Modeling of plasma behavior in a plasma electrode Pockels cell

We present three interrelated models of plasma behavior in a plasma electrode Pockels cell (PEPC). In a PEPC, plasma discharges are formed on both sides of a thin, large-aperture electro-optic crystal (typically KDP). The plasmas act as optically transparent, highly conductive electrodes, allowing uniform application of a longitudinal field to induce birefringence in the crystal. First, we model the plasma in the thin direction, perpendicular to the crystal, via a one-dimensional fluid model. This yields the electron temperature and the density and velocity profiles in this direction as functions of the neutral pressure, the plasma channel width, and the discharge current density. Next, me model the temporal response of the crystal to the charging process, combining a circuit model with a model of the sheath which forms near the crystal boundary. This model gives the time-dependent voltage drop across the sheath as a function of electron density at the sheath entrance. Finally, we develop a two dimensional MHD model of the planar plasma, in order to calculate the response of the plasma to magnetic fields. We show how the plasma uniformity is affected by the design of the current return, by the longitudinal field from the cathode magnetron, and by fields from other sources. This model also gives the plasma sensitivity to the boundary potential at which the top and bottom of the discharge are held. We validate these models by showing how they explain observations in three large Pockels cells built at Lawrence Livermore National Laboratory     

A one-dimensional fluid simulation of a magnetized DC discharge including the non-uniform effects of the magnetic field

A one-dimensional simulation of a magnetized DC discharge is conducted based on the fluid equations that consider the non-uniform effects of the magnetic field. We apply a dielectric relaxation scheme (DRS) as an efficient numerical algorithm that was recently developed for simulation of low-temperature process plasmas. The simulation results, such as the spatial profiles of ion densities, electron densities, electron fluxes, and electric fields, are compared with the simulation results for which the non-uniform effects of the magnetic field are neglected.     

Modeling of argon discharge characteristics of planar-type surface wave plasmas in an electron fluid model

The planar-type surface wave plasma (SWP) device permits the generation of high-density and uniform processing plasmas via 2.45-GHz microwave power without the application of an external magnetic field. In the present study, the discharge characteristics in the SWP device were analyzed using a two-dimensional numerical simulation code, and the results were compared with experimental observations. The simulation code is based on the finite-difference time-domain (FDTD) method for the microwave field and on the electron fluid model for the argon discharge plasma. Experimental measurements were performed, and they showed that the surface-wave discharge at a filling pressure of 10-100 mtorr has characteristic electron-density distributions that have a peak at approximately 2 cm from the surface. This characteristic of the electron density profiles, as well as the electron temperature profiles in the plasma, is reproduced by the simulation code, albeit with some discrepancies. In order to reduce the effects of these discrepancies, intentional changes in the electron heat conductivity were introduced, and the adiabatic assumption was found to result in a reasonable electron temperature profile. The effects of the alumina window thickness were also investigated in the simulation.     

真空二极管辐射微波的机理分析

在研究真空开关的过程中,发现真空二极管能辐射出宽带微波.这种器件只由带触发装置的阴极和平板阳极组成,不存在金属波纹慢波结构,所以真空二极管的辐射机理与等离子体填充微波器件不同,不能直接套用等离子体填充微波器件的相关理论.本文描述了真空二极管产生辐射的物理过程,建立了真空二极管辐射的数学模型,通过求解波动方程得到产生辐射的色散关系,并绘制出了色散曲线.将理论分析得到的色散曲线与已经测得的微波辐射进行比较,两者能很好地符合.理论分析和实验结果表明,电子束和磁化等离子体的相互作用是真空二极管产生微波辐射的原因.     

高功率微波与等离子体相互作用理论和数值研究

研究高功率微波与等离子体的相互作用,对于微波放电和电磁兼容研究均具有重要意义.基于波动方程、等离子体的流体力学方程以及波尔兹曼方程,建立高功率微波脉冲与等离子体相互作用的理论模型,并结合等离子体的特征参数,采用时域有限差分方法分析了等离子体电子密度和高功率微波传输特性的变化.结果表明,由于高功率微波的电子加热作用,等离子体中的非线性效应明显,发生击穿使得等离子体电子密度增大,从而导致微波的反射增强,透过率降低.所提出的模型和相关结果对于高功率微波和电磁脉冲防护具有指导意义.     

