离子束-本底等离子体的产生及束上下游鞘层分布的实验研究

发布时间：2018-08-15 11:33

【摘要】：离子束-等离子体系统在等离子体与材料表面相互作用等应用以及等离子体集体波动和不稳定性的激发和传播等基础研究中有广泛应用。束-等离子体系统中等离子边界鞘层结构特性就是其中一个典型的基本问题。前人在利用离子束-等离子体系统研究波动激发或鞘层结构等问题时,对离子束的能量以及束流所占的比例均未做到接近独立的控制。前人曾在理论和实验上研究过束对离子鞘的影响,但是对束上下游鞘层结构的不对称性却未曾研究。为了研究束-本底等离子体中的鞘层特性,本论文的主要工作在双等离子体装置的源室(source chamber)和靶室(target chamber)中间加两层隔离栅网并通过其偏压差在靶室形成离子束-本底等离子体系统,并用自制校准的减速栅网式离子能量分析仪诊断离子分布函数。通过调节相关的放电参数以实现离子束能量和束流比例的控制。在此基础上重点研究了离子束对金属平板的上下游鞘层结构的影响。主要结论如下：1.实验区中的离子分布函数含有双峰结构,一群各向同性的本底离子和一群有定向能量的束离子。通过调节两隔离栅网压差或放电电压可以几乎独立地控制束离子的能量,而这两个放电参数对束流比例的影响甚微。束流比例主要由热阴极加热电流决定,该参数对束能量的影响较弱,因此通过控制热阴极加热电流并辅以两隔离栅网偏压差或放电电流的适当调节可实现束流比例的独立控制。但中性气体气压的改变却造成束能量和束流比例同时变化。该实验为束-等离子体系统中的鞘层结构和波动激发等基础研究提供了独立控制束能量和比例的方法。2.发射探针诊断的鞘层电势分布结果显示,处于束-等离子体系统中的平板上下游的鞘层是非对称的,下游鞘比上游厚。离子束的出现会使上下游的鞘层收缩,鞘厚会随着离子束能量及束密度的增加而变薄。将平板换成栅网,则由于存在透射率,栅网的上下游鞘层始终比平板情况薄得多。另外,气压的升高导致束能量降低和密度下降,从而导致上下游鞘层变厚。本论文的另一部分工作是涉及磁化等离子体的。磁化等离子体中静电离子回旋波(EICW)在近乎垂直磁场的方向上传播,前人对EICW的所有实验几乎全在Q-machine中进行,产生的等离子体是细长的柱,激发方式单一不可控。为此,本工作搭建了一个稳态的磁镜装置(SMM),其直径远大于前人实验的磁化等离子体,轴线中心磁场强度超过1200高斯。这为EICW的激发以及磁化等离子体鞘层结构等基础研究提供了实验平台。在一个小的测试用真空室中利用热阴极灯丝放电产生磁化等离子体,用小栅网加外部驱动信号,开展了EICW激发的预研实验。得到以下结论：1.在热阴极放电过程中,热丝所在磁面内电子温度呈现双温,一群对应本底电子,一群对应初级电子,而热丝中间电子温度只呈现单温,对应本底电子,说明初级电子被磁场约束只能沿着磁面运动从而电离中性气体,产生的等离子体扩散至其他位置。2.利用小栅网激发垂直磁场方向传播的波动,测得该波的频率随磁场的增强而增大,获得其色散关系与EICW较符合,从而验证了外部驱动信号激发大横向尺度的EICW的可行性。
[Abstract]:Ion beam-plasma systems have been widely used in the fundamental research of plasma-material interaction and the excitation and propagation of plasma collective wave and instability. The energy of ion beam and the proportion of the beam to the sheath have not been controlled independently when the plasma system is studied for wave excitation or sheath structure. The main work of this paper is to add two isolation grids between the source chamber and the target chamber of the double plasma device and form an ion beam-background plasma system in the target chamber through its bias difference. Ion distribution is diagnosed by a self-calibrated deceleration grids ion energy analyzer. The main conclusions are as follows: 1. The ion distribution function in the experimental region contains bimodal structure, a group of isotropic background ions and a group of isotropic background ions. Beam ion with directional energy. The energy of the beam ion can be controlled almost independently by adjusting the voltage difference or discharge voltage between two isolated grids, and the effect of these two discharge parameters on the beam current ratio is negligible. The beam current ratio is mainly determined by the heating current of the hot cathode, which has a weak influence on the beam energy. Therefore, the heating current of the hot cathode can be controlled by controlling the heating current of the hot cathode. The beam proportions can be controlled independently by adjusting the bias difference between two isolated grids or the discharge current properly. However, the change of neutral gas pressure results in the simultaneous change of beam energy and beam proportions. 2. The results of sheath potential distribution diagnosed by emission probe show that the sheath is asymmetrical upstream and downstream of the plate in the beam-plasma system, and the sheath is thicker than upstream. The sheath shrinks with the appearance of ion beam, and the sheath thickness becomes thinner with the increase of ion beam energy and beam density. The sheath of the grid is always much thinner than that of the plate in the presence of transmittance. In addition, the increase of pressure leads to the decrease of beam energy and density, which leads to the thickening of the sheath. In this work, a steady-state magnetic mirror device (SMM) is constructed. The diameter of the SMM is much larger than that of the magnetized plasma, and the magnetic field intensity in the center of the axis is more than 1200 Gauss. It provides an experimental platform for the basic research of EICW excitation and magnetized plasma sheath structure. In a small test vacuum chamber, the magnetized plasma is produced by hot cathode filament discharge and the EICW excitation experiment is carried out by using a small grid plus external driving signal. The following conclusions are obtained: 1. The temperature of the electron in the magnetic plane of the hot wire is two-temperature, one group corresponds to the background electrons and one group corresponds to the primary electrons, while the temperature of the intermediate electrons in the hot wire is only one-temperature, which corresponds to the background electrons. The frequency of the wave increases with the increase of the magnetic field, and the dispersion relation is in good agreement with the EICW, which verifies the feasibility of the external driving signal exciting large-scale EICW.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O53

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