开放空间低气压微波脉冲放电氮气等离子体电子行为研究

发布时间：2018-10-17 12:30

【摘要】：微波产生等离子体的技术很早就应用在工业生产中,并且最近人们对微波放电产生的等离子体特性也进行了深入的讨论。但是人们大多关注微波产生等离子体之后的稳定状态的作用,并不关注放电过程中等离子体形成的过程及此时等离子体状态的变化。另一方面,微波气体击穿的研究在很多微波传输或者微波器件的研究都有涉及,但是大多数针对微波气体击穿的研究的目标在于防止微波产生击穿(打火)的效果,所以该类研究大多关注微波击穿时微波本身的参数和环境因素,对微波击穿之后的形态并不详细讨论和研究。本文研究的主要目的是详细讨论从微波击穿开始,到产生等离子体,最后形成稳定的等离子体的全过程。而研究这整个过程的核心点在于研究电子在微波电场的作用如何得到能量,损失能量以及如何产生更多的电子最终形成稳定等离子体。由于该过程是相当复杂的过程,所以简化该过程中的非重要因素是非常必须的。在实验中本文选择开放空间的条件去除了电子与容器壁的相互作用,并且背景气体的状态更加稳定,同时使得电子的扩散效应占据主导地位,但是该开放空间的条件也导致了微波放电过程中等离子体的形成将是一个电子运动远快于离子运动的过程。同时在实验中选择低气压氮气作为背景气体将使得电子与氮分子的碰撞过程更加重要,这样避免了在极低的气压下电子与电子和电子与离子的碰撞过程会产生剧烈的影响或者在极高的气压下氮分子之间碰撞作为主要碰撞过程出现。本文采取脉冲式微波进行放电可以清晰和直观地分辨放电过程中从击穿到产生等离子体到最终形成稳定等离子体的过程。脉冲式微波放电也为测量和诊断提供了清晰的参考。在理论上,本文首先从单电子运动模型出发,导出扩散效应下单电子在微波电场中运动的流体方程,并讨论该方程的适用条件和注意事项。然后详细讨论电子与氮分子的各个碰撞过程,氮分子之间的碰撞行为、电子与电子和电子与离子的碰撞行为以及电子的自扩散行为。并针对开放空间低气压的条件对各类碰撞行为进行讨论,分析其适用条件,最后就电子的碰撞行为对单电子模型进行补充说明。最后从统计的角度出发,将单电子模型拓展到全域模型(Global Model)中,并采用统计的观点对电子的碰撞行为进行具体分析,讨论该模型适用的条件。最后通过实验测量结果与理论计算的结果进行对比,分析和讨论开放空间低气压条件下,微波脉冲放电产生等离子体中电子温度和电子密度的变化过程及其受影响的因素。实验中采用气压300Pa,气体温度300K氮气作为背景气体进行放电。放电微波采用C波段5.3GHz的微波聚焦产生等离子体,脉冲宽度为1μs,重复频率为500Hz,峰值功率为600-800kW。在诊断上主要采用X波段的连续波进行微波透射诊断以得到电子密度的变化,同时采用发射光谱诊断对氮分子的振动和转动温度进行测量。最后通过实验结果与理论结果对比发现在微波击穿到形成稳定等离子体的过程中,电子温度会率先迅速上升达到一个较高的值,然后较高的电子温度促使电离过程剧烈发生,形成雪崩效应,此时电子密度开始急剧上升。在电子密度上升至放电微波频率对应的临界密度的时候,等离子体开始对放电微波的进入产生阻碍,这导致进入等离子体的微波电场开始衰减,而微波电场的衰减将导致电子从微波电场中吸收的能量减少,从而使电子温度下降。电子温度的下降又会导致电离率下降,从而导致电子密度的增长速度下降。最终在电子数目的得失与电子能量的得失上达到一个平衡,即形成稳定的等离子体。整个放电过程是一个非平衡和非线性的过程,但是究其物理本质是等离子体态本身与微波相互作用的结果,这也是等离子体态相对于其它三种状态的特性。
[Abstract]:The technology of microwave generating plasma has been widely used in industrial production, and the plasma characteristics of microwave discharge have been discussed in depth recently. However, most attention is paid to the stabilization of plasma after microwave generation, and it is not concerned about the process of plasma formation during the discharge process and the change of plasma state at this time. On the other hand, the research on the breakdown of microwave gas is related to many microwave transmission or microwave devices, but most of the research on the breakdown of microwave gas is to prevent the effect of microwave generating breakdown (sparking). Therefore, most attention is paid to the parameters and environmental factors of the microwave itself at the time of microwave breakdown, and the morphology after microwave breakdown is not discussed and studied in detail. The main purpose of this paper is to discuss the whole process of generating plasma and finally forming stable plasma from the beginning of microwave breakdown. The core point of this process is to study how electrons gain energy, loss of energy, and how to generate more electrons eventually form stable plasma in the microwave electric field. Since the process is a quite complex process, it is very necessary to simplify the non-important factors in the process. in the experiment, the condition of the open space is selected to remove the interaction between the electrons and the wall of the container, and the state of the background gas is more stable, and meanwhile, the diffusion effect of the electrons is dominant, However, the condition of the open space also results in the formation of the plasma in the process of microwave discharge will be the process of electron motion much faster than the ion motion. meanwhile, selecting low-pressure nitrogen as a background gas in the experiment will make the collision process of electrons and nitrogen molecules more important, This avoids dramatic effects on the collision of electrons with electrons