Evolution Of Modes In The Pulsed Dielectric Barrier Discharges At Atmospheric Pressure

In recent years, the cold atmospheric-pressure plasmas have been widely applied in various fields, and the relative mechanisms have become the hot spot in gas discharge and non-thermal plasmas. Dielectric barrier discharge (DBD) is considered to be an effective method of generating the atmospheric-pressure non-thermal plasmas. Among discharge modes of the atmospheric-pressure pulsed DBDs, uniform discharge modes including glow discharge and Townsend discharge have been the expected ones because of their advantage generating the non-thermal plasmas with bulk mass. For the reported studies on atmospheric-pressure DBD, continuous sinusoidal voltage is usually used as a driving source. With the development of pulsed power technology, the atmospheric-pressure DBD excited by repetitive voltage pulses (pulsed DBD) presents its particular advantages. Although some studies on atmospheric-pressure pulsed DBD have been made experimentally and numerically in recent years, there are many problems to need solving, such as the reasonable and general explanations for the formationes of both glow discharge and Townsend discharge as well as the transformation between these two discharges, the effects of discharge parameters on the evolution of uniform discharge modes in one circle of the applied voltage pulse, and the physical and chemical processes in the discharges.In this thesis, the atmospheric-pressure pulsed DBD has been systematically investigated by means of numerical simulation with the use of a one-dimensional fluid model, and the main contents and results are summarized as follows:1. Fundamental concepts of the gas discharge and non-thermal plasma and the background of atmospheric-pressure pulsed DBD are systematically described. The emphasis is placed on the modes and actualities of atmospheric-pressure pulsed DBD.2. According to the considered discharge structure, the one-dimensional (1-D) fluid model and the corresponding SG numerical algorithm are described in detail. Following these, the space-time evolutions of the discharge voltage, the discharge current density, the electric filed, and the particles density can be simulated.3. Using the 1-D fluid model, the atmospheric-pressure pulsed DBDs have been studied in nitrogen, helium, and helium-nitrogen gas, respectively. According to the simulated space-time evolutions of the electron, ion, and the electric field, the evolution laws of the discharge modes in one cycle of the applied voltage pulse have been obtained. The results are as follows:Affected by penning ionization, weak glow mode, Townsend mode, and glow mode appear successively in one cycle of atmospheric-pressure pulsed DBD in nitrogen; For atmospheric-pressure pulsed DBDs in helium and in helium-nitrogen, glow mode and sub-normal glow mode appear, when the discharge current densities reach their peaks.4. The atmospheric-pressure pulsed DBD in nitrogen has been further investigated. The effects of frequency f,pulse width tw, rising time tr (=falling time tf), and the dielectric thickness ds on the mode evolution of atmospheric-pressure pulsed DBD are systematically analyzed. It is found that the change of the discharge mode will take place, when changing one of these discharge parameters. When the frequency is 0.5 kHz, the discharge mode evolves into Townsend mode from weak glow mode, then it evolves into weak glow mode; When the frequency increases to 2 kHz, after the discharge mode evolves into Townsend mode from weak glow mode, it evolves into glow mode; When the frequency continues increasing to 5 kHz, the evolution law of the discharge modes becomes that the discharge mode evolves into glow mode from weak glow mode, then it evolves into sub-normal glow mode. When the pulse width is 200 ns, only Townsend mode appears in the discharge; When the pulse width increases to 500 ns, the discharge mode evolves into Townsend mode from weak glow mode, then it evolves into glow mode; When the pulse width continues increasing to 1000 ns, after weak glow mode, Townsend mode, and glow mode appear successively in the process of discharge, the discharge mode evolves to sub-normal glow mode. When the rising time is 10 ns or 100 ns, weak glow mode, Townsend mode, and glow mode appear successively in one cycle of the applied voltage pulse; When the rising time increases to 200 ns, the discharge mode evolves into Townsend mode from weak glow mode, then it evolves into weak glow mode. When the dielectric thickness is 0.05 cm, weak glow mode, Townsend mode, glow mode, and subnormal glow mode appear successively in the process of discharge. When the dielectric thickness increases to 0.1 cm, the discharge mode evolves into Townsend mode from weak glow mode, then it evolves into glow mode; When the dielectric thickness continues increasing to 0.3 cm, the discharge mode evolves into Townsend mode from weak glow mode.