摘要:电子回旋共振离子推力器(electron cyclotron resonance ion thruster, ECRIT)离子源内等离子体分布会影响束流引出, 而磁场结构决定的ECR区与天线的相对位置共同影响了等离子体分布. 在鞘层作用下, 等离子体中的离子或电子被加速对壁面产生溅射, 形成壁面离子或电子电流, 造成壁面磨损和等离子体损失, 因此研究壁面电流与等离子体特征十分重要. 为此本文建立2 cm ECRIT的粒子PIC/MCC (particle-in-cell with Monte Carlo collision)仿真模型, 数值模拟研究磁场结构对离子源内等离子体与壁面电流特性的影响. 计算表明, 当ECR区位于天线上游时, 等离子体集中在天线上游和内外磁环间, 栅极前离子密度最低, 故离子源引出束流、磁环端面电流和天线壁面电流较低. ECR区位于天线下游时, 天线和栅极上游附近的等离子体密度较高, 故离子源引出束流、天线壁面电流和磁环端面电流较高. 腔体壁面等离子体分布与电流受磁场影响最小. 关键词:电子回旋共振离子推力器/ 磁场结构/ 粒子模拟/ 壁面电流
English Abstract
Numerical simulation of influence of magnetic field on plasma characteristics and surface current of ion source of 2-cm electron cyclotron resonance ion thruster
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11875222)
Received Date:09 October 2020
Accepted Date:01 December 2020
Available Online:25 March 2021
Published Online:05 April 2021
Abstract:The cathode-less miniature electron cyclotron resonance ion thruster (ECRIT) has the advantages of long-life and simple-structure. In the ECRIT ion source, the plasma distribution will affect the beam extraction, and the relative position of the ECR layer determined by the magnetic field structure and the flat-ring antenna together affect the plasma distribution. Due to the sheath, the ions or electrons in the plasma will be accelerated to sputter the surface of wall and induce plasma loss. It is important to investigate the wall currents and plasma characteristics. Therefore, particle-in-cell with Monte Carlo collision (PIC/MCC) model is established in this article to study the influence of the magnetic field structure on the plasma and wall current characteristics of 2-cm ECRIT ion source. The calculation results show that the electrons are confined near the ECR layer of antenna by the magnetic mirror, which leads the plasma to be distributed near the ECR layer. When the ECR layer is located on the upstream side of the flat-ring antenna, the plasma is concentrated between the antenna and magnet rings, and the ion density in front of the grid is lowest, which results in a lower ion beam current extracted from ion source and a lower current on the surface of magnetic ring and antenna. When the ECR layer is located on the downstream side of the flat-ring antenna, the plasma density near the upstream side of the antenna and grid is high, which results in higher ion beam current extracted from the ion source and higher current on the surface of antenna and magnetic ring. The plasma distribution and the total wall current of the ion source are affected weakly by the magnetic field structure. In this magnetic field structure, the ion sputtering on the flat-ring antenna is serious. Although such a magnetic field design can increase the extracted ion beam current, it will shorten the working life of the ion source. In the future, when designing a new thruster, it is necessary to weigh the ion current of extraction and lifetime to select the appropriate magnetic field structure. Keywords:electron cyclotron resonance ion thruster/ magnetic field structure/ particle-in-cell with Monte Carlo collision simulation/ surface current
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2.1.物理模型
如图1所示, 2 cm ECRIT离子源由圆柱腔体、环形天线、磁轭、内外环形永磁体和双栅极组成, 其中两磁环分布在离子源底部, 其间环向均布8个进气孔; 4.2 GHz的微波能量通过环形天线馈入放电室, 在0.15 T的磁场区域形成ECR区[15]. 在ECR区, 电子绕磁力线的回旋运动和微波电场变化同步产生谐振, 由此微波电场加热电子形成高能电子, 高能电子和原子发生碰撞激发和电离, 从而形成ECR等离子体, 等离子体中的离子经由双栅极被高速引出. 图1中的H1, W1, H2和W2分别表示外磁环高度、外磁环宽度、内磁环高度和内磁环宽度. L1为天线环形段下表面与屏栅上表面之间的距离, L2为内磁环下表面与天线环形段上表面之间的距离, 本文中不同磁场结构中L1和L2的值相同. 定义A, B, C和D分别表示天线环形段的内表面, 下表面, 外表面和上表面; E, F分别表示内磁环端面、外磁环端面; G表示腔体内表面. 图 1 2 cm ECRIT离子源结构示意图 Figure1. Schematic diagram of the 2 cm ECRIT ion source internal structure.
22.2.计算模型 -->
2.2.计算模型
图2为2 cm ECRIT离子源的气体放电图像. 从图2中可以看出, z轴周围有一个圆形的等离子体区域, 其亮度较均匀, 环形天线的阴影区域是可见的. 由图可见, 离子源内的等离子体分布近似轴对称分布. 文献[9]也表明2 cm ECRIT离子源中静磁场轴对称, 高频场接近于轴对称性. 基于以上分析, 计算域等离子体参数分布具有轴对称性, 因此将放电室内的三维的环形电离区简化为二维轴对称区域. 图 2 2 cm ECRIT离子源的放电形貌 Figure2. Discharge image of 2 cm ECRIT ion source.