Effect of collision parameter on magnetized electronegative plasma sheath structure

Abstract

The structure of an electronegative plasma sheath in an oblique magnetic field is investigated. Moreover, the collisions between positive ions and neutral particles are taken into account. It is assumed that the system consists of hot electrons, hot negative ions, and cold positive ions. Also the negative ions and the electrons are assumed to be described by the Boltzmann distributions of their own temperatures, and the accelerated positive ions are treated by the continuity and momentum balance equations through the sheath region. In addition, it is assumed that the collision cross section has a power law dependence on the positive velocity. After theoretical derivation, an exact expression of sheath criterion is obtained. The numerical simulation results include the density distributions of the positive ions for different invariable ion Mach numbers satisfying Bohm criterion, and the comparison of net space charge distribution between variable and invariable ion Mach numbers. Furthermore, three kinds of charged particle densities, the net space charges, and the spatial electric potentials in the sheath are studied numerically for different collision parameters under the condition of the fixed ion Mach number. The results show that the ion Mach number has not only the lower limit but also the upper limit. The ion Mach number affects the sheath structure by influencing the distribution of the positive ion density, and different conclusions can be obtained because ion Mach number is adopted as variable or invariable value when discussing the effects of the other variables which can result in a variety of the ion Mach numbers on the sheath formation. The reason is that the actual sheath structure modification brought on by the variation of a parameter can be divided into two parts. One is the sheath formation change caused directly by the variation of the parameter, and the other is the sheath formation change caused by the Bohm criterion modification which the variation of the parameter results in. Therefore, an identical ion Mach number should be adopted when studying the direct effects of a parameter variety on plasma sheath structure. In addition, it is concluded that the collisions between positive ions and neutral particles make positive ion density curve higher and electron density curve lower than the case without collisions. Negative ion density does not change significantly no matter whether there exists collision. Besides, there is a peak in the profile of the net space charge while in the presence of ion-neutral collision, and the net space charge peak moves toward the sheath edge. The spatial potential increases and the sheath thickness decreases on account of the presence of the collisions between ions and neutral particles.