离子初始速度对等离子体鞘层厚度的影响

本文从理论上研究了离子初始速度对等离子体鞘层厚度的影响,建立了理论计算公式和数值模拟方法,考察了离子初始速度对鞘层厚度的影响因子。在离子初始能量为3.2 eV情况下,用粒子模拟程序进行了数值模拟,模拟与理论计算得到的空间电场分布十分接近。     

托卡马克刮削层等离子体-器壁间过度层中的离子碰撞效应(英文)

在考虑粒子碰撞的情况下,利用二流体模型研究了等离子体与器壁之间的相互作用。首先,应用类似于分析势阱中粒子运动的方法研究了德拜 (Debye) 鞘层判据和边界条件,结果表明,原来用于推导鞘层判据 (Bohm'scriterion) 的方法在有些情况下是不适用的,而且求得鞘层判据的新的表示式,其中包含碰撞的影响。此外分析了磁预鞘层 (magnetic presheath) 边界的离子流和预鞘层宽度之间的关系,发现在一定的碰撞频率下,沿磁力线方向的离子速度随预鞘层宽度的增加而减小,其规律类似于指数下降,因此可以用离子速度的e-倍衰减长度 (e-foldlength) 定义预鞘层宽度,基于这一定义计算了边界离子流与碰撞之间的函数关系。     

DPF中子源中子产生机理成因分析

基于等离子体的动量和能量守恒方程,利用一维雪耙模型,描绘出等离子体焦点装置放电电流壳层的动力学行为,得到整个电流壳层在两极间的运动情况,计算了电流壳层的加速段时间,计算结果与实验符合得很好.实验测量了等离子体鞘的反箍缩速度、加速阶段速度、聚焦阶段箍缩速度和崩溃阶段速度及中子发射异向性因子,均证明焦点装置中子产生机制主要由束靶机制控制.     

ITER高能alpha粒子轨道损失模拟研究

基于蒙特卡罗方法和无碰撞模型,采用测试粒子模拟方法,研究ITER等离子体中高能alpha粒子在无磁扰动和有磁扰动情况下分布与损失规律。结果表明:在无磁扰动条件下,alpha粒子的损失比例约为0.83%,这些损失的alpha粒子的初始位置主要位于靠近等离子体边界(最外层闭合磁面)的地方;在低频有磁扰动条件下,随着磁扰动幅度的增加,磁岛宽度增加,高能alpha粒子的损失率相应增加,但是其损失率仍然比较小(2%);损失的高能alpha粒子的初始飞行方向大部分为逆等离子体电流方向,而它们在飞出等离子体边界时的飞行方向大部分为顺等离子体电流方向,所有损失的alpha粒子几乎都是从等离子体赤道面以下飞出。     

考虑共振电荷转移的二维等离子体离子收集过程

对考虑了共振电荷转移碰撞过程的二维ＡＶＬＩＳ离子引出收集问题进行了数值模拟，分析结果表明：碰撞主要发生在鞘层中；影响碰撞损失率的主要因素是极板间距，应选择小于３ｃｍ的收集板间距；离子引出时间不是判断碰撞损失率的标准。     

石墨表面的热等离子体去污

利用搭建的热等离子体实验装置,对重金属镉表面沾污的石墨样片进行去污处理,并采用X射线荧光分析去污前后的沾污程度。通过改变样片沾污方式、去污时间和输出电流,对热等离子体的干法去污能力进行了探讨。研究结果表明,热等离子体的去污率与其剥离厚度息息相关,且两者都随着输出电流和去污时间的增大而增大。当输出电流为60 A、去污时间为100 s时,滴加式和浸泡式样片的去污率分别为92%和65%,剥离厚度均达到了几十微米。在同样的实验条件下对热等离子体湿法去污进行了探索,发现在相同输出电流时,随着去污时间的增加湿法去污比干法去污具有更高的去污率,为不同去污场合提供了更多的选择。     

平行极板间等离子体RF共振特性研究

为探索用于ＡＶＬＩＳ的更为有效的离子引出方法,对等离子的ＲＦ共振离子出引方法进行了理论研究,用ＰＩＣ方法对其了模拟。结果表明：在弱磁场下可产生鞘层－等离子体振荡,其振荡频率与理论一致;外加与振荡频率王牟的交流电压后将发生共振,其振荡幅度大幅增加,电流加大,共振在等离子体中产生的最大电位幅可高于外加电压幅度几倍;与离子引出方法中传统的平行板电场相比,ＲＦ共振法使得出出的离子流加大,离子引出时间显著     

不同初始温度下氩等离子体演化过程研究

本文对不同初始温度下氩等离子体中主要粒子随时间的演化规律进行数值模拟,跟踪各主要成分粒子的浓度随时间的变化,得到放电后氩等离子体中主要带电粒子和中性粒子浓度随时间的变化规律。计算结果表明,氩等离子体化学反应达到平衡所需时间随初始温度升高逐渐减少,活性粒子Ar⁺浓度随时间不断增大,Ar^*、Ar^**浓度随时间先增大后减小,最终趋于稳定值达到平衡状态。     

