Simulation On Glow Discharge Mode And Homogeneity At Atmospheric Pressure

Atmospheric pressure glow discharges (APGDs) have been hot topics in recent years since they do not require expensive vacuum equipment, exhibit homogeneous characteristics, and show great potential in a wide-spread range of industrial applications. Many studies have been carried out but some important parameters such as electron energy distributions (EEDFs) cannot be evaluated appropriately; therefore the theoretical models should be improved. Moreover, the APGDs can be achieved only when special conditions are satisfied; hence optimal means for realizing homogeneous glow Discharges at atmospheric pressure should be explored.In this dissertation,1D/2D self-consistent fluid models were developed for APGDs, and the evolutions of EEDFs in the plasma were obtained quantitatively. Based on these models, characteristics for both Townsend discharge and glow discharge, and the transition characteristics between them are studied. More importantly, the means for achieving APGDs are explored theoretically:(a) Studying the influences of the external frequency on the discharge modes; (b) Studying the improvement of AC-driven discharge homogeneity with extra ratio-frequency (RF) voltage source; (c) Studying discharge characteristics driven by combined RF and short pulse power sources; (d) Studying pulse-modulated RF discharges. In Chapter one, the background of our study is presented. In the following, the research works are briefly described as follows:An improved theoretical model by Boltzmann Equation and fluid equations is established with which the EEDFs and electron temperature in APGDs can be evaluated. The simulation results in Chapter Two show that EEDFs and electron temperature in different regions of glow discharge vary greatly in time and space domains; in contrast, the difference between the characteristic regions in Townsend discharge is small. In the case of a glow discharge, the cathode fall region contains largest percentage of high-energy electrons, and electron temperature is several times higher than that in any other region; while for the Townsend discharge, the EEDFs and electron temperature in the discharge gap are similar. It is found that the Townsend mode turns into the glow mode when the external voltage increases over a certain critical value. With the increase of the external voltage, the transition characteristics of discharge current density and gas voltage in both Townsend and glow discharges are obtained. Moreover, it is also found that a minimum of the critical voltage exists in the relationship curve of external voltage vs the discharge gap spacing.In Chapter three, the influences of working frequency on the discharge mode are studied. The numerical simulations show that when the driven frequency increases in the range (1 kHz-100 kHz), three discharge modes, Townsend discharge, glow discharge, and local glow discharge, appear in turn. The Townsend discharge happens in the lowest frequency range part, and it then turns into the glow discharge when the voltage frequency increases over a critical value. The reason for the discharge mode transition is that most charged particles cannot be confined in the discharge gap when a low frequency power is applied; charged particles can be confined more efficiently when a higher frequency power is applied—a glow discharge will happen. The glow discharge changes to a local glow mode when the frequency is raised to a high value. At such a high frequency discharge, the density of the charged particles in space, especially in the cathode fall region, increases and the electric field rises, the cathode sheath becomes thinner, the discharge channels form in local regions.Local glow discharges are one of the most common modes at atmospheric pressure DBDs driven by the AC source, it is very important to investigate means to improve the discharge homogeneity. In chapter four, we find out that the discharge homogeneity can be improved by adding a low-amplitude RF voltage. The RF voltage can help to confine charged particles in the discharge space. These charged particles offer enough seed electrons for the discharge in the whole discharge space. Thus, the possibility of the local breakdown is reduced; in other words, a homogeneous glow discharge could happen more easily.The RF power can produce high-density plasmas at lower excited voltage, but the power cannot be converted efficiently and excessive heat is produced; pulse source can produce plasmas in short time, while the densities of charged particles could not be maintained for long time. So in Chapter five, the discharge driven by combined low-amplitude RF and high-amplitude trapezoidal short pulse voltages are studied. It is found that high density plasmas can be produced by a high pulse voltage in short time, and be sustained by a low RF voltage. The mechanism for the increase in the plasma density can be attributed to a strong localized electric field induced by the applied short pulse; the strong electric field generates a great number of high energy electrons and the other charged particles, which subsequently generate more electrons and ions. After the pulse is switched off, the current density reduced quickly while the plasma density drops off more slowly. Applying short pulses can enhance the plasma density without consuming much more power, and the discharge also operates in RF glow mode.Pulse-modulated RF discharges have already exhibited many advantages in semiconductor area, and some works have been done experimentally. In chapter six, the pulse-modulated RF discharges are investigated using 2D fluid model. The results show that stable self-organized discharge patterns could be obtained as long as the imposed pulse has a proper period and duty cycle. Moreover, with a fixed pulse period, the breakdown happens in local channels when a low-duty-cycle pulse is used; the number of the discharge channels increases with the increment of duty cycle; the homogeneous glow mode appears when the duty cycle of the pulse is high enough. In the high duty cycle case, more charged particles are generated during a long power-on time. Enough residual charged particles serve as seeds for the next breakdown, which increase the chance of homogeneous discharge. In addition, the diffusion of ions plays an important role in formation of breakdown channels; its role is discussed in details.