Saturation current densities at the critical point of a spherical probe in a moving collisional plasma with negative ions or dust grains

The problem is considered of the probe diagnostics of a moving, weakly ionized, collision-dominated plasma containing singly charged negative ions or dust grains (heavy multicharged ions). Based on an asymptotic analysis, expressions are obtained that describe the saturation current densities of the electrons and of the charged particles of other species at the point where the plasma flowing around a spherical probe in the laminar boundary layer regime stagnates.     

rf Electrode sheath formation near a concave electrode

In this work we present experimental results concerning electrode sheath and ion flux formation near a concave electrode with the dimension of a cavity comparable to the electrode sheath length. It is shown that the secondary electron emission can play a crucial role in plasma molding over the electrode surface. It is also observed that plasma has a tendency to "self-leakage" in electrode cavities.     

二次电子发射和负离子存在时的鞘层结构特性

 建立了包括电子、离子、器壁发射二次电子以及负离子多种成分的等离子体无碰撞鞘层的基本模型,讨论了二次电子发射和负离子对1维稳态等离子体鞘层结构的影响,并且分析了多种成分等离子体鞘层内的二次电子和负离子的相互作用。结果表明：二次电子发射系数的增加和负离子含量的增加,都将导致鞘层的厚度有所减小;二次电子发射系数超过临界发射系数之后,鞘层不再是离子鞘。随着器壁材料二次电子发射系数的增加,鞘层中的负离子密度分布也逐渐增加;负离子的增加,导致二次电子临界发射系数有所增加。另外,在等离子体鞘层中二次电子发射和负离子的存在,也影响着鞘层中电子的放电特性与器壁材料的腐蚀。     

Comparison of the directional characteristics of swift ion excitation for two small biomolecules: glycine and alanine

The characteristics of ions that enter the plasma sheath with an oblique incident angle have been investigated in the presence of an external magnetic field. The ion dynamics in a collisional and collisionless magnetized plasma sheath have been numerically calculated by using a fluid model. Several values for the ion velocity at the sheath edge, orientation and strength of the magnetic field and the ion-neutral collision frequency have been considered. The results show that in a collisionless magnetized plasma sheath, the behaviour of ions that obliquely enter the sheath with some specific velocities at the sheath edge and at some specific orientations and strengths of magnetic field, is more complicated than that of ions with normal entrance angles. For the oblique entrance of ions, the weak magnetic fields cause some fluctuations in ion velocity around its boundary value, i.e. the ion velocity does not accelerate. However, the numerical calculations show that the ion dynamics in a collisional magnetized plasma sheath are the same for both normal and inclined entrance of ions into the sheath.     

KInetic Description of the Positive Column for Medium Knudsen-Numbers

We analyze the influence of collisions on a plasma column with cylindrical symmetry in the pressure regime between the collision-dominated and the free-fall case. A model is investigated which can handle arbitrary contributions from ion-neutral-collisions. One finds an analytic solution for the ion-distribution-function which depends functionally on the potential. This electrical potential is calculated numerically — with consideration of the space-charge-layer near the wall — and selfconsistently from the Poisson-equation. From the calculation of the ion-distribution-function, of density, flux and temperature of the ions as well as of the potential variation, we discuss the influence of the model parameters. The results are compared with corresponding hydrodynamic calculations.     

高温辐射场作用下的原子过程

等离子体中辐射能量密度与物质能量密度的比值是区分等离子体原子过程性质的重要参量. 根据这个参量, 等离子体中的原子过程可分为碰撞占优和辐射占优两种典型类型. 数值模拟发现碰撞占优和辐射占优的原子过程有不同的性质: 碰撞占优的等离子体能够很快达到LTE状态; 辐射占优等离子体的束缚电子温度、电离度和自由电子温度存在不同的弛豫时间尺度, 存在某种形式的准平衡状态.     

梯形管内壁等离子体离子注入鞘层扩展MATLAB-PIC数值模拟

基于MATLAB利用Particle-in-cell模型,对梯形管内壁等离子体离子注入过程,进行了二维数值模拟.计算结果表明在中心电极附近出现了"阳极鞘层",该鞘层内部不存在离子,而且在鞘层边缘离子密度最高.在上下管壁上的离子注入剂量呈现"m"形分布.通过对注入过程中等离子体密度分布和不同时间段管壁不同位置离子注入剂量的跟踪,发现"阳极鞘层"扩展行为是导致"m"形分布的原因.由于梯形管形状的不对称性,"阳极鞘层"的边缘向梯形长底方向扩展较快.在注入初始时刻离子注入的能量很低,随着时间延长离子能量逐渐升高,这是由离子初始位置决定的.可见梯形管自身形状决定了鞘层形状和最终的离子注入能量和剂量分布.     

