T. Kamimura

Dynamics of Two-Dimensional Solitary Vortices in a Low- β Plasma with Convective Motion

Numerical studies of the Hasegawa-Mima equation, derived in the context of drift waves but equivalent to the quasi-geostrophic vortex potential equation for Rossby waves, show the stable properties of solitary vortices which are two dimensional, localized, steady and translating solutions of this same equation. A solitary vortex can propagate only in the direction ( x -direction) perpendicular to the density gradient. When this solitary vortex solution is inclined at some angle with respect to the x -axis, its propagation direction oscillates in the x and y plane. In two dimensional collisions, i.e. head-on collision and overtaking, solitary vortices interact two-dimensionally and recover their initial shapes at the end of both types of collisions.

Compact Helical System physics and engineering design

This paper reports on the Compact Helical System designed for research on transport in a low-aspect-ratio helical system. The machine parameters were chosen on the basis of a physics optimization study. Considerable effort was devoted to reducing error fields from current feeds and crossovers. The final machine parameters are as follows: major radius of 1 m; minor radius of the helical field coil of 0.313 m; plasma aspect ratio A{sub p} = 5; pole number and toroidal period number of the helical field coil of l = 2 and m = 8, respectively; and helical pitch modulation of {alpha}{sup *} = 0.3.

Kelvin-Helmholtz instability and vortices in magnetized plasma

Analytic theory and implicit particle simulations are used to describe the evolution of the Kelvin–Helmholtz instability and the plasma vortices driven by a nonuniform E×B flow velocity. Formulas for plasma convection arising from the self‐consistent plasma electric field give the rate of anomalous momentum transport across the magnetic field. The momentum transport is shown to be controlled by the tilting angle of the elliptical vortex with respect to the direction of the parallel flow. Three stages of evolution are investigated and formulas for the final vortex state are given.

Simulation studies of the collisionless tearing instabilities

Collisionless tearing instabilities and the consequent enhanced transport are investigated theoretically and also numerically, using a two‐and‐one‐half‐dimensional particle simulation code in a slab geometry. The effects of electrostatic fields on the instability are also considered. The initial current is found to diffuse along the perturbed magnetic field lines, the observed diffusion along the perturbed magnetic field lines, the observed diffusion coefficient being in good agreement with the theoretical prediction. The growth of the instability is observed to divide into three phases; a linearly unstable phase, a quasi‐stable phase, and a nonlinear phase similar to the Rutherford phase. Electrostatic effects have a tendency to enhance the tearing mode growth rate. In multi‐mode tearing, a coalescence of magnetic island is observed.

On boundary conditions for a simulation plasma in a magnetic field

Abstract Various methods of treating particles at the boundary walls in a magnetized simulation plasma are studied extensively. Especially, Lee and Okudas ‘reflection scheme’ and the numerical instability caused by this scheme alone are investigated in detail. Methods (Method I, Method II, and the random reflection method), which eliminate the numerical instability, are proposed. They are simple and straightforward, and introduce neither a density gradient nor a surface current next to the walls. Lee and Okudas complete method is stable by virtue of the introduction of a ‘smoothing scheme,’ and our methods provide an alternative.

Kinetic effects on the convective plasma diffusion and the heat transport

Cross field heat and particle transport due to the thermally excited convective cell mode is investigated both by theoretical work and by particle simulations. It is shown that there is a parameter range where the heat diffusion coefficient D H is considerably less than the particle diffusion coefficient D P . This results from the fact that high energy particles in a velocity distribution diffuse much more slowly than the low energy particles because the fluctuating electric fields are averaged over their finite Larmor radii. The magnetic field scaling of D H / D P is also discussed. The results imply that ions in a thermonuclear reactor should be less affected by turbulence than are electrons as appears to be the case.

Stochastic diffusion in the standard map

Abstract A numerical observation of stochastic diffusion in the standard map has been carried out in the domain of small stochastic parameter A c A A c is given as (2 π ) −1 ×0.9716 = 0.1546. Multiple periodic accelerator modes manifest their pronounced sharp resonant effect in the stochastic diffusion process even in the region of small stochastic parameter, where the fundamental accelerator modes are not allowed to exist.

Double layer formation caused by contact between different temperature plasmas

The formation of an electrical potential difference between hot and cold plasmas is studied by means of a particle simulation. It is found that a double layer structure is formed on the hot‐plasma side of the contact surface between the cold and hot plasmas. The potential difference is given approximately by φDL∼Teh/2e, where Teh and e are the hot‐electron temperature and the electronic charge, respectively. The double layer is accompanied by a negative potential dip on the low potential side (cold plasma) of the double layer, the depth of which is φdip∼2Tec/e, where Tec is the cold‐electron temperature. Interestingly, however, the positive potential difference and the negative potential dip are created independently.

Laminar electrostatic shock waves generated by an ion beam

Strong laminar electrostatic shock waves have been experimentally observed when an ion beam is injected into a collisionless plasma. The structure of the shock is qualitatively different from one with a trailing wave train. A density depression follows behind the shock front, and no trailing wave train due to wave dispersion is found. A significant amount of ions reflected from and transmitted through the shock front form a precursor. The critical Mach number above which no shock is formed is found to be 1.5. Numerical simulations reported here reproduce the experimental observations very well. An analysis based on the water‐bag model accounts for the observed value of the critical ion‐beam velocity which gives the critical Mach number. It also points out that the reflected ions play an essential role in the persistence of the shock.

Implicit particle simulation of electromagnetic plasma phenomena

A direct method for the implicit particle simulation of electromagnetic phenomena in magnetized, multi-dimensional plasmas is developed. The method is second-order accurate for ω Δt < 1, with ω a characteristic frequency and time step Δt. Direct time integration of the implicit equations with simplified space differencing allows the consistent inclusion of finite particle size. Decentered time differencing of the Lorentz force permits the simulation of strongly magnetized plasmas in the limit of zero perpendicular temperature. A Fourier-space iterative technique for solving the implicit field corrector equation, based on the separation of plasma responses perpendicular and parallel to the magnetic field and longitudinal and transverse to the wavevector, is described. Wave propagation properties in a uniform plasma are in excellent agreement with theoretical expectations. Applications to collisionless tearing and coalescence instabilities further demonstrate the usefulness of the algorithm.