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The nonlinear interaction between drift turbulence and zonal flows plays a key role in determining the turbulence amplitude, spatiotemporal scale and subsequent transport and is therefore of significant interest in magnetic fusion research. We summarize multipoint time-domain probe studies, frequency-domain studies, and direct visualization studies of this important process. The fluctuation-induced Reynolds stress is shown to be consistent with the observed shear flow; spatially resolved direct measurement of nonlinear exchange of energy between drift fluctuations and zonal flows and direct imaging show how the drift instability generates isotropic vortex structures which then propagate outwards towards the zonal flow shear layer, where they are stretched and thinned, and then incorporated into the shear layer, thereby maintaining it against damping mechanisms. The drift fluctuation-zonal flow nonlinear interactions, shearing rate, and cross-field transport are consistent with a critical gradient transport model. [ABSTRACT FROM AUTHOR]

This work is divided into two different lines of research. The first one is devoted to the gyrokinetic simulations of ITG linear instabilities and ZF-GAM damping in multiple-helicity magnetic field configurations. The multiple helicity terms are required to study inward-shifted scenarios in LHD and are responsible for the improvement of the neoclassical helical ripple transport. It is thought that they also enhance the residual ZF in these inward shifted scenarios, thus reducing also the anomalous transport. These studies are to be the starting point of full non-linear simulations. In the second part, numerical simulations of Electron Bernstein Waves (EBW) are performed in order to justify the experimentally observed increase of the stored energy in the Compact Helical System (CHS) in NIFS, when EC waves are launched into a comparatively over-dense plasma. Calculations of ray-tracing, mode-conversion and power deposition of OXB converted waves are presented. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Collisionless time evolution of zonal flows in ion and electron temperature gradient turbulence in toroidal plasmas is investigated. The responses of the zonal-flow potential to the initial perturbation and to the turbulence source are determined from the gyrokinetic equations combined with the Poisson equation, A novel gyrofluid model is presented, which properly describes the zonal-flow time evolution and reproduces the same residual zonal-flow levels as predicted by the gyrokinetic model. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]