Dc Glow Discharge Positive Column And Cathode Sheath Simulation Study

In this dissertation work, three major approaches in the area of plasma physics were used to study the physico-chemical processes of d. c. glow discharges. The first one is a kinetic model, which is used to deal with the positive column of a hydrogen glow discharge. The second approach is a fluid model, which is used to deal with the transport processes of nitrogen ions in the cathode sheath of a nitrogen glow discharge. The third one is the Monte Carlo simulation in dealing with the energy and angle distributions of nitrogen ions in the cathode sheath of a nitrogen glow discharge.In the kinetic modeling of a hydrogen glow discharge positive column, primary physico-chemical processes of six species (H2, H, H+, H2+, H3+ and electron), were taken into account, including elastic collisions, inelastic collisions and some chemical reactions. The results show that the electron energy distribution function obviously departs from a Boltzmann. And as E/N increases, the distribution tail moves to higher energy. And the tail moves towards higher energy, too, as H atom proportion increases at the same E/N. From the simulation, the electron, H atom and hydrogen ions densities rises with increasing E/N. Among three positive ions, H3+ is the dominant ionic species over all E/N examined. And H" density is higher than that of H2+. H2 fractional dissociation also rises with increasing E/N.The transport processes of nitrogen ions (N2+, N+) in the cathode sheath of a nitrogen glow discharge have been studied by using a fluid model which includes the consideration of momentum transfer between ions and neutral molecules. The variations of ion and electron concentrations, the sheathpotential and the electrical field strength along the cathode sheath have been investigated under different gas pressures and initial ion proportions. The results show that the ion densities decrease as ions move from the sheath edge to the cathode. Expecially, the ion densities decrease very fast nearby the sheath edge. And the electron density decreases even faster than ions. At the same sheath position, N+ and N2+ densities increase with the gas pressure. When the initial proportion of N+ and N2+ is 1 to 1 , N2+ density is always higher than N+ in the whole cathode sheath. Under the same sheath potential decline, the electrical field strength increases with the decreasing sheath thickness.The Monte Carlo simulation of nitrogen ions in the cathode sheath of a nitrogen glow discharge shows that, as gas pressure increases, nitrogen ions with lower energy and the ions scattered at larger angle will be more populated. And the variation of gas pressure influences N2+ energy distribution than that of N+ and N+ angle distribution than that of N2+ more obviously. Under the same pressure, with nitrogen ions moving toward cathode, ion energy distributions move towards the higher energy area, however, ions angle distributions move toward smaller angle. When the sheath thickness decreases, the similar variation trend was found, which indicates that high electric strength has strong acceleration and focus effect to the ions.