrofluid model in slab geometry. It is found that the multiscale system goes through five distinct phases and is then saturated in a dynamic quasi-steady state. During the nonlinear evolution, the macroscale TM and the microscale ITG turbulence can mutually destabilize each other. On the one hand, the fluctuation level of the turbulence is greatly raised when the magnetic island grows beyond a threshold. The contributions of different scale fluctuations to heat conductivity are calculated. Although the macroscale long wavelength TM plays a dominant role in inducing heat transport in comparison with micro turbulence, the secondary harmonics of the TM have a considerable effect on causing heat pinch. On the other hand, the island growth is significantly enhanced through increasing the ITG as the island width increases above a critical value or the island propagating velocity is reduced below a critical value. The underlying mechanisms of the mutual destabilizations are identified. In addition, the generation of zonal flows and the associated turbulent transport in the multiscale interaction process are analysed in detail.

Effect of shear flow on the multi-scale nonlinear interaction in plasmas is numerically investigated by using a self-consistent Landau-fluid model. Dual roles of shear flow in the process are discovered, significantly suppressing micro-scale fluctuations and dramatically promoting macro-scale fluctuations. Furthermore, its similar dual roles in turbulent transport are also demonstrated. The novel underlying mechanism for the nonlinear promotion is identified as the formation of a large vortex flow inside magnetic island, which as a common phenomenon have been often observed in space and magnetic fusion plasmas. The theoretical prediction on the threshold of shear flow based on an analytical modeling is verified via numerical simulations.

Zonal flow behaviour and its effect on turbulent transport in tokamak plasmas with reversed magnetic shear are investigated by global fluid simulations of electrostatic ion temperature gradient driven turbulence. It is found that for high q (safety factor) turbulent transport is high even at a minimum-q region because oscillatory zonal flows called geodesic acoustic modes (GAMs) are dominant. The turbulent transport is reduced in the neighbourhood of a minimum-q surface when q is low enough to damp the GAMs. The difference in zonal flow behaviour causes a difference in turbulent transport and may trigger the formation of ion internal transport barriers in reversed shear tokamaks.

Characteristics of ETG-driven turbulence dominated by zonal flows (ZFs) and nonlinearly generated large scale structures with low toroidal/poloidal wave numbers are investigated by gyrofluid simulations in slab geometry. Main results found in this research are as follows. (i) The electron transport analysis in various parameter regions shows that the electron heat flux was not necessarily enhanced as the electron temperature gradient increases. (ii) The ZFs change the characteristics of turbulence from a ‘homogeneous’ structure to an ‘inhomogeneous’ one, in which micro-scale vortices and nonlinearly generated macro-scale vortices coexist in different radial zones, exhibiting a two-scale nature in turbulence. (iii) The fractal dimension is simultaneously reduced in any radial region with an increase in the ratio of ZF energy to that of total fluctuations. Namely, macro-scale vortices, which are the origin of low fractal dimension and play a role in saturating ZFs, survive in the system without causing large thermal transport by adjusting the phase relation.

No abstract