Research of the interaction among different spatio-temporal scale turbulence and transport structure

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Based on the idea that various prominent structures in thermonuclear fusion plasmas like tokamak are established resulting from complex interaction among different spatio-temporal scale fluctuations, we studied fundamental principles of the interaction in turbulent transport process. The main results are as follows :1) Based on theory and simulation using gyro-fluid model, we clarified the dynamics of zonal flows and its structures produced by nonlinear interaction of micro-scale electron temperature gradient (ETG) turbulence. We investigated a "partition law" between turbulent fluctuation and zonal fluctuation and found that the partition is controlled by magnetic shear. Specifically, we found that in weak and/or reversed magnetic shear plasmas a large portion of fluctuation energy is efficiently transferred to zonal fluctuation in high pressure gradient regime and then anomalousness of turbulent transport disappears.2) We studied the effects of the micro-scale ETG-driven zonal flows … More observed in the above process (1) on macro-scale ion temperature gradient (ITG) turbulent transport and found that an intermittent behavior is exhibited in the heat diffusivity near the ITG critical gradient. This may results from complex interplay among ITG turbulence, ITG-driven zonal flow and also micro-scale ETG-driven zonal flow. The result indicates that treating key physical processes with different spatio-temporal scales in wide wave number and frequency space simultaneously is important.3) In order to clarify the relation between secondary generated large scale convective structure and structure of maternal turbulence, we developed a modulational instability analysis and found that the convective structure is significantly influenced by the structure of turbulence. Namely, zonal flows are preferentially excited from radially elongated turbulence structure, whereas streamers are produced from poloidally elongated turbulence structure. Thus, the secondary large scale structure is found to be controlled by the structure of maternal turbulence.4) In order to characterize the transport dynamics where the turbulent component and zonal component are mixed, we investigated statistical quantities such as probability distribution function (PDF), fractal dimension, wavelet spectrum, bi-coherence, etc. We found that the fractal dimension is significantly reduced in plasmas dominated by zonal flows, suggesting that the suppressed transport is sustained by rather coherent process. Less