| The atmospheric-pressure non-thermal plasmas have great application prospect in the fields of nanotechnology, biomedicine, environmental protection et al. Dielectric barrier discharge (DBD) is considered as an effective method to generate the atmospheric-pressure non-thermal plasmas. Especially, with the development of pulsed power technology, the atmospheric-pressure DBD excited by repetitive voltage pulses (called the pulsed DBD) has attracted much attention because of its particular advantages.In applications, the transition between the discharge modes of the atmospheric-pressure pulsed DBDs is of great significance for the generation of large-scale uniform plasmas. But, the related reports are spare. In addition, the plasmas in the pulsed DBD can provide the reactive oxygen species (ROS) with high density when a small amount of oxygen is added to the working gas. The ROS plays a significant role in many biomedical applications, such as sterilization, wound healing, and cancer therapy, to which the related mechanisms have become the hot spot in the application of plasmas in biomedicine internationally. However, due to the limitation of means of diagnosis, the mechanisms of the generation and dissipation of the ROSs are not clear for the complicated and nonlinear system formed by the atmospheric-pressure pulsed DBD, and there are still many basic problems left open. Therefore, numerical simulation is a useful tool for these.In this thesis, several subjects have been systematically investigated by means of numerical simulation with the use of a one-dimensional fluid model. These subjects include the transition between the two discharge modes, i.e. the atmospheric pressure glow discharge (APGD) and the atmospheric pressure Townsend DBD (APTD), and the electrical characteristics of the atmospheric-pressure pulsed DBD in pure He, the mechanisms of ROS evolution, and the dependences of ROS on the discharge parameters for the atmospheric-pressure pulsed DBD in He-02 mixture. The discharge parameters include the parameters of the pulsed voltage and those of the discharge configuration. Here, it should be pointed out that in all the simulations below, there are two discharges with opposite polarity in each period of the applied voltage pulse, one discharge occurs at the rising edge of the applied voltage pulse and the other at the falling edge of the applied voltage pulse. The main contents and results are summarized as follows:(1) The two discharge modes of the atmospheric-pressure pulsed DBD in pure He have been investigated numerically, based on a 1-D fluid model. The time evolutions of both the discharge voltage and discharge current density have been calculated. The spatial distributions of ion density, electron density, and electric filed at the time point where the first discharge current reaches its maximum (hereafter, called the current peak time) have been obtained, and the characteristics of these distributions have been analyzed. Also, the influences of the parameters of both the applied voltage pulse and the discharge configuration on the two discharge modes have been investigated in detail. Due to the above, the following results are obtained.The discharge current density in APGD mode is evident large, compared to that in APTD mode. At the current peak time, the electric field distorts near the momentary cathode (MC), and there is respective peak for the electron density and ion density nearby MC, forming the cathode sheath. In addition, in a wide area near the momentary anode (MA) the electron density is very close to the ion density, leading to the quasi-neutral plasma bulk. In APTD, there is no sheath near MC, and the ion density and electron density are low in the gap, but the ion density is obviously large in contrast with the electron density. The increase of the voltage growth rate can induce the transition of the discharge mode from APTD to APGD. With the use of efficient large gap, the APGD is easier to be driven. And the increase of the capacitance of dielectrics (by decreasing the dielectric thickness or by increasing the dielectric constant) can also induce the transition of the discharge mode from APTD to APGD. In practice, the transition of the discharge mode results from the combined effects of the operation parameters. When other discharge parameters are fixed, the parameter set consisted of the voltage growth rate, the gap width, and the dielectric thickness are obtained for indicating the area where the discharge is in the corresponding mode, which is of importance for the control of the discharge mode.