Two-Dimensional Structure and Particle Pinch in Tokamak H Mode

Two-dimensional structures of the electrostatic potential, density, and flow velocity near the edge of a tokamak plasma are investigated. The model includes the nonlinearity in bulk-ion viscosity and turbulence-driven shear viscosity. For the case with the strong radial electric field (H mode), a two-dimensional structure in a transport barrier is obtained, giving a poloidal shock with a solitary radial electric field profile. The inward particle pinch is induced from this poloidal asymmetric electric field, and increases as the radial electric field becomes stronger. The abrupt increase of this inward ion and electron flux at the onset of L- to H-mode transition explains the rapid establishment of the density pedestal, which is responsible for the observed spontaneous self-reorganization into an improved confinement regime

Two-Dimensional Model Including the Mechanism of a Poloidal Shock Structure and Geodesic Acoustic Mode in Toroidal Plasmas

In H-mode plasmas, two-dimensional steep structures of the electrostatic potential and density are formed when a large poloidal flow exists, whose formation mechanism has been studied to obtain a quantitative understanding of the particle transport in H-mode transport barriers. The previous two-dimensional model is extended to investigate parallel flow dynamics when potential and density distributions do not satisfy the Boltzmann relation. The extended model includes the generation mechanism of a poloidal shock structure and geodesic acoustic mode, whose competitive formation can be studied

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

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

Accessibility to a Double-Peaked Er Shear Layer Structure by Double Electrode Biasing in Tokamak Plasmas

Bifurcation of the radial electric field in the tokamak edge, which is induced by electrode biasing, is studied. A case of multiple electrodes is investigated in order to obtain a structure of multiple peaks in the radial electric field. It is found that a double-peaked structure is accessible with an applied voltage rampup, allowing the possibility of obtaining double transport barriers

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

Transition Probability to Turbulent Transport Regime

Transition phenomena between thermal noise state and turbulent state observed in a submarginal turbulent plasma are analyzed with statistical theory. Time-development of turbulent fluctuation is obtained by numerical simulations of Langevin equation which contains hysteresis characteristics. Transition rates between two states are analyzed. Transition from turbulent state to thermal noise state occurs in entire region between subcritical bifurcation point and linear stability boundary.Comment: 9 pages, 6 figures, to be published in Plasma Phys. Control. Fusio

Theoretical Analysis of Transport Barriers in Helical Systems

lntroduction The Abstract A set of one-dimensional model equation in helical system is analyzed including the electric field bifurcation. The spatial and temporal evolutions of the temperature and the electric field are examined. A spatial structure which is related with the edge transport barrier is duscussed. A self-generated oscillation of the edge temperature and the heat flux loss takes place under the constant heat flux from the core. The oscillation occurs near the transition boundary. The transport barrier in the inner region is found in the high confinement state