Ion extraction and charge exchange in laser isotope separation

In the atomic vapor laser isotope separation (AVLIS) process, a vapor is ionized by pulsed laser beams, and the ions are extracted by negatively biased collectors. We compute the unsteady dynamics of the photoplasma using a two-dimensional (2-D) particle-in-cell (PIC) code. Collisions between ions and neutral species are simulated by a Monte Carlo technique. The plasma dynamics is visualized by snapshots of particle positions showing the directions of their velocities. The three kinds of particles (electrons, photo-ions, and ions created by charge exchange) are marked by different colors. The graphic outputs illustrate the motion of the electrons toward the anodes, the vertical drift of the plasma, its erosion by the transient ion sheath, and nonselective ionization by charge exchange     

多成分电负性碰撞等离子体的鞘层玻姆判据

采用双流体模型研究了多成分的电负性碰撞等离子体鞘层的玻姆判据,讨论了一维稳态情形下不同的带电粒子对鞘层玻姆判据的影响。采用拟牛顿法,得到了数值解。结果表明：玻姆判据存在上限和下限。二次电子发射系数越大,离子的马赫数越大;负离子的含量越多,离子的马赫数越小。鞘层中离子的温度、离子与中性粒子的碰撞以及离子的带电量对玻姆判据的上下限的取值都有一定的影响。     

Asymptotic theory of boundary layers of weakly ionized thermalplasmas on emitting electrodes

The paper deals with collision-dominated boundary layers of weakly ionized thermal plasmas on emitting electrodes. The case considered is when the dominating ionization mechanism in the plasma is ionization by electron impact, and the dominating recombination mechanism is recombination with an electron as a third body. The ratio of the Debye length to the recombination length is treated as a small parameter, and the method of matched asymptotic expansions is employed. Analytical formulas have been obtained for the distributions of the number densities of ions and electrons and of the electrostatic potential in each asymptotic zone. Formulas have been obtained describing the voltage drop in the boundary layer as a function of the density of the electric current coming from the plasma to the electrode     

Modal focusing effect of positive and negative ions by a three-dimensional plasma-sheath lens

A complex focusing effect of positive and negative ions caused by the sheath forming to biased electrodes that interface insulators has been found by solving in three dimensions the potential distribution and ion kinetics within the sheath. Thus, intrinsically correlated with the sheath shape, certain electrical charges are focused on the surface, forming well defined patterns named modal lines and modal spots. Their superposition to the previously reported discrete focusing leads to a total flux that represents a "fingerprint" of the entire sheath on the electrode surface. The ion flux pattern is developed experimentally on the surface of square and octagonal electrodes exposed to Ar/SF(6) and CF4 plasmas. Present results are of high potential importance for fundamental studies concerning sheath formation and charge kinetics and also in a wide range of plasma applications.     

Sheath characteristics in multi-component plasma with negative ions

Properties of steady state ion sheath formed in front of a negatively biased metal plate under the influence of negative ions have been investigated in collisionless argon/SF₆ plasma. This experiment is carried out at a fixed discharge voltage and fixed filament heating power. In this experiment, the decrement in plasma pre-sheath potential drop as well as positive ion drift velocity toward the plate is experimentally recorded in the presence of negative ions. It is also found that the plasma positive ion density and plasma electron temperature decrease in the presence of negative ions. These factors attribute to the decrease of ion current toward the plate. Hence the usual ion sheath expands. Article presented at the International Conference on the Frontiers of Plasma Physics and Technology, 9–14 December 2002, Bangalore, India.     

脉冲偏压上升沿特性对等离子体浸没离子注入鞘层扩展动力学的影响

等离子体浸没离子注入(PIII)是用于材料表面改性的一种廉价高效、非视线的技术.采用等离子体粒子模型，通过假设电子密度服从Boltzmann分布，求解Poisson方程和Newton方程，跟踪离子在等离子体鞘层中的运动形态及特性并进行统计分析，研究了不同上升速率和形状的6种波形上升沿对鞘层时空演化、离子注入能量和剂量的影响.结果表明，在PIII过程中，脉冲上升沿影响了等离子体鞘层的扩展，且不同波形诱导的鞘层厚度间存在最大差值.电场强度在鞘层的外边缘区域存在陡降区，离子的运动为非匀加速过程.可以通过调整脉冲 关键词： 等离子体浸没离子注入 鞘层 粒子模型 上升沿     

Theory of sheath in a collisional multi-component plasma

The aim of this brief report is to study the behaviour of sheath structure in a multi-component plasma with dust-neutral collisions. The plasma consists of electrons, ions, micron size negatively charged dust particles and neutrals. The sheath-edge potential and sheath width are calculated for collisionally dominated sheath. Comparison of collisionless and collisionally dominated sheath are made.     

