纵磁作用下真空电弧单阴极斑点等离子体射流三维混合模拟

真空电弧的特性直接受到从阴极斑点喷射出的等离子体射流的影响,对等离子体射流进行数值仿真有助于我们深入了解真空电弧的内部物理机制.然而,磁流体动力学和粒子云网格仿真方法受限于计算精度和计算效率的原因,无法有效地应用于真空电弧等离子体射流仿真模拟.本文开发了一套三维等离子体混合模拟算法,并在此基础上建立了真空电弧单阴极斑点射流仿真模型,模型中将离子作宏粒子考虑,而电子作无质量流体处理,仿真计算了自生电磁场与外施纵向磁场作用下等离子体的分布运动状态.仿真结果表明,单个阴极斑点情况下真空等离子体射流在离开阴极斑点后扩散至极板间,其整体几何形状为圆锥形,离子密度从阴极到阳极快速下降.外施纵向磁场会压缩等离子体,使得等离子体射流径向的扩散减少并且轴线上的离子密度升高.随着外施纵向磁场的增大,其对等离子体射流的压缩效应增强,表现为等离子体射流的扩散角度逐渐减小.此外,外施纵向磁场对等离子体射流的影响也受到电弧电流大小的影响,压缩效应随电弧电流的增加而逐渐减弱.     

等离子体融断开关磁场Hall渗透机制的模拟研究

 利用自行研制的2-1/2维全电磁柱坐标粒子模拟程序对等离子体融断开关磁场渗透机制进行了模拟研究。模拟结果表明在磁场Hall渗透机制特征长度远远小于等离子体离子的无碰撞趋肤深度的条件下,等离子体内部磁场渗透过程主要由电子流体运动的Hall项来控制。对于等离子体空间分布存在较大的密度梯度的物理问题,必须考虑二维空间特性对磁场渗透速度的影响。在磁场已渗透经过的等离子体区域中,等离子体呈现非电中性,离子受静电场的作用会加速运动到达阴极,最终形成真空鞘层。     

镀膜陶瓷真空室脉冲磁场穿透特性研究

介绍了合肥电子储存环新凸轨注入系统中将采用的超高真空陶瓷镀钛真空室的研制状况。对穿过真空室镀膜的脉冲磁场特性进行了理论分析与计算。讨论了不同的镀膜厚度及方式对脉冲磁场均匀性的影响,用跟踪程序计算了注入过程磁场径向不均匀性对束流轨道的扰动,给出了涡流引起的磁场径向不均匀性最大允许值。     

镀膜陶瓷真空室脉冲磁场穿透特性研究

 介绍了合肥电子储存环新凸轨注入系统中将采用的超高真空陶瓷镀钛真空室的研制状况。对穿过真空室镀膜的脉冲磁场特性进行了理论分析与计算。讨论了不同的镀膜厚度及方式对脉冲磁场均匀性的影响,用跟踪程序计算了注入过程磁场径向不均匀性对束流轨道的扰动,给出了涡流引起的磁场径向不均匀性最大允许值。     

非平衡磁控溅射系统离子束流磁镜效应模型

为了研究非平衡磁控溅射沉积系统的等离子体特性，采用常规磁控溅射靶和同轴约束磁场构成非平衡磁控溅射沉积系统.在放电空间不同的轴向位置，Ar放电，02Pa和150V偏压条件下，采用圆形平面离子收集电极，测量不同约束磁场条件下的饱和离子束流密度.研究结果表明，在同轴磁场作用下，收集电极的离子束流密度能达到饱和值9mA/cm2左右，有利于在沉积薄膜的过程中产生离子轰击效应.根据磁流体理论分析了同轴约束磁场形成的磁镜效应和对放电过程的影响机理.实验与模型计算结果的比较表明，模型从理论上表达了同轴磁场约束对非平衡磁控溅射等离子体特性的影响规律. 关键词： 等离子体 金属薄膜/非磁性 磁控溅射 磁镜     

