Ye Gong

Numerical studies of collisionless and collisional sheath evolution in plasma source ion implantations

In plasma source ion implantations, a target immersed directly in a uniform plasma is biased with a high negative voltage pulse. Then an expanding plasma sheath forms between the plasma and the solid target surface, and implants ions into the material. In this paper, the evolution of collisionless and collisional sheaths in different geometries, and for various gas pressures, has been numerically studied using a one-dimensional fluid model. Collisions are included in the model through a collisional drag term in the equation of ion motion. Besides the sheath evolution, plots for the average kinetic energy as well as the current density of ion implantations at the target surface are presented. For the collisional sheath, it is found that the final sheath thickness in planar and spherical (or cylindrical) geometries differs from each other significantly. Also, computational results of the collisional sheath in different geometries are in good agreement with those obtained by analytic models.

The trap of particles in plasma sheath

The traps of isolated dust particles under the action of electrostatic, gravitational, ion-drag, and neutral collision forces are investigated near the boundary of a dusty plasma. It is shown that in the lower sheath (above the lower electrode) the particle traps can exist only for the negatively charged particles. The positions of trapped particle are determined by grain size and forces on it and there are upper and lower critical values for the trapped particle. In the upper sheath (under the upper electrode) there is balance force for the positively charged particles but it is not a stable balance. Still negatively charged particles can be trapped in the upper sheath. In addition, it was shown that two sized grains are trapped in the same position.

Analysis of the effect of external gas flow on helical instabilities of arc plasmas

The effect of external gas flow on helical instability of arcs is studied by using an analytical method. The magnetohydrodynamic equations in an electrostatic approximation serve as the starting point of the theory. Using a linear time-dependent perturbation theory, the analytic expression that corresponds to the term of a gas flow stabilizing effect is deduced, and the growth rate of the helical instability is given. It is found that in the short wavelength perturbation case, the effect of external gas flow is large, so it is beneficial for stabilizing arcs. However, its effect is very small for intermediate and long wavelength perturbations. At the same time, numerical results show that the stabilizing effect of the flow on helical instability increases with increasing shear viscosity of external gas flow.

An electrostatic magnetohydrodynamics theory for resistive-viscous helical instabilities of arc discharges

A simplified linear analysis for resistive-viscous magnetic helical instabilities of arc discharges in a cylindrical plasma is developed. Based on a set of electrostatic magnetohydrodynamic (MHD) equations, resistive-viscous m=1 modes with an external axial magnetic field are studied. Explicit analytic results are obtained, from which the growth rate and the stability criterion can be shown, and the electrostatic assumption can be justified. In comparison with the previous channel model calculations, this analytic treatment can provide a simplified model for instability estimates, while avoiding artificial assumptions and misorderings in the energy equation.A simplified linear analysis for resistive-viscous magnetic helical instabilities of arc discharges in a cylindrical plasma is developed. Based on a set of electrostatic magnetohydrodynamic (MHD) equations, resistive-viscous m=1 modes with an external axial magnetic field are studied. Explicit analytic results are obtained, from which the growth rate and the stability criterion can be shown, and the electrostatic assumption can be justified. In comparison with the previous channel model calculations, this analytic treatment can provide a simplified model for instability estimates, while avoiding artificial assumptions and misorderings in the energy equation.

The stabilizing effect of external gas flows on the arc helical instability

The effect of gas flow on the helical instability of arcs is studied by using a linear time dependent perturbation theory in this paper. Based on a set of electrostatic magnetohydrodynamic equations, the corresponding equations and the boundary conditions are derived in the presence of a vortical gas flow, combined by a rotation and an axial component (the rotating flow and the axial flow). The effects of rotating flow, axial flow, and both of them on the arc stability are discussed, respectively. The marginal Maecker’s number and the growth rate of the helical instability are given. Computational results show that the gas flow stabilizes the arc helical instability. In particular, the axial flow (the axial component) plays a more important role in practical applications.

Helical instability of arcs with electrical conductivity of parabolic distribution under electrostatic approximation

Abstract A simplified linear analysis of the helical instability of arc discharges in a cylindrical plasma is given. The starting equations in describing the movement of an arc plasma are the electrostatic magnetohydrodynamic (MHD) equations. The terms ρ m d h/ d t and ∂P/∂t in the energy equation are neglected under the electrostatic approximation. Thus, brief analytic expressions are obtained for the terms of the thermal stabilizing Ω p , the self-magnetic field destabilization Ω s , the external magnetic field destabilization Ω e and the boundary effect Ω b , from which the growth rate and the stability criterion can be shown. In the case of the applied uniform magnetic field, theoretical results are in good agreement with the experimental measurements. At the same time, we have calculated the growth rate of the instability generated by the self-magnetic field. The results are in perfect agreement with those of complicated calculations in previous works.