Reduced Fluid Model Research Articles

Nonlinear elongation of two-dimensional structures in electron temperature gradient driven turbulence

The nonlinear evolution of the electron temperature gradient (ETG) driven mode can be described with a simple, two-dimensional reduced fluid model, similar to that used for the thermal Rossby wave system. Consistent with ballooning mode structure, primary instability drive with a strong anisotropy in wave number (i.e., ky≫kx) is considered for the inviscid limit of the ETG model. The amplitude equation, describing the initial envelope modulations of this system, is derived using reductive perturbation methods. The dynamics of the intensity field variance in radial and poloidal directions, i.e., the two diagonal elements of the covariance tensor), which follow from the amplitude equation, are investigated in an attempt to determine the basins of attraction for forming zonal flow and streamer secondary structures. It is found that the focusing (or diffracting) effect of Reynolds stress is essentially stronger in the radial direction than it is in the poloidal direction. Further analysis of the structure in the radially elongated limit of the amplitude equation yields interesting results, such as a poloidally localized sheared soliton solution. The approach used here is broadly applicable.

Growth and stabilization of drift-tearing modesin weakly collisional plasmas

In the limit where the electron drift-wave frequency exceeds theelectron-ion collision frequency, drift-tearing modes are found to growwith a linear growth rate independent of resistivity and proportional tothe product of the electron inertial skin depth and the ion sound Larmorradius. The stabilization of these modes in collisionless andsemi-collisional regimes is investigated. The stabilization mechanism isrelated to the coupling and propagation of drift-acoustic perturbationsaway from the reconnecting mode-rational surface. Analytic and numericalsolutions of the four-field reduced fluid model in the slab geometryapproximation with constant electron temperature and negligible iontemperature are presented. The actual stability threshold can occur atvalues of the normalized tearing mode stability parameter Δ' ashigh as 102.\\pacs{52.35.P}}\\fnm{1}{Permanent address: Istituto Nazionale Fisica della Materia, Department of Energetics, Politecnico di Torino, Italy

Nonlinear behavior of multiple-helicity resistive interchange modes near marginally stable states

Nonlinear behavior of resistive interchange modes near marginally stable states is theoretically studied under the multiple-helicity condition. Reduced fluid equations in the sheared slab configuration are used in order to treat a local transport problem. With the use of the invariance property of local reduced fluid model equations under a transformation between the modes with different rational surfaces, weakly nonlinear theories for single-helicity modes by Hamaguchi [Phys. Fluids B 1, 1416 (1989)] and Nakajima [Phys. Fluids B 2, 1170 (1990)] are extended to the multiple-helicity case and applied to the resistive interchange modes. Nonlinear amplitude equations of the multiple-helicity modes are derived, from which the convective transport in the saturated state is obtained. It is shown how the convective transport is enhanced by nonlinear interaction between modes with different rational surfaces compared with the single-helicity case. We confirm that theoretical results are in good agreement with direct numerical simulations.

A generalized reduced fluid model with finite ion-gyroradius effects

Reduced fluid models have become important tools for studying the nonlinear dynamics of plasma in a large aspect-ratio tokamak. A self-consistent nonlinear reduced fluid model, with finite ion-gyroradius effects is presented. The model is distinctive in being correct to O(( ρi/a)2) and in satisfying an exact, relatively simple energy conservation law.