考虑溅射和碰撞损失的j×B离子引出收集

研究了j×B法离子引出收集过程。影响引出过程收集率的两个主要因素是碰撞和溅射。对引出过程中收集板的收集情况进行了数值模拟,给出了收集板电参数对引出时间、碰撞与溅射的影响。收集板电参数对引出时间和收集率有很大影响,电流js及其变化率的增加对离子引出是有利的。总收集率随等离子体密度增大而减小。收集板间距对引出的影响较为复杂,增大极板间距,且限制准直孔径,引出效果得以改善。     

10cm×30cm矩形射频离子束源的研制

本文介绍了射频(Radio frequency,RF)感应耦合等离子体(Inductive couple plasma,ICP)离子束源的设计研究.该射频离子束源可工作于Ar,在使用四栅引出系统时,可获得100-1000 eV的离子束.当射频功率为900 W,在Ar为工作气体时,束流可达到600 mA.在束流为120 mA时,距源26 cm处,在主轴方向27 cm的范围内不均匀性小于±6%.该离子束源可作为大面积离子束刻蚀、离子束抛光等的离子束源.     

Frequency Matching Effects on Characteristics of Bulk Plasmas and Sheaths for Dual-Frequency Capacitively Coupled Argon Discharges： One-Dimensional Fluid Simulation

A one-dimensional fluid model is proposed to simulate the dual-frequency capacitively coupled plasma for Ar discharges. The influences of the low frequency on the plasma density, electron temperature, sheath voltage drop, and ion energy distribution at the powered electrode are investigated. The decoupling effect of the two radio-frequency sources on the plasma parameters, especially in the sheath region, is discussed in detail.     

Sheath Structures of Strongly Electronegative Plasmas

The sheath structures of strongly electronegative plasmas axe investigated on basis of the accurate Bohm criterion obtained by Sagdeev potential. It is found that the presheath transition between the bulk plasma and the sheath almost does not exist there, and that distributions of electrons, negative and positive ions in the sheath form a pure positive ion sheath near the boundary of the electrode. Furthermore, the density distribution of space net charge has a peak near the sheath edge, the spatial potential within the sheath falls faster, and the sheath thickness becomes thinner.     

Study of Characteristics of the Radio-Frequency Sheath over a Substrate with a Circular Trench

Since processed substrates usually exhibit nonplanar surface structures in Micro-Electro-Mechanical-Systems (MEMS) etching, a two-dimensional (2D) fluid model is developed to simulate the characteristics of the sheath near a conductive substrate with a circular trench, which is placed in an argon discharge powered by a radio-frequency (rf) current source. The model consists of 2D time-dependent fluid equations, the Poisson equation, and a current balance equation that can self-consistently determine the instantaneous voltage on the substrate placed on a powered electrode. The effects of both the aspect ratio (depth/width) and the structure of the trench on the characteristics of the sheath are simulated. The time-averaged potential and electric field in the sheath are calculated and compared for different discharge parameters. The results show that the radial sheath profile is not uniform and always tends to adapt to the contour of the substrate, which is believed to be the moulding effect. Affected by the structure of the substrate surface, the potential and electric field near the inner and outer sidewalls of the trench exhibit obvious non-uniformity, which will inevitably lead to non-uniformity in etching, such as notching. Furthermore, with a fixed amplitude of the rf current source, the potential drops and the sheath thickness decrease with an increase in aspect ratio.     

Two-Dimensional Fluid Simulation of Collisional Plasma sheath over rf Powered Electrode with Cylindrical Hole

The characteristics of collisional radio-frequency (rf) sheath dynamics over an electrode with a cylindrical hole is simulated by means of a self-consistent model which consists of two-dimensional time-dependent fluid equations coupled with Poisson equation. In addition, an equivalent-circuit model is coupled to the fluid equations in order to self-consistently determine relationship between the instantaneous potential at the rf-biased electrode and the sheath thickness.Two-dimensional profiles of the potential, the ion fluid velocity, and the distributions of the ion and electron densities within the sheath are computed under various discharge conditions, such as the discharge powers and the gas pressures. The results show that the existence of the cylindrical hole on the electrode significantly affects the sheath structure and generates a potential trap in the horizontal direction, which is particularly strong when the sheath thickness is comparable to the depth of the hole. Moreover, it is found that the collisional effects have a significant influenc eon the sheath characteristics.     

Collisional Sheath in the Electronegative Radio-Frequency Plasma

A model of collisional RF sheath with negative ions is discussed in this paper. The influences of collision and negative ions on the parameters of the sheath are studied through numerical simulation. It is found that when the collision coefficient increases and the RF power is fixed, the electrode potential and sheath electric field potential increase, the electrode current and thickness of the sheath decrease. When the negative ion content changes, the same phenomenon Occurs,     

Determination of plasma parameters in rf glow-discharge plasma

The Langmuir probe measurements were carried out in a planar rf discharge of N₂ and Ar gases. The dc characteristic I-U curves are calculated from the measured rf characteristic I-U curves for frequencies 10-60 MHz. The measured parameters such as electron temperature and electron density are compared with the simulated results. At gas pressures 10-40 Pa, the sheath thickness at the powered electrode was proportional to f^−0.5 for simulation and f^−2/3 by using the electrical parameters of the probe measurements and to p^−0.5 for both cases.     

