Dust in near-wall magnetized plasma

Abstract

The experimental work on dust in near-wall plasmas has attracted considerable interest in the last few years. Dust particles can carry both negative as well as positive charge depending upon the ambient plasma conditions. The charged dusts not only levitate over a negatively charged wall due to balance between the gravitational and electrostatic forces but also move under the action of fields and flows in the plasma. The controlled experiment with massive dust grains in the plasma allows us to investigate the spatial and temporal scales inaccessible by probe techniques. The micrometre-sized dust has been utilized as a diagnostic tool to investigate the plasma edge characteristics. Dust occurs in almost every plasma device and interactions of plasmas with near-wall impurities and/or dust significantly affects the efficiency and lifetime of such devices. Often presence of the magnetic field is inevitable in all such investigations and, therefore, it is important to study the effect of such a field on the near-wall plasma. Here, the charge on the dust, plasma potential, and plasma density in near wall region are calculated self-consistently. The electrons are assumed non-Boltzmannian and the effect of electron magnetization and electron-atom collisions on the dust charge is calculated in a self-consistent fashion. For various plasma magnetization parameters viz. the ratio of the electron and ion cyclotron frequencies to their respective collision frequencies, plasma - atom and ionization frequencies, the evolution of the plasma potential and density in the near wall region is investigated. It was found that the profiles of the plasma parameters change significantly not only when the ion cyclotron frequency is increased by an order of magnitude but also by the angle of the field orientation. When the magnetic field is perpendicular to the wall, grains of smaller size stay deeper inside the sheath in comparison with the case when the field is directed parallel to the wall. With the increase in the ion-magnetization level, and the angle between the field and the wall, the plasma flow velocity increases. The grains acquire less negative charge, so grains of larger size can stay inside the near wall plasma.