外扩型电磁场控制筒形阴极内等离子体放电输运特性的仿真研究

筒形阴极由于具有向内放电的特性,可改善高功率脉冲磁控溅射技术放电不稳定、溅射材料离化率差异大等缺陷.然而其产生的等离子体仅能依靠浓度差扩散的方式向基体运动,沉积速率并没有明显改善,尤其是在远离阴极区域.使用外扩型磁场对离子运动进行引导,可实现等离子体的聚焦和远距离输运,从而减少离子损失,提高沉积效率.本文从模拟和实验的角度对磁场的布局与设置进行研究,并获得不同磁场条件下的等离子体空间和时间输运特性及其对薄膜沉积的影响.结果表明电磁场的引入不仅可以大幅提高筒形阴极内等离子体的引出效率,实现不同程度的引出或聚焦,而且对等离子体放电也产生明显的增强或减弱,可根据不同的需求或材料进行精确调控.通过控制磁场,可获得较强的Hi PIMS放电和较高的沉积速率,实验结果与仿真预测相符合.该工作完善了Hi PIMS沉积技术在沉积效率上的不足,拓宽了筒形阴极的溅射工艺窗口和适用范围,有助于Hi PIMS更进一步的推广与应用.     

磁场调控型离子源的设计与实验

磁场调控型离子源在离子源等离子体扩散空间中引入轴向强脉冲磁场,磁场起两方面的作用,一是形成潘宁放电效应,使原子、气体分子碰撞电离效率增加;二是在脉冲强磁场的作用下,强轴向磁场将质量较轻的离子约束在轴线上,对质量较重的金属离子约束能力较弱,导致其在等离子体膨胀引出通道中碰壁损失,能够提升引出轻离子的比例。开展了磁场调控的离子源放电结构、强脉冲螺线管磁场以及引出束流光学结构的设计;测量分析了引出离子流强和离子打靶束斑形貌。研究结果表明,强轴向磁场通过等离子体对混合离子成分的筛选作用,可有效提高引出离子流强中的轻离子成分比例。     

圆形复合式磁控溅射阴极设计及其放电特性模拟研究

基于二维有限元算法使用COMSOL软件对圆形复合式磁控溅射阴极的磁场进行了计算,结合Matlab优化工具箱分别采用遗传算法和模拟退火算法对圆形复合式磁控溅射阴极的结构进行优化,得到靶材利用率达到最大的最优结构.对得到的最优化磁控阴极,基于自洽粒子模拟方法,使用VSim软件对不同工况下的放电特性进行了模拟.研究发现随着磁场非平衡度的增加,阴极表面电势降落最大的位置和等离子体聚集的位置,沿着阴极表面外沿不断向阴极中心移动,阴极表面磁场的强度不断减小.随着磁场非平衡度的增加,等离子体密度先增加后减小,鞘层厚度先减小后增加,等离子体的密度和鞘层厚度不仅与磁场非平衡度有关,而且与磁场强度有关.最后根据粒子模拟的结果,对复合式磁控阴极的靶材刻蚀深度进行了研究.研究发现,在优化前后靶材的刻蚀范围从60 mm扩展至整个靶面,极大地提高了靶材利用率.     

用于真空弧等离子体放电阴极温度场测量的多光谱高温计

阴极表面温度是真空弧等离子体放电过程中一个重要参数,对真空弧等离子体的形成、电极腐蚀预测、热传导以及离子源的寿命都有重要影响。真空弧离子源的阴极具有目标小,放电过程快等特点,其温度的测量,对于时间分辨率和空间分辨率要求都很高,阴极表面温度的测量技术的欠缺,使得仅靠理论解析获得的结果难以得到验证。并且等离子体放电过程中测量仪器极易受到弧光的影响,如何避免放电过程中等离子体的辐射也是采用辐射法测量阴极表面温度要考虑的问题。这无疑给其温度场的测试研究带来困难。针对脉冲真空弧等离子体开展阴极表面温度测试实验有着重要意义,在分析了真空弧等离子体放电特性以及背景辐射特性和等离子体放电阴极测温的实际需求,本文基于高速CCD相机研制了一种新型的多光谱高温计。该高温计采用单色高速CCD相机,主要避免RGB彩色相机不能完全滤除背景辐射的弧光。为使用单色CCD相机实现多光谱辐射测温,设计了高温计的光学系统,该系统采用4孔径分光系统。将4种不同波长的滤光片嵌入到1个滤光片中。该研究设计的高温计可用于2 000~6 000 K的等离子体温度测量。并在中国工程物理研究院电子工程研究所进行现场测试,测试过程中将研制的高温计,通过外部触发形式对等离子体放电过程进行跟踪拍摄,高温计完全拍摄到等离子体放电过程。利用真空弧等离子体金属电极阴极放电的实测数据对高温计进行了验证。实验结果表明,设计的新型多光谱高温计能够用于测量真空弧等离子体放电时阴极温度场信息,测量的温度值低于放电电极的沸点温度,与等离子体放电过程中出现气化现象相符,说明高温计测的是等离子体放电阴极的温度。     

