Study On Characteristics Of Atmospheric-Pressure Pulsed Dielectric Barrier Discharges In Pure Helium And Helium-Oxygen Mixture

The atmospheric-pressure non-thermal plasmas have been widely used in industrial applications due to its higher electron energy, lower ion energy, and the active particles. The dielectric barrier discharge (DBD) is considered as an effective way of producing the atmospheric-pressure low-temperature plasmas and much attention has been focused on it. 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 the 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 explanation for the mechanism of the discharge, the more knowledge for some discharge behaviors, and the relationship between characteristics of discharges and conditions.To this end, 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. Describe fundamental concept of the plasma, background and significance of the atmospheric-pressure non-thermal plasmas as well as the ways of discharge are briefly introduced. The emphases is placed on the background and significance of the pulsed dielectric barrier discharge.2. The classifications of the way on the charged particles creation and destruction are briefly introduced. The ways of the ionization and recombination in the discharges are presented in detail. These are foundation for the simulation.3. The1-D fluid model of simulating the pulsed DBD in pure helium at atmospheric-pressure has been presented. Using this model, the characteristic quantities which refer to discharge voltage, discharge current density, and particles density as a function of time have been calculated systematically. In addition, the spatial distributions of ion density, electron density, and electric filed at different time points have also been obtained. Based on the above, the characteristics and mechanism of the discharge have been analyzed.4. The effects of the frequency on the electrical characteristics in a dielectric barrier discharge (DBD) in pure helium excited by repetitive voltage pulses with the frequency f from5to500kHz have been systematically investigated by means of the1-D fluid model under different operating conditions including gap width dg, secondary electron emission coefficient y, dielectric thickness ds, and dielectric constant εr. The important characteristic quantities of describing the pulse discharge, i.e., discharge current density Jg, averaged electron density Nave, averaged dissipated power density Pave, and the axial distributions of both electron density Ne and electron temperature Te, are calculated and analyzed in detail. The present work gives the following significant results.Nave increases with increasing f and Pave is almost proportional to f. Especially, there is a characteristic frequency of about50kHz, namely, when f is lower than about50kHz, Jm1(peak value of Jg in the first discharge) decreases evidently with increasing f and then changes very slightly for fs is larger than about50kHz. Also, with the increase off, Jm2(peak value of Jg in the second discharge) increases very slightly for small ds and the reverse is true for large ds. The frequency dependence of Jm2for different εrs is similar to that by changing ds. For a fixed frequency, the change of Jm1with y is very small, which differs from that at low frequencies. In addition, Jm1and Jm2decrease with increasing ds and increase with increasing, but there are almost constant△Jm1and△Jm2for high frequencies. When the frequency is larger than about50kHz, there will be the second peak of Ne outside the cathode sheath in the first discharge, and both large ds and small εr can result in the formation of the evident peak of Ne nearby the momentary anode.5. The differences of discharges between pure helium and helium-oxygen gas are briefly introduced. As oxygen is electronegative gas, the discharge current decreases and the main charged particles is different from those for pure helium. For the discharge in pure helium, the negative ion is electrons and the number of positive ion He2+is larger than He+. In helium-oxygen gas with the addition of0.5%oxygen, the variety of charged particles increases and the main function charged particles has changed, in which the dominating positive ions are He2+and O2+, and the dominating negative ions are O3-and O2-. The density of electron is much smaller than that of pure helium, inducing weak impact ionizations and in turn leading to the decreasing discharge current.