Study on the characteristics of non-Maxwellian magnetized sheath in Hall thruster acceleration region

The secondary electron emission (SEE) and inclined magnetic field are typical features at the channel wall of the Hall thruster acceleration region (AR), and the characteristics of the magnetized sheath have a significant effect on the radial potential distribution, ion radial acceleration and wall erosion. In this work, the magnetohydrodynamics model is used to study the characteristics of the magnetized sheath with SEE in the AR of Hall thruster. The electrons are assumed to obey non-extensive distribution, the ions and secondary electrons are magnetized. Based on the Sagdeev potential, the modified Bohm criterion is derived, and the influences of the non-extensive parameter and magnetic field on the AR sheath structure and parameters are discussed. Results show that, with the decrease of the parameter q, the high-energy electron leads to an increase of the potential drop in the sheath, and the sheath thickness expands accordingly, the kinetic energy rises when ions reach the wall, which can aggravate the wall erosion. Increasing the magnetic field inclination angle in the AR of the Hall thruster, the Lorenz force along the $x$ direction acting as a resistance decelerating ions becomes larger which can reduce the wall erosion, while the strength of magnetic field in the AR has little effect on Bohm criterion and wall potential. The propellant type also has a certain effect on the values of wall potential, secondary electron number density and sheath thickness.     

Modeling of magnetized collisional plasma sheath with nonextensive electron distribution and ionization source

The properties of an atmospheric-pressure collisional plasma sheath with nonextensively distributed electrons and hypothetical ionization source terms are studied in this work. The Bohm criterion for the magnetized plasma is extended in the presence of an ion–neutral collisional force and ionization source. The effects of electron nonextensive distribution, ionization frequency, ion– neutral collision, magnetic field angle and ion temperature on the Bohm criterion of the plasma sheath are numerically analyzed. The fluid equations are solved numerically in the plasma–wall transition region using a modified Bohm criterion as the boundary condition. The plasma sheath properties such as charged particle density, floating sheath potential and thickness are thoroughly investigated under different kinds of ion source terms, contributions of collisions, and magnetic fields. The results show that the effect of the ion source term on the properties of atmosphericpressure collisional plasma sheath is significant. As the ionization frequency increases, the Mach number of the Bohm criterion decreases and the range of possible values narrows. When the ion source is considered, the space charge density increases, the sheath potential drops more rapidly, and the sheath thickness becomes narrower. In addition, ion–neutral collision, magnetic field angle and ion temperature also significantly affect the sheath potential profile and sheath thickness.     

Modelling of electronegative collisional warm plasma for plasma-surface interaction process

An electronegative collisional plasma having warm and massive positive ions, non-extensive distributed electrons and Boltzmann distributed negative ions is modelled for the plasma-surface interaction process that is used for the surface nitriding. Specifically the sheath formation is evaluated through the Bohm's criterion, which is found to be modified, and the variation of the sheath thickness and profiles of the density of plasma species and the net space charge density in the sheath region in addition to the electric potential. The effect of ion temperature, nonextensivity and collisional parameter is examined in greater detail considering the collisional cross-section to obey power-law dependency on the positive ion velocity. The positive ions are found to enter in the sheath region at lower velocities in the collisional plasma compared to the case of collision-less plasma; this velocity sees minuscule reduction with increasing nonextensivity. The increasing ion temperature and collisional parameter lead to the formation of sheath with smaller thickness.     

Kinetic-theory-based investigation of electronegative plasma–wall transition with two populations of electrons

Kinetic theory has been employed to investigate the magnetized plasma-sheath structure and its characteristics in the presence of more than one species of negatively charged particles: hot electrons,cold electrons, and negative ions. The cold electrons and negative ions are considered to obey a Maxwellian distribution, whereas the hot electrons follow a truncated Maxwellian distribution. The Bohm sheath condition has been extended for the case of more than one species of negatively charged particles, in which the concentration of hot electrons has a crucial role in achieving the Bohm velocity. The thermal motion of hot electrons is much higher compared to cold electrons and negative ions, such that the variation of hot electron concentrations and the temperature ratio of hot to cold electrons play a key role in the determination of the plasma-sheath parameters: particle densities,electrostatic potential, the flow of positive ions towards the wall, and sheath thickness. We have estimated the deviation of the resultant drift velocity of positive ions on the plane perpendicular to the wall from the parallel component at the presheath–sheath interface. It is found that the deviation between the two velocity components increases with an increase in the obliqueness of the magnetic field. Furthermore, the results obtained from the kinetic trajectory simulation model are compared with the results obtained using a fluid model; the results are qualitatively similar, although the potential varies by less than 4% in terms of the magnitude at the wall.