Plasma density decay of vacuum discharges after current zero

In vacuum circuit breakers the post-arc current caused by the remaining ions and electrons in the contact gap is an indication of the residual ionization and its decay. It coincides with the formation of a positive space charge sheath in front of the new cathode, which grows toward the new anode. In a vacuum test chamber an arc (1.5-15 kA RMS) is drawn between high current electrodes of the spiral type. At different times after current zero a transient recovery voltage is applied across a separate pair of high voltage electrodes. In contrast to real circuit breakers, where the transient recovery voltage reappears between the arcing contacts, this separation allows the study of residual plasma free from the thermal stress and melting on the contact surfaces. From the post-arc current across these electrodes, in comparison with a mathematical model of sheath growth, the density of the charge carriers can be evaluated. Such values and their temporal decay are presented     

The Boundary Layer between Electrodes and a Thermal Plasma

The electrical boundary layer between an isothermal, weakly ionized plasma and a plane electrode is discussed. Following a formulation of the complete problem, the governing equations are solved in the quasi-neutral continuum approximation to give explicit results for the ionization nonequilibrium layer. This allows three critical current densities to be identified: the first when a cathode must emit electrons; the second at which the anode potential fall changes sign from negative to positive; the third at which the anode must emit ions. The neglected space charge and inertia terms are evaluated to give conditions distinguishing collision-dominated and collisionless sheaths. Using an estimation for the location of the plasma-sheath boundary, the potential falls associated with the ionization nonequilibrium layer are evaluated. The results are illustrated for the case of combustion MHD plasmas.     

碰撞对两种正离子的电负性磁鞘影响的数值研究

建立包括两种正离子的电负性磁鞘的流体模型,利用四阶龙格库塔法数值求解描述一维稳态等离子体鞘层的方程组,考察离子与中性粒子碰撞对一维稳态等离子体鞘层的影响.结果表明:鞘边Ar⁺与He⁺的含量比值与碰撞参数对离子马赫数的取值范围都有影响.鞘边负离子含量越少,碰撞对鞘层中带电粒子密度的影响越明显.并且随碰撞参数的加大,鞘层中电子、负离子的密度下降越快,两种正离子的密度则呈现不同的波动变化.鞘边负离子含量越多,碰撞对鞘层中两种正离子的速度影响就越明显.此外,碰撞参数越大对鞘边δ越大的鞘层中的带电粒子密度影响越大.     

Influence of vacuum space on formation of potential sheath in plasmas

Properties of potential sheaths developed in plasmas are investigated in terms of the plasma Debye length and the dimension of vacuum space. Biased plasma potential and the potential profile depend very sensitively on the geometrical configuration of plasma and vacuum space. The potential sheath is never developed near electrodes in high-density plasmas where the Debye length is much less than the dimension of the vacuum space. In this case, most of the potential drops occur in the vacuum space and almost no electric field exists inside the plasma. Parametric investigation of the potential sheath in terms of the vacuum-space and plasma dimensions is carried out.     

磁化等离子体的鞘层结构

采用流体方法数值研究了斜磁场作用下的等离子体鞘层结构.模拟结果显示磁场对鞘层结构有显著的影响,特别是在磁鞘边界附近离子的密度分布产生了明显的变化.分别讨论了离子垂直入射和斜入射两种情况,当离子斜入射进鞘层时,其运动情况要相对复杂一些.在适当的条件下,洛伦兹力的作用使离子在某些区域产生相对聚集,离子密度分布产生波动. 关键词： 鞘层 等离子体 磁场     

Variations of Plasma Parameters and Sheath Thickness Due to the Extraction of Ion and Electron Flux in a Double Plasma Device

Influence of charged particle extraction on plasma parameters and on ion sheath has been investigated in a double plasma device. When ions are extracted from the plasma, the plasma density as well as the positive ion flux into the sheath increases. As a result a sheath contraction takes place. Again, in case of electron extraction, it is found that the plasma density as well as the positive ion flux into the sheath decreases. As a result a sheath expansion takes place. Furthermore, it is observed that the floating potential of a plate can be controlled by extracting charged particles from the plasma. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)