Turbulent Transport Reduction by Zonal Flows: Massively Parallel Simulations

Abstract

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...

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