and electrons and ions at very low pressures or collisions between nitrogen molecules as a major impact process at very high gas pressures. In this paper, pulsed microwave discharge can clearly and intuitively distinguish the process from breakdown to generation of plasma to stable plasma in the process of discharge. Pulse microwave discharge also provides a clear reference for measurement and diagnosis. In this paper, based on the single-electron motion model, the fluid equation of the electron in the microwave electric field is derived by the diffusion effect, and the applicable conditions and precautions of the equation are discussed. The collision behavior between electrons and nitrogen molecules, the collision behavior between electrons and electrons and ions, and the self-diffusion behavior of electrons are discussed in detail. According to the condition of the open space low air pressure, the collision behavior is discussed, the applicable conditions are analyzed, and finally the single electronic model is supplemented by the collision behavior of the electrons. At last, from the statistical point of view, the single electronic model is extended to the Global Model, and the collision behavior of the electrons is analyzed in a statistical way to discuss the applicable conditions of the model. Finally, the change of electron temperature and electron density in plasma generated by microwave pulse discharge and its influence factors were analyzed and discussed by comparing the experimental results with the results of theoretical calculation. In the experiment, a gas pressure of 300Pa and a gas temperature of 300K were used as background gas to discharge. The plasma has a pulse width of 1. m u.m, a repetition frequency of 500Hz and a peak power of 600-800kW. In the diagnosis, the X-band continuous wave is mainly used for microwave transmission diagnosis to obtain the change of electron density, and the vibration and rotation temperature of nitrogen molecules are measured by emission spectrum diagnosis. Finally, by comparing the experimental results with the theoretical results, it is found that in the process of microwave breakdown to the stable plasma, the electron temperature will rise rapidly to a higher value, then the higher electron temperature will cause the ionization process to occur violently, forming the avalanche effect. At this time, the electron density begins to rise sharply. when the electron density rises to a critical density corresponding to the discharge microwave frequency, the plasma starts to interfere with the entry of the discharge microwave, which causes the microwave electric field entering the plasma to begin to decay, and the attenuation of the microwave electric field will result in a reduction in the energy absorbed by the electrons from the microwave electric field, thereby lowering the electronic temperature. The decrease in the electron temperature will also cause the ionization rate to drop, resulting in a decrease in the growth rate of the electron density. and finally achieving a balance on the gain and loss of the electronic number and the gain and loss of the electronic energy, namely forming a stable plasma. The whole discharge process is a non-equilibrium and non-linear process, but its physical nature is the result of the interaction between the plasma state itself and the microwave, which is also the characteristic of the plasma state with respect to the other three states.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O53

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