Investigation of Electron Kinetics in Atmospheric-Pressure Microplasmas through Particle-in-Cell Simulation

Abstract

In this dissertation, three kinds of electron kinetics of atmospheric-pressure microplasmas have been studied using particle-in-cell simulation with a Monte Carlo collision. The first topic deals with an electron heating mode transition induced by ultra-high frequency in single-frequency (SF) atmospheric-pressure microplasmas. Interestingly, this discharge mode transition is accompanied by non-monotonic evolution of electron kinetics such as effective electron temperature, plasma density, and electron energy on the electrode. In this study, the highest flux of energetic electrons (ε > 4 eV) usable for tailoring the surface chemistry in atmospheric microplasmas is obtained at the specific frequency (400MHz), where an optimal trade-off is established between the amplitude of sheath oscillations and the power coupled to electrons for sub-millimeter dimensions (200 µm). The second topic includes a comparative study of electron kinetics between SF microplasmas and their equivalent dual-frequency (DF) microplasmas with matching effective frequencies in atmospheric-pressure helium discharges. The effective-frequency concept helps in analyzing DF microplasmas in a fashion similar to SF microplasmas with effective parameters. In this study, the plasma characteristics such as the plasma potential, density, and EEPFs of the SF microplasma and its DF counterpart were almost the same. However, the oscillating sheath edge was pushed further into the electrode for a substantial fraction of the time and the sheath width decreased in DF microplasmas. As a result, the transport of the energetic electrons in atmospheric microplasmas is enhanced in DF microplasmas as compared to SF microplasmas. The third topic deals with a formation of secondary energetic electrons induced by abnormal heating mode in pulsed microwave-frequency atmospheric microplasmas. We found that additional high electron heating takes place only during the first period of the ignition phase at the retreating sheath for sub-millimeter dimensions. During this period, electrons are unable to follow the abrupt retreating sheath by diffusion alone. Thus, a self-consistent electric field is induced to drive electrons toward the electrode to follow the abrupt retreating sheath. These behaviors result in the abnormal electron heating mode, which produces higher energy (ε > 50 eV) electrons at the electrode.