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

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

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

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

Neutral particle drag on parallel flow shear driven instability

The neutral drag effect on the parallel velocity gradient driven instability (PVG) in the presence of density inhomogeneity is theoretically investigated. The dispersion relation of PVG mode with the effect of the density gradient and neutral particle drag is derived, and its solution is analytically obtained. The neutral particle drag gives rise to the phase shift between parallel flow and electrostatic potential fluctuations and modifies the parallel compression. As a result, the stability of the PVG mode changes. It is found that the neutral particle drag does not only reduce but also enhances the instability. Specifically, near the marginal condition, the neutral particle effect suppresses the density gradient effect, and the parameter region where the PVG mode is linearly unstable significantly expands

Spatio-temporal dynamics of turbulence trapped in geodesic acoustic modes

The spatio-temporal dynamics of turbulence with the interaction of geodesic acoustic modes (GAMs) are investigated, focusing on the phase-space structure of turbulence, where the phase-space consists of real-space and wavenumber-space. Based on the wave-kinetic framework, the coupling equation between the GAM and the turbulence is numerically solved. The turbulence trapped by the GAM velocity field is obtained. Due to the trapping effect, the turbulence intensity increases where the second derivative of the GAM velocity (curvature of the GAM) is negative. While, in the positive-curvature region, the turbulence is suppressed. Since the trapped turbulence propagates with the GAMs, this relationship is sustained spatially and temporally. The dynamics of the turbulence in the wavenumber spectrum are converted in the evolution of the frequency spectrum, and the simulation result is compared with the experimental observation in JFT-2M tokamak, where the similar patterns are obtained. The turbulence trapping effect is a key to understand the spatial structure of the turbulence in the presence of sheared flows

Developments of frequency comb microwave reflectometer for the interchange mode observations in LHD plasma

We have upgraded the multi-channel microwave reflectometer system which uses a frequency comb as a source and measure the distribution of the density fluctuation caused by magneto-hydro dynamics instability. The previous multi-channel system was composed of the Ka-band, and the U-band system has been developed. Currently, the U-band system has eight frequency channels, which are 43.0, 45.0, 47.0, 49.0, 51.0, 53.0, 55.0, and 57.0 GHz, in U-band. Before the installation to the Large Helical Device (LHD), several tests for understanding the system characteristics, which are the phase responsibility, the linearity of output signal, and others, have been carried out. The in situ calibration in LHD has been done for the cross reference. In the neutral beam injected plasma experiments, we can observe the density fluctuation of the interchange mode and obtain the radial distribution of fluctuation amplitude

Structure formation in parallel ion flow and density profiles by cross-ferroic turbulent transport in linear magnetized plasma

In this paper, we show the direct observation of the parallel flow structure and the parallel Reynolds stress in a linear magnetized plasma, in which a cross-ferroic turbulence system is formed [Inagaki et al., Sci. Rep. 6, 22189 (2016)]. It is shown that the parallel Reynolds stress induced by the density gradient driven drift wave is the source of the parallel flow structure. Moreover, the generated parallel flow shear by the parallel Reynolds stress is found to drive the parallel flow shear driven instability D\u27Angelo mode, which coexists with the original drift wave. The excited D\u27Angelo mode induces the inward particle flux, which seems to help in maintaining the peaked density profile