The dynamics of turbulence-driven E B zonal ows has been systematically studied in fully 3-dimensional gyrokinetic simulations of microturbulence in magnetically-conned toroidal plasmas using recently available massively parallel computers. Linear ow damping simulations exhibit an asymptotic residual ow in agreement with recent analytic calculations. Nonlinear global simulations of instabilities driven by temperature gradients in the ion component of the plasma provide key rst principles results supporting the physics picture that turbulence-driven uctuating E B zonal ows can signicantly reduce turbulent transport.Turbulence shear suppression by E B ows is the most likely mechanism responsible for the transition to various forms of enhanced connement regimes observed in magnetically-conned plasmas [1]. Understanding the physical mechanisms of turbulence suppression processes [2,3] here is what controls the generation of the ows and how strongly the ows aects the turbulent transport which is believed to arise from electrostatic pressuregradient driven instabilities. These highly complex nonlinear phenomena can be most eectively investigated by n umerical experiments. One of the most promising approaches is gyrokinetic particle-in-cell simulation [6] which suppress the rapid gyromotion of a charged particle about the magnetic eld line. By making use of recent advances of new low-noise numerical algorithms and by taking advantage of the exciting opportunities oered by high-end massively parallel computing power, it has been able to reproduce key features of turbulent transport observed at the core of tokamak plasmas.The present n umerical experiments clearly demonstrate that turbulence-driven uctuating E B zonal ows play a crucial role in regulating nonlinear saturation and transport levels. This is in agreement with previous toroidal gyrokinetic and gyrouid (a uid model with kinetic eect) simulations of instabilities driven by iontemperature-gradient (ITG) in a local geometry which follows a magnetic eld line [7{9]. However, previous global gyrokinetic simulations, which treat the whole plasma volume, either did not include [10] or did not observe [11,12] signicant eects of these self-generated ows. Since local simulations are restricted to a uxtube domain with radially periodic boundary conditions and since they rely on the assumption of scale separation between the turbulence and equilibrium proles, the key issues of transport scaling and eects of steep pressure proles in transport barriers can only be eectively studied in global simulations. In this report, nonlinear simulation results from a newly developed global gyrokinetic code [13] yield the important conclusion that turbulencedriven uctuating E B ows can signicantly reduce the anomalous transport. In order to understand this key process, the dynamics of E B ows have been systematically analyzed. Linear ow damping simulations exhibit a time asymptotic residual ow in agreement with a recent analytic calculation [14]. The present nonlinear global simulation...

Results from 3D global gyrokinetic particle simulations of ion temperature gradient driven microturbulence in a toroidal plasma show that the ion thermal transport level in the interior region exhibits significant dependence on the ion-ion collision frequency even in regimes where the instabilities are collisionless. This is identified as arising from the Coulomb collisional damping of turbulence-generated zonal flows.52.25. Fi, 52.35.Ra, 52.65.Tt Understanding the physical mechanism responsible for the turbulent transport observed in magnetized plasmas is crucial for developing techniques to improve confinement. In particular, ion thermal transport in the core region of a tokamak plasma is believed to arise from electrostatic pressure-gradient driven microinstabilities [1]. In most previous studies, ionion collisions have been assumed to have little or no effect on the microinstabilities most likely to be responsible for the ion thermal transport, such as ion-temperature-gradient (ITG) modes. This is because the temperature in present day major tokamak core plasmas is so high that the ion-ion collision frequency is much smaller than the characteristic frequency of the ITG mode (e.g., linear growth rate or nonlinear decorrelation rate, which is of the order of the ion diamagnetic frequency). Consequently, most theory based ion thermal diffusivities do not contain explicit dependence on the ion-ion collisionality [2,3].Current investigations indicate that ion-ion collisions can enhance turbulent transport via Coulomb collisional damping of turbulence-generated ¢ shear flows. These zonal flows [4], which are linearly stable ¼ modes, are nonlinearly driven by the flux-surface-averaged, radially local current modulations and are mainly in the poloidal direction for high aspect ratio devices. The shear decorrelation [5,6] by these small scale flows results in the reduction of turbulence and transport. Since the turbulence is regulated by zonal flows, the turbulent transport can depend on ion-ion collisions which damp poloidal flows through the "neoclassical" effects.In this letter, we report gyrokinetic particle simulation [7] results which show that the ion thermal transport from electrostatic ITG turbulence depends on ion-ion collisions for representative tokamak core plasma parameters using the global gyrokinetic toroidal code (GTC) [8]. The collisionalitydependence of the turbulent transport comes from the neoclassical damping of zonal flows. The fluctuations and transport exhibit bursting behavior with a period corresponding to the collisional damping time of poloidal flows. These results are contrary to the usual assumption that core ion transport is "collisionless". The fact that the change of the ion heat conductivity with collision frequency cannot be attributed to the change in the linear growth rate or mode spectrum places considerable limitations on the applicability of most of the existing transport models that are based on an oversimplified ¾ type mixing length rule.In the experiments, despite the di...

A new type of particle simulation model based on the gyrophase-everageo Vlasov and Poisson equations is presented. The reduced system, in which particle gyrations are removed from the equations of motion while tne finite Larmor radius effects are still preserved, is most suitable for studying low frequency microinstabiIittes in magnetized plasmas. Tte resulting gyrokinetit plasma is intrinsically quasineutral for >," « p 1= p,(T e /Tj) 1/2 ]. Thus, without the troublesome space charge waves in the simulation, we can afford to use much larger time steps (o H At < 1) and grid spacings (AXj/pg < 1) at a much reduced noise level than we would have for conventional particle codes, where °H * ^ki/ k i^D / 'P^c > pe' and k n K< k r F ur t nerrnore > il is feasible to simulate an elongated system (l.,, » L x) with a tnree-djmensional grid using the present model without resorting to the usual mode expansion technique, since there is essentially no restriction on the si2e of Ax,, in a gyrokinetic plasma. The new approach also enables us to further separate the time and spatial scales of the simulation from those associated with global transport through the use of multiple spatial scale expansion. Thus, the model can be a very efficient tool for studying anomalous transport problems related to steady-state drift-wave turbulence in magnetic, confinement devices. It can also be applied to other areas of plasma physics.