电子回旋共振等离子体密度均匀性的数值研究

基于漂移扩散近似,在轴对称假设下,对电子回旋共振等离子体源腔室内的等离子体建立了二维流体模型.采用有限差分法对所建立的模型进行了自洽数值模拟,得到了等离子体密度均匀性随时间演化的数值结果.通过对数值结果的分析,研究了背景气体压强、微波功率和磁场线圈电流对等离子体密度均匀性的影响.研究表明,在电离初期,电子密度的均匀性好于离子密度的均匀性.在电离后期,离子密度的均匀性好于电子密度的均匀性.随着背景气体压强的增大,电子密度和离子密度的均匀性都在增加,且离子密度的均匀性增加的更快.随着微波功率的增大,电子密度和 关键词： 等离子体密度均匀性 背景气体压强 微波功率 磁场线圈电流     

The effects of a small transverse magnetic field upon acapacitively coupled RF discharge

A capacitively-coupled RF argon discharge at a pressure of 10 mTorr with a plate separation of 7.5 cm has been studied both experimentally and using a one-dimensional particle in cell simulation with Monte Carlo collisions. A magnetic field of 0 to 60 G is applied in the direction parallel to the capacitor plates. In the simulation it was found that as the magnetic field was increased such that the electron cyclotron orbit radius of the hot electrons became smaller than of the order of the discharge length, the electron heating in the bulk of the discharge increased. The dominant electron heating mechanism was observed to change from a stochastic sheath to a bulk ohmic electron heating mode, with a variation of field from a to 10 G. This was accompanied by a drop in the plasma density at small magnetic fields, which was also observed experimentally. At higher magnetic fields the plasma density was found to increase, A detailed discussion of the simulation results is presented drawing comparisons with the experimental results, with which there is good agreement, and a simple magnetohydrodynamic model for the bulk heating     

霍尔效应推力器放电双稳态机理研究

实验发现霍尔效应推力器在自励磁模式下具有两个稳定的放电工作点,且运 行过程中在这两个工作点上往复跳变,很大程度上影响了推力器通道内等离子体的放 电物理过程及其综合性能.本文结合推力器放电磁安特性曲线与励磁电流曲线的相互关 系,给出了推力器放电双稳态特性形成的物理机理．在此基础上提出了通过改变励磁电 流曲线斜率,使推力器稳定工作于单放电工作点的方法,结合一维动态流体模型给予了物 理解释,并通过实验加以验证.     

两种外磁场形式对介质面次级电子倍增的抑制

利用自编1D3V PIC程序,数值研究了不同外加磁场方式对次级电子倍增抑制的物理过程,给出了次级电子数目、平均能量、密度、运动轨迹、渡越时间、介质表面静电场及沉积功率等物理量时空分布关系。模拟结果表明:不同方向外加磁场抑制次级电子倍增的机理有所不同。轴向外加磁场利用电子回旋运动干扰微波电场对电子加速过程,使其碰壁能量降低以达到抑制二次电子倍增的效果;横向外加磁场利用电子回旋漂移过程中,电子半个周期被推离介质表面（不发生次级电子倍增）,半个周期被推回介质表面（降低电子碰撞能量）的作用机理,达到抑制二次电子倍增的效果。讨论了横向磁场在回旋共振下,电子回旋同步加速导致回旋半径增大,电子能量持续增加的特殊过程。两种外加磁场方式都可以通过增加磁场达到进一步抑制次级电子倍增的目的。轴向外加磁场加载容易,但对磁场要求较高;横向外加磁场需要磁场较低,但加载较为困难。     

MHD plasma physics in rail accelerators for hydrogen-pelletinjection in fusion reactors

The behavior of the electromagnetic and thermal quantities in a plasma arc placed between two conducting rails is analyzed. The plasma hydrogen drives the hydrogen pellets for the refueling of magnetic fusion reactors. Considering the general equations of electromagnetism and of plasma fluid dynamics and assuming steady-state conditions in a frame which is moving at the same rate as the plasma arc armature, a one-dimensional model is deduced. The effects of an applied magnetic field on the behavior of all flow variables are investigated. Results indicate that the adverse effects of plasma arc heating can be reduced by the application of a magnetic-induction field normal to the current path in the armature. At the maximum acceleration pressure (30 bar) applicable to the hydrogen pellet in the proposed one-dimensional model, the arc temperature at the pellet backend falls from 20000 to 14000 K when a magnetic induction of about 5 T is applied     

