Search

It is argued that the usual understanding of the suppression of radial turbulent transport across a sheared zonal flow based on a reduction in effective transport coefficients is, by itself, incomplete. By means of toroidal gyrokinetic simulations of electrostatic, ion-temperature-gradient turbulence, it is found instead that the character of the radial transport is altered fundamentally by the presence of a sheared zonal flow, changing from diffusive to anticorrelated and subdiffusive. Furthermore, if the flows are self-consistently driven by the turbulence via the Reynolds stresses (in contrast to being induced externally), radial transport becomes non-Gaussian as well. These results warrant a reevaluation of the traditional description of radial transport across sheared flows in tokamaks via effective transport coefficients, suggesting that such description is oversimplified and poorly captures the underlying dynamics, which may in turn compromise its predictive capabilities. [ABSTRACT FROM AUTHOR]

Because experiment/model comparisons in magnetic confinement fusion have not yet satisfied the requirements for validation as understood broadly, approaches to validating mathematical models and numerical algorithms are recommended as good practices. Previously identified procedures, such as, verification, qualification, and analysis of errors from uncertainties and deficiencies, remain important. However, particular challenges intrinsic to fusion plasmas and physical measurement therein lead to identification of new or less familiar concepts that are also critical in validation. These include the primacy hierarchy, which tracks the integration of measurable quantities, and sensitivity analysis, which assesses how model output is apportioned to different sources of variation. The use of validation metrics for individual measurements is extended to multiple measurements, with provisions for the primacy hierarchy and sensitivity. This composite validation metric is essential for quantitatively evaluating comparisons with experiments. To mount successful and credible validation in magnetic fusion, a new culture of validation is envisaged. [ABSTRACT FROM AUTHOR]

A global normal mode stability analysis on the effect of radially sheared azimuthal flow and radially sheared magnetic fields on magnetohydrodynamic (MHD) instabilities in Z-pinch plasmas is presented. A linearized set of ideal MHD equations allows the investigation of plasmas with both magnetic shear and flow shear included in the plasma equilibrium. The stabilizing effects of sheared azimuthal flow and sheared axial magnetic field are presented. Here we show that radial shear in the functional form of a vortex is particularly efficient at reducing instability growth rates. [ABSTRACT FROM AUTHOR]

The development of global magnetohydrodynamic (MHD) instabilities in Z-pinch plasmas has been studied with a three-dimensional hybrid simulation model. Plasma equilibria without and with axial sheared flow, and with different values of the parameter ϵH which appears as a coefficient before the Hall term in dimensionless nonideal MHD equations, have been considered. Increasing the parameter ϵH leads to larger simulation growth rates for both in =0 sausage and in = 1 kink modes. The hybrid simulations do however show that axial sheared flow severely curtails the linear and nonlinear development of both sausage and kink instabilities. In these respects, the hybrid simulations are in qualitative agreement with linear Hall MHD results. Moreover, in the nonlinear stage, long wavelength modes dominate the excited wave spectrum when the parameter ∊H is small. For the larger value of the parameter ϵH, small-scale structures do however develop nonlinearly in the excited wave spectrum at late times. [ABSTRACT FROM AUTHOR]

The effect of externally imposed sheared toroidal rotation on ion temperature gradient turbulence (ITG) and magnetohydrodynamic (MHD) stability in a large aspect ratio tokamak has been studied using nonlinear gyrokinetic particle-in-cell and linear reduced MHD computer calculations. Compared to sheared poloidal rotation, it is found that more sheared toroidal rotation is needed to reduce or suppress ITG turbulence and sheared toroidal rotation appears to always be linearly destabilizing for the paradigmatic resistive internal kink MHD mode studied. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]