(2) The electrical characteristics of the pulsed DBDs in He-O2 mixture at atmospheric pressure and the generation of ROS as well as the corresponding oxygen concentration effects have been numerically investigated. The characteristics of both the discharge voltage and discharge current density and the spatial distributions of electron, charged particles, and ROS at the current peak time have been calculated and analyzed. The oxygen concentration effects on the characteristics quantities of the pulsed DBDs in He-O2 mixture at atmospheric pressure have been studied. These characteristics quantities refer to the discharge current density, the spatial distributions of the electric field, the averaged dissipated power, the averaged electron density, and the averaged electron temperature. The mechanisms of the evolution of the main ROS and the effects of oxygen concentration on these ROS have been explored. Based on the above, the following results are given.At the current peak time, in the gap the densities of the charged particle He2- and O2- are much larger than those of other particles. These two densities are very close to each other and reach their maximums about at the central position of the gap. Moreover, the spatial distributions of both the electric field and all of the particles indicate that the discharge is in APGD mode.The ground state atomic oxygen density is the largest one in all the ROS. With the increase of the oxygen concentration, the peak value of discharge current density decreases and the position of peak shifts backwards, leading to the decrease of the averaged electron density and the averaged electron temperature. In addition, the averaged densities of the ground state atomic oxygen O and the excited oxygen molecule SDO increase first and then decrease, the averaged density of Ozone O3 increases, and the averaged density of the excited atomic oxygen O(’D) decreases. Under the given discharge parameters,0.5% is an optimal oxygen concentration which results in the maximal density of ROS.(3) The influences of the parameters of the pulsed voltage on the characteristics of the atmospheric pressure DBDs in He-02 mixture, the averaged densities of ROSs, and the optimal oxygen concentration have been investigated. These parameters refer to the rising time, amplitude, and frequency of the pulsed voltage. The following results are obtained.With the increase of rising time of the pulsed voltage, the discharge becomes weak, namely, the current density peak values of the two discharges decrease, the averaged electron temperature and the averaged electron density decrease, and the amplitude of the electric field in the cathode sheath at the current peak time decrease, but the thickness of the cathode sheath does not change. Moreover, the averaged dissipated power, the densities of four main ROSs, i.e. O, SDO, O3 and O(1D), and the optimal oxygen concentration decrease.When increasing the amplitude of the pulsed voltage, the discharge becomes strong which means the decrease of the current density peak values of the two discharges, the averaged electron temperature and the averaged electron density increase, the amplitude of the electric field in the cathode sheath at the current peak time increases, the cathode sheath narrow down, and the averaged dissipated power and the densities of four main ROSs increase. Especially, the optimal oxygen concentration becomes large.Under the different oxygen concentrations, the increase of frequency of the pulsed voltage leads the following. The first discharge becomes weak, and the second one gets strong at frequencies below 50 kHz and then weak. There is very weak effect on the averaged electron density and the electric field in the gap, but the averaged electron temperature and the averaged densities of four main ROSs decrease. In addition, the optimal oxygen concentration is changed.(4) The influence of the parameters of the discharge configuration on the characteristics of the atmospheric pressure pulsed DBDs in He-02 mixture, the averaged densities of ROSs, and the optimal oxygen concentration have been systematically investigated. These parameters include the dielectric thickness, the relative permittivity of the dielectric, and the gap width. The following results are obtained.With the increase of the dielectric thickness, the discharge becomes weak, the averaged electron temperature and the averaged electron density decrease, the amplitude of the electric field in the cathode sheath at the current peak time decreases, the cathode sheath broaden, and the averaged dissipated power and the densities of the four main ROSs decrease. But, there is no change for the optimal oxygen concentration and it is about 0.5%.Increasing the relative permittivity, the discharge become strong, the averaged electron temperature and the averaged electron density increase, the amplitude of the electric field in the cathode sheath at the current peak time increases, the cathode sheath narrow down, and the averaged dissipated power and the densities of the four main ROSs increase. Similarly, there is no change of the optimal oxygen concentration and it is about 0.5%.The increase of the gap width results in the following. The current density peak values of the two discharges, the averaged electron temperature, and the averaged electron density increase first and then decrease. In addition, the averaged dissipated power, the averaged density of the ROSs under the different oxygen concentrations, and the optimal oxygen concentration decrease. |
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