大气压脉冲调制射频氩气放电等离子体特性的数值研究

基于等离子体流体理论,建立了大气压脉冲调制射频氩气放电的一维流体模型。通过数值模拟的方 法研究了放电参数(放电电极间距、射频频率)对氩等离子体特性的影响。研究结果表明：当电压固定时,随着电 极间距的增加,等离子体区逐渐增大,最大电子密度也增加,在 0.20cm 达到最大值后略有降低;放电电流密度 与输入功率密度随着电极间距的增加而增加;鞘层区电子温度随着电极间距的增加而降低;在脉冲开启前期,等 离子体区电子温度随着电极间距的增加而增加,但当脉冲开启后期,电极间距对等离子体区电子温度影响较小。 不同射频频率下最大电子密度随电极间隙的增加而减小,也具有一个最优值。      

高功率微波输出窗内侧击穿动力学的PIC/MCC模拟研究

高功率微波在受控热核聚变加热、微波高梯度加速器、高功率雷达、定向能武器、超级干扰机及冲击雷达等方面有着重要的应用.本文针对高功率微波输出窗内侧氩气放电击穿过程,建立了二次电子倍增和气体电离的一维空间分布、三维速度分布（1D3V）模型,并开发了相应的PIC/MC程序代码.研究了气压、微波频率、微波振幅对放电击穿的影响.结果表明：在真空情况下,介质窗放电击穿只存在二次电子倍增过程;在低气压和稍高气压时,二次电子倍增和气体电离共存;在极高气压时,气体电离占主导.给出了不同气压下电子、离子的密度和静电场的空间分布.此外还观察到,在500 mTorr时,随着微波振幅或微波频率的变化,气体电离出现的时刻和电离产生的等离子体峰值位置有较大差异,尤其是当微波频率（GHz）在数值上是微波振幅（MV/m）的2倍时,气体电离出现的较早.     

Equilibrium profiles for RF-plasma sources with ponderomotiveforces

An analysis is given of the influence of the electron ponderomotive force on the equilibrium plasma profiles of partially ionized, radio frequency discharge sources, The ponderomotive force can be written as a gradient of a potential varying with the square of the RF field in the plasma and is largest for electrons, The impact of this electron ponderomotive force on density and electrostatic potential profiles is demonstrated using a one-dimensional analytic model with supporting numerical solutions and a two dimensional fluid simulation. For nearly collisionless plasmas the ponderomotive force is valid when ω_ce^h/ω<1 where ω_ce ^h is the electron cyclotron frequency due to the RF magnetic field and ω is the RF driving frequency, In processing plasmas with parameters that satisfy this validity criteria, the equilibrium density profiles are weakly modified, For nearly collisionless processing plasmas with parameters such that ω_ce^h /ω>1, the ponderomotive force, is modified by other nonlinear force terms that need to be evaluated     

射频容性耦合等离子体放电特性的光谱诊断

利用流体模型模拟和发射光谱实验诊断相结合的方法,研究了中等气压、中等功率下射频容性耦合等离子体的放电特性。理论上,采用基于流体模型的COMSOL软件仿真,建立一维等离子体放电模型,以Ar气为工作气体,研究了不同气压以及不同射频输入功率下等离子体电子温度和电子密度的分布规律。实验上,依据仿真模型设计制作了相同尺寸的密闭玻璃腔体和平板电极,采用13.56 MHz射频放电技术电离腔体内的工作气体Ar气,测量了不同气压、不同射频输入功率时放电等离子体的发射光谱。通过分析和选择适当的Ar Ⅰ和Ar Ⅱ的特征谱线,分别利用玻尔兹曼斜率法以及沙哈-玻尔兹曼方程计算了等离子体的电子温度与电子密度,并结合模拟仿真结果对光谱诊断结果进行了修正。结果表明：当气体压强为300~400 Pa、输入功率为600~800 W时,等离子体近似服从玻尔兹曼分布,此时利用光谱法得到的等离子体参数与仿真结果相符合。仿真模拟与光谱实验诊断相结合的方法可初步诊断出中等气压下等离子体的放电参数,增加了玻尔兹曼斜率法和沙哈-玻尔兹曼方程在等离子体放电中的使用范围,扩大了光谱法在低电子密度容性耦合等离子体参数诊断的应用场合,为中等气压容性耦合等离子体在工业与军事上的应用研究提供了重要物理状态的分析手段。     

