Simulation of resistive drift wave turbulence in a linear device

The three-field reduced magnetohydrodynamic (MHD) model is extended to describe the resistive drift wave turbulence in a linear device. Using this model, the linear eigenmode analysis has been performed to identify the unstable modes, which give an estimation of a necessary condition for the turbulence excitation in the Larger Mirror Device designed by Kyushu University. The parameter scan predicts the experimental condition for the excitation of the resistive drift wave turbulence. It is found that ion?neutral collision strongly stabilizes the resistive drift wave. A nonlinear simulation has also been performed to examine the saturation amplitude of the resistive drift wave turbulence

Self-sustained annihilation of magnetic islands in helical plasmas

The evolution of the magnetic island which is induced by the resonant deformation by external currents in helical systems (such as the large helical device (LHD) [A. Iiyoshi, Phys. Plasmas 2, 2349 (1995)]) is analyzed. The defect of the bootstrap current, caused by the magnetic island, has a parity which reduces the size of the magnetic island, if the bootstrap current enhances the vacuum rotational transform. The width of magnetic island can be suppressed to the level of ion banana width if the pressure gradient exceeds a threshold value. This island annihilation is self-sustained. That is, the annihilation continues, for fixed beta value, until the external drive for island generation exceeds a threshold. The effects of the reversal of the direction of the bootstrap current and of the sign of radial electric field are also investigated. The possibility of the neoclassical tearing mode in the LHD-like plasma is discussed

Stability of Externally Driven Magnetic Islands in a Helical Plasma

Influence of external resonant magnetic perturbation (RMP) on a helical plasma is numerically investigated, using a set of reduced magnetohydrodynamic equations. Coexistence of the resistive interchange mode and RMP is simulated. In nonlinear simulations, saturated magnetic islands by the RMP typically show two states: oscillating small islands and locked large islands. In the former state, rotation of magnetic islands by neoclassical transport-driven poloidal flows disturbs growth of islands. On the other hand, in the latter state, locking of poloidal flows due to the RMP and growth of islands occur simultaneously. It is found that the curvature driven current enhances magnetic reconnection, and width of the large islands overcomes that of vacuum islands

A mass-energy-conserving discontinuous Galerkin scheme for the isotropic multispecies Rosenbluth--Fokker--Planck equation

Structure-preserving discretization of the Rosenbluth-Fokker-Planck equation is still an open question especially for unlike-particle collision. In this paper, a mass-energy-conserving isotropic Rosenbluth-Fokker-Planck scheme is introduced. The structure related to the energy conservation is skew-symmetry in mathematical sense, and the action-reaction law in physical sense. A thermal relaxation term is obtained by using integration-by-parts on a volume integral of the energy moment equation, so the discontinuous Galerkin method is selected to preserve the skew-symmetry. The discontinuous Galerkin method enables ones to introduce the nonlinear upwind flux without violating the conservation laws. Some experiments show that the conservative scheme maintains the mass-energy-conservation only with round-off errors, and analytic equilibria are reproduced only with truncation errors of its formal accuracy

Selective formation of turbulent structures in magnetized cylindrical plasmas

The mechanism of nonlinear structural formation has been studied with a three-field reduced fluid model, which is extended to describe the resistive drift wave turbulence in magnetized cylindrical plasmas. In this model, ion-neutral collisions strongly stabilize the resistive drift wave, and the formed structure depends on the collision frequency. If the collision frequency is small, modulational coupling of unstable modes generates a zonal flow. On the other hand, if the collision frequency is large, a streamer, which is a localized vortex in the azimuthal direction, is formed. The structure is generated by nonlinear wave coupling and is sustained for a much longer duration than the drift wave oscillation period. This is a minimal model for analyzing the turbulent structural formation mechanism by mode coupling in cylindrical plasmas, and the competitive nature of structural formation is revealed. These turbulent structures affect particle transport

Drift reduced Landau fluid model for magnetized plasma turbulence simulations in BOUT++ framework

Recently the drift-reduced Landau fluid six-field turbulence model within the BOUT++ framework has been upgraded. In particular, this new model employs a new normalization, adds a volumetric flux-driven source option, the Landau fluid closure for parallel heat flux and a Laplacian inversion solver which is able to capture n=0 axisymmetric mode evolution in realistic tokamak configurations. These improvements substantially extended model's capability to study a wider range of tokamak edge phenomena, and are essential to build a fully self-consistent edge turbulence model capable of both transient (e.g., ELM, disruption) and transport time-scale simulations.Comment: 26 pages, 14 figure

Flow field control to mitigate airborne sea salt adhesion on bridge girders

In order to realise effective maintenance and enhanced durability of structures, it is important to also reduce corrosion of bridges by airborne sea salt. The objective of this study is to reduce airborne sea salt adhesion amount on steel girder bridges by employing aerodynamic countermeasures. The study bridge is a typical metropolitan highway bridge with 8 I-shaped steel girders located in Japan. Aerodynamic countermeasure devices are employed to change the flow field around the bridge structure in an attempt to reduce wind velocity normal to the bridge girders. Devices existing on urban bridges such as noise barriers, median barriers, and facilities for passage of drainage pipes and electric cables, modelled as horizontal plates, are modified and investigated for their ability to reduce airborne sea salt adhesion amount. As additional devices, vertical plates are installed to change the flow separation and their applicability is also studied. Computational fluid dynamics is employed for flow field simulations and airborne sea salt adhesion amount is estimated by the improved concentration flux method. Findings indicate that horizontal plates and vertical plates significantly reduce airborne sea salt adhesion amount. Noise barriers and median barriers can also reduce airborne sea salt adhesion amount