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

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

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

Relaxation of Coaxial Nonneutral Magnetized Plasmas

Abstract A variational principle is applied to the relaxation of pure electron plasma in a strong axial magnetic field. The initial cylindrical shell structure of electrons can be unstable against Kelvin Helmholtz instability, and the plasma shape relaxes to its final state having a diffused profile. The shape of the plasma distribution in the final state is given based upon the anzats of the minimum enstrophy, and an experimentally-testable formula is obtained

Propagation direction of geodesic acoustic modes driven by drift wave turbulence

Selection rule of the radial propagation direction of geodesic acoustic modes (GAMs) is investigated. Here, we study the influence of nonlinear coupling with drift wave turbulence on the propagation direction of GAMs. Based on wave-kinetic equation for the turbulence, the phase-space dynamics is numerically solved and the nonlinear saturated states are obtained, where the phase-space consists of the real space and the wavenumber space. A wave pattern of the GAM in a nonlinear saturated state varies to form a standing wave, outward and inward propagating waves, depending on the peak radial wavenumber of the turbulence. The impact of nonlinear coupling with turbulence is discussed by deriving the GAM dispersion relation that includes the effect of the turbulence

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

Nonlinear Excitation of Subcritical Instabilities in a Toroidal Plasma

In a collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase space. However, nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave-wave coupling can provide a seed, which is significantly below the threshold, but can still grow by (and only by) the collaboration of fluid and kinetic nonlinearities. By modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave-wave coupling equations, it is shown that this new kind of subcritical instability can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. The model is applied to the bursty onset of geodesic acoustic modes in a LHD experiment. The model recovers several key features such as relative amplitude, time scales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, driven by this mechanism of combined fluid and kinetic nonlinearities

Calibration of Setting of Mach Probes by Observing GAM Oscillations

The influence of relative displacement of Mach probe (which is placed near the top of magnetic surface) on the interference of signals is discussed. An error can arise in measured value of poloidal electric field. The Mach number perturbation at the GAM frequency has an interference from the density perturbation. The interference from the density perturbation can propagate to all of Mach number measurement. By observing the signals associated with GAM oscillations, the error in setting the probe arrays can be detected. This result can be applied to correct the positioning of probes