面向复杂求解域的高效粒子网格/蒙特卡罗模型与阳极层离子源仿真

等离子体仿真是研究等离子体放电特性的重要手段,特别是阳极层离子源,其放电结构的几何特性对等离子体特性的作用很难通过实验手段进行系统研究.然而,传统仿真模型一般是针对离子源进行整体建模,离子源的阴阳极几何轮廓形成的复杂求解域,导致模型的计算效率和收敛性较差.鉴于此,将离子源结构仿真与等离子体仿真分离,首先利用磁镜原理将离子源内外阴极大小、形状和相对位置等一系列阴极几何参数简化为磁镜比R_m和磁镜中心磁感应强度B₀两个磁镜参数,并在此基础上,建立了高效粒子网格/蒙特卡罗模型,将收敛时间由1.00μs缩短到0.45μs,大幅提升了计算效率和稳定性.进一步利用该模型系统研究了阳极层离子源放电结构的几何特性对等离子体特性的影响规律,发现R_m=2.50, B₀=36 mT时磁镜对等离子体约束效果最佳,当放电中心的位置与内外阴极间磁镜中心重合时,不仅能够输出高密度离子束流,同时可大幅减少阴极刻蚀,并保证内外阴极的刻蚀平衡.     

Improvement of the efficiency of a glow discharge-based ion emitter with oscillating electrons

Ion emission from the plasma of a low-pressure (≈5×10⁻² Pa) glow discharge with electrons oscillating in a weak (≈1 mT) magnetic field is studied in relation to the cold hollow cathode geometry. A hollow conic cathode used in the electrode system of a cylindrical inverted magnetron not only improves the extraction of plasma ions to ≈20% of the discharge current but also provides the near-uniform spatial distribution of the ion emission current density. The reason is the specific oscillations of electrons accelerated in the cathode sheath. They drift in the azimuth direction along a closed orbit and simultaneously move along the magnetic field toward the emitting surface of the plasma. A plasma emitter with a current density of ≈1 mA/cm² over an area of ≈100 cm² designed for an ion source with an operating voltage of several tens of kilovolts is described.     

A physical model of the low-current-density vacuum arc

A one-dimensional (1-D) physical model of the low-current-density steady-state vacuum arc is proposed. The model is based on the continuity equations for ions and electrons and the energy balance for the discharge system; the electric potential distribution in the discharge gap is assumed to be nonmonotonic. It is supposed that the ion current at the cathode is generated within the cathode potential fall region due to the ionization of the evaporated atoms by the plasma thermal electrons having Boltzmann's energy distribution. The model offers a satisfactory explanation for the principal regularities of a hot-cathode vacuum arc with diffuse attachment of the current. The applicability of the model proposed to the explanation of some processes occurring in a vacuum arc, such as the flow of fast ions toward the anode, the current cutoffs and voltage bursts, and the backward motion of a cathode spot in a transverse magnetic field is discussed     

Energy distributions of electrons emitted from tungsten tips covered by diamond-like films

The energy distribution of electrons emitted from the surface of diamond-like pointed cathodes under the action of a high electric field is reported. Diamond-like coatings are applied on thin tungsten tips by ion-beam evaporation in an ultrahigh vacuum. The structure of the carboniferous films covering the tungsten tips is examined by field-emission microscopy. The stability of the field-emission cathode current is considered, and the Fowler-Nordheim I-V characteristics are presented. Based on the results obtained, a model of field-emission cathode covered by a thin diamond-like coating that explains the energy distributions is suggested.     

Determination of the specific ion erosion of the vacuum arc cathode by measuring the total ion current from the discharge plasma

The specific ion erosion γ_i of cathodes made of C, Mg, Al, Ti, Co, Cu, Y, Mo, Cd, Sm, Ta, W, Pt, Pb, and Bi is determined by measuring the total ion current from the vacuum arc plasma. It is demonstrated that the ratio of the total ion current to the discharge current, α_i in a vacuum arc varies from 5 to 19%, depending on the cathode material. It is found experimentally that the ion current fraction α_i is inversely proportional to the atomic bond energy of the cathode material. It is shown that an increase in the total ion current extracted from the discharge plasma when applying an external magnetic field to the cathode region of the discharge is related solely to the appearance of ions with higher charge numbers in the plasma, while the magnitude of the specific ion erosion γ_i remains unchanged.     

