Recent progress on turbulence and multi-scale interactions in tokamak plasmas

Studies on nonlinear interactions and multi-scale physics are crucial in the physical understanding of the turbulence and transport in the complex nonlinear system of fusion plasmas. This paper reviews recent studies on the turbulence and the related transport. The prominent results are mainly from tokamaks, such as HL-2A, Tore Supra and J-TEXT. First, turbulence behaviors and its role in transport will be introduced. Critical gradients in heat and particle transports have been observed and reproduced by simulations. Turbulence modes and its role in the transport are identified using advanced diagnostics. The measured turbulence transition is consistent with the observation of particle convection reversal and the simulation prediction. In addition, the excitation of an electromagnetic turbulence by either edge self-accumulated or externally seeded impurities has been observed. The impurity-driven turbulence plays an essential role in regulating the impurity transport. Then, the effect of plasma flows on turbulence and multi-scale interactions (micro-turbulence, meso-scale fluctuations, and large scale MHD activities) will be presented. Moreover, a new meso-scale electrostatic fluctuation has been observed. The results indicate that geodesic acoustic modes and magnetic fluctuations can transfer energy through nonlinear synchronization. At last, the enhancement of nonlinear interactions by external actuators will be reported. Theoretical studies in comparisons with experimental findings will be discussed.