不同磁路电子回旋共振离子源引出实验

空间推进所用的电子回旋共振离子源(ECRIS)应具有体积小、效率高的特点. 本文研究的ECRIS使用永磁体环产生磁场, 有效减小了体积, 该离子源利用微波在磁场中加热电子, 电子与中性气体发生电离碰撞产生等离子体. 磁场在微波加热电子的过程中起关键作用, 同时影响离子源内等离子体的约束和输运. 通过比较四种磁路结构离子源的离子电流引出特性来研究磁场对10 cm ECRIS性能的影响. 实验发现: 在使用氩气的条件下, 特定结构的离子源可引出160 mA的离子电流, 最高推进剂利用率达60%, 最小放电损耗为120 W·A^-1; 所有离子源均存在多个工作状态, 工作状态在微波功率、气体流量、引出电压变化时会发生突变. 离子源发生状态突变时的微波功率、气体流量的大小与离子源内磁体的位置有关. 通过比较不同离子源的引出离子束流、放电损耗、气体利用率、工作稳定性的差异, 归纳了磁场结构对此种ECRIS引出特性的影响规律, 分析了其中的机理. 实验结果表明: 保持输入微波功率、气体流量、引出电压不变时, 增大共振区的范围、减小共振区到栅极的距离, 离子源能引出更大的离子电流; 减小共振区到微波功率入口、气体入口的距离能降低维持离子源高状态所需的最小微波功率和最小气体流量, 提高气体利用率, 但会导致放电损耗增大. 研究结果有助于深化对此类离子源工作过程的认识, 为其设计和性能优化提供参考.     

The effects of ion-neutral collision frequency on the plasma sheath dynamics for oblique entrance of ions into the sheath

Recently it has been shown that the magnetic field has significant effects on the characteristics of ions which enter the collisionless plasma sheath with an oblique incidence angle. Here, we have investigated these effects in collisional plasma sheath. Considering the ion-neutral collision, the basic equations of fluid model in a plasma sheath have been numerically solved in the presence of an external magnetic field where the ion collision frequency has a power law dependency to the ion flow velocity. The results show that the effects of magnetic field on the plasma sheath dynamics strongly depend on the ion-neutral collision frequency and its dependency to ion flow velocity.     

磁控直流辉光等离子体放电特性

本文利用单探针诊断等离子体参数来研究自行设计的磁控直流辉光等离子体实验装置的放电特性, 从而得出电子密度与气体压强、电子密度分布与磁场位型以及磁场强度等的关系. 另外, 用有限元的方法对线圈通电产生的磁场进行数值计算, 模拟出不同接线方式的两种磁场位型分布. 通过实验得出这两种不同的位型的磁场均对等离子体的状态有一定的“控制”作用, 而且这种“控制”作用与现有理论相符合.     

Conditions for the existence of a positively charged structure in a Penning discharge

An analytical model is developed for the high-current form of a low-pressure glow discharge in a magnetic field. Expressions are derived for the critical magnetic induction and critical pressure, below which it becomes impossible for this form of discharge to exist. It is shown that the transition from the high-voltage form to the high-current form of discharge with increasing pressure is not attributable to an increase in the ionization rate, but to an increase in the drift velocity of plasma electrons across the magnetic field. Estimates based on the expressions derived in the article agree in order of magnitude with the experimental data. It is shown that the region in which discharge exists can change considerably in the presence of electron emission. Zh. Tekh. Fiz. 68, 56–63 (July 1998)     

Coaxial Discharge and Transverse Magnetic Field

The study deals with the effect of an applied transverse magnetic field on the dynamics and parameters of the focused and expanded plasma in a coaxial discharge. The experimental results were found with a 3 kJ Plasma focus device of a Mather geometry. The discharge takes place in hydrogen gas with base pressure of 0.5 Torr. The experiments are conducted with a 10 kV bank voltage, which corresponds to 100 kA peak discharge current with rise time 8 μs. Helmholtz magnetic coils are placed outside the expansions chamber to produce a transverse magnetic field with intensity 280 G perpendicular to the plasma expanded from the coaxial electrodes. The investigations have shown that the plasma flow along the expansion chamber axis is restricted when applying the externally transverse magnetic field and the maximum axial velocity of the expanded plasma is decreased by 33%. X-ray probe has been used to measure the focused plasma electron temperature (T_e). The experimental results and the calculations showed that T_e is decreased from 2.2 keV to 800 eV with the application of a transverse magnetic field. The expanded plasma electron temperature and density have been measured by an electric double probe, the results cleared that the expanded plasma electron temperature is decreased by 2.6 times while its density is increased by 9 times, when a transverse magnetic field is applied.