Cathode spot dynamics on pure metals and composite materials inhigh-current vacuum arcs

An investigation has been carried out of cathode spot dynamics in a triggered vacuum arc in a demountable chamber. A rectangular current pulse of 1-5 kA, 1-5 ms has been used. Sufficient statistics were collected. The expansion of a cathode spot ring on a clean, pure metal surface was corroborated to be a retrograde movement in the self-magnetic field which obeys the same law as the movement of a single spot in an external magnetic field. The influence of a contact gap of 0.5-8 mm and current on the dynamics of cathode spots was investigated. The gap dependence of the proportional coefficient between the spot velocity and magnetic field in the case of a pure copper cathode was obtained. A phenomenon was discovered, where a group of cathode spots form in the short arcs on the CuCr cathodes after a transition diffuse arc stage. The follow-up investigation revealed that a close interrelation exists between the cathode and anode processes in short arcs. This interrelation is responsible for the appearance of the discovered phenomenon. Short-circuit performance tests conducted for a commercial vacuum interrupter proved cathode spot group formation to be responsible for the interruption failure at short contact gaps     

Performance Improvements in the Relativistic Magnetron: The Effect of DC Field Perturbations

The effect of using a shaped or transparent cathode to enhance key performance parameters in the Michigan magnetron, such as output power, power efficiency, and mode purity, was examined using a massively parallel electromagnetic particle-in-cell code Improved Concurrent Electromagnetic Particle-In-Cell. In simulations, we saw a dramatic increase in the range of magnetic field and input voltage over which the magnetron functions, an increase in output power, an increase in efficiency, an elimination of mode competition, and immediate start-up for both the shaped and transparent cathodes relative to the standard cathode. These simulations also revealed that the mechanism that is responsible for the improved performance that accompanies the transparent cathode may not be RF-azimuthal-field penetration but rather perturbations in the “dc” field resulting from the design of the novel cathodes themselves.      

The Influences of the Magnetic Field Strength on the Magnetic Confinement of Primary Electrons in an Ion Source

This paper deals with the simulation of trajectories of energetic electrons under the influence of a multi-cusp magnetic field produced by a permanent magnet assembly in a ring cusp geometry which is designed for supporting a plasma based ion source. The primary electrons are assumed as being emitted from a cathode located coaxially with the cylindrical discharge chamber. The volume of the region occupied by the electron trajectories and the magnetic trapping time were evaluated and plots are presented showing the effects of the surface magnetic field strength of the magnets on the volume and the mean confinement time of primary electrons in the ion source.     

空心阴极直流放电的二维自洽模型描述和阴极溅射分析

建立了空心阴极放电的二维自洽理论模型,理论研究了气压为50—120Pa,电压为150—300V的范围内Ar空心阴极放电特性、粒子密度和电离速率空间分布,特别考察了影响阴极溅射分布有关因素:阴极面上的电场、离子流和离子密度的沿阴极截面的空间分布.研究结果不仅证实了在所讨论的范围内,空心阴极效应明显存在而且发现归一化电离速率的空间分布形状强烈依赖于气压.通过研究电场、离子流和离子密度的空间分布解释了空心阴极溅射型离子激光器中不均匀阴极溅射的现象来源于阴极面附近的电场、离子流和离子密度的不均匀分布 关键词： 空心阴极放电 自洽模型 气体激光 阴极溅射     

Numerical simulations of the reditron

The reflected-electrons discrimination microwave generator (reditron) is a high-power, narrow-band, and single-mode microwave generator that makes exclusive use of the oscillatory character of the virtual-cathode of a relativistic electron beam. The complex, nonlinear character of the virtual-cathode device necessitates particle-in-cell plasma simulation techniques. Investigations indicate two sources of the radiation: (1) the trapped electrons reflexing between the real and virtual cathodes, and (2) the oscillation of the virtual cathode. In the conventional design, the two mechanisms coexist and interfere with each other destructively, causing degradation of the efficiency of microwave generation. The authors have investigated a configuration with a slotted, thick anode and an external magnetic field, which effectively eliminates the reflexing electrons. Two-dimensional particle-in-cell simulations showed that such a configuration exploits the oscillation of the virtual cathode exclusively, and it generates single-mode, narrow bandwidth, and high-power microwave radiation with a potential efficiency over 10%. It was found that further optimization could be achieved by the use of a density (current) modulated electron beam at appropriate frequencies