Magnetic field-aligned (parallel) sheared plasma flows in space, laboratory, and fusion-oriented plasmas have been recognized to be the origin of plasma instabilities and turbulences. We have experimentally demonstrated that the drift-wave, ion-cyclotron, and Kelvin-Helmholtz instabilities are excited and suppressed by the parallel flow shears, where the destabilizing and stabilizing mechanisms are well explained by a plasma kinetic theory. In this experimental investigation, however, it is difficult to change the shape of the flow shear (width, location, and direction) and the plasma parameters such as the ion to electron temperature ratio, which is very effective in the growth rate of the shear-driven instabilities. In order to understand the experimental results in detail and to clarify the essential mechanism of the shear-driven instabilities, we have attempted to perform a particle simulation.
We adopt a three dimensional electrostatic particle simulation with a periodic boundary model in which an external uniform magnetic field directs to the positive z direction. The initial electron velocity distribution is a stationary Maxwellian and the initial ion velocity distribution is a shifted-Maxwellian with the drift speed v_di in the positive z direction. Two cases of the ion flow patterns are used in our present simulation. In the first case, the ion flow is uniform in the system. In the second case, on the other hand, it is non-uniform in the x direction, namely, there exists the parallel flow shear. According to the time evolutions of the spatial Fourier modes, several unstable modes in the range of ion-cyclotron frequency are observed in the presence of the parallel flow shear, while the fluctuations are not observed in any modes in the absence of the shear. Since the spatially averaged velocity of sheared ion flow is the same as the velocity of the uniform ion flow, which can not excite the ion-cyclotron instability, the observed instability is considered to be enhanced by the effects of the parallel flow shear.