碰撞等离子体中电中性条件对单探针测量的约束

在碰撞等离子体中使用单探针测量系统测得的伏安特性曲线会发生畸变. 实验中分别测量了非碰撞条件下(20 Pa)和碰撞条件下(400 Pa)氩气(Ar)电感耦合等离子体的单探针伏安特性曲线, 并进一步通过在400 Pa下测量不同位置处的单探针伏安特性曲线和引入干扰电极调节整体电中性约束的程度的方法来分析研究曲线的畸变现象. 结果表明碰撞等离子体中整体电中性约束条件会对非碰撞探针测量系统进行约束, 并通过理论分析和实验证明:在单探针测量过程中, 以真空室壁为电位参考点, 等离子体将通过改变自身等离子体电位来满足整体电中性的要求, 此时, 单探针测得的伏安特性曲线就不是理论上的单探针的鞘层伏安特性.     

Dynamics of a collisional ion sheath

Experiments on collisional ion sheaths are carried out by applying a pulsed negative bias on a disc electrode immersed in a collisional plasma. The pulse is characterized by a linear rise, followed by a constant voltage phase and then exponential decay. The measured currents to the electrode are compared to predictions from a dynamic collisional ion sheath model which is developed from the basic two fluid equations. The parameter determining the degree of collisionality is also defined. The agreement between the two in the rising and the flat top phases of the pulse is found to be good. Some residual discrepancies as well as the disagreement in the decay phase are discussed.     

光场电离类镍氪等离子体参数研究

计算了１０ＨＺ、１４０ｆｓ、５０ｍＪ、８００ｎｍ圆偏振光场电离产生的类镍氪等离子体参数,着重研究了电离激光强度对等离子体参数的影响,并由此确定了适合于电子撞激发类镍氪３１．９ｎｍ超紫外（ＸＵＶ）激光系统的最佳激光强度。     

Deshaping of the laser beam observed on the collisionally induced fluorescence of Na atoms in the nozzle expansion

The excitation spectrum ofD ₁ fluorescence and collisionally inducedD ₂ fluorescence of sodium in the early stage of the beam formation near the nozzle is studied. We observe strong reabsorption and imprisonment ofD ₁ photons influencing collisionally inducedD ₂ fluorescence. For high pressure of sodium vapour the excitation profile ofD ₂ fluorescence is dominated by the collisional broadening effect and shows unusual maxima and minima. These we connect with the laser beam focusing and defocusing effect, changing the number of photons penetrating the high density region.     

Nonlinear propagation of fast and slow magnetosonic waves in collisional plasmas

Low-frequency fast and slow magnetosonic waves propagating in electron ion plasmas with damping effects through ions and neutral atoms collisions are investigated. Linear wave analysis is performed to obtain dispersion relation. The reductive perturbation method is applied and it is shown that fast and slow modes of nonlinear magnetosonic wave are governed by damped Korteweg-de Vries (DKdV) equation in the presence of ion neutral collisions in plasmas. The analytical solution of DKdV soliton is presented under the assumption of weak collisional effects and numerical solutions of DKdV equation are also obtained using two-level finite difference scheme with the help of Runge–Kutta method at different plasma parameters. The damping of nonlinear fast and slow magnetosonic wave structures at different times are discussed in the context of space plasma situations where ions and neutral atoms collisions exist.     

霍尔推进器中磁化二次电子对鞘层特性的影响

为进一步研究霍尔推进器壁面二次电子发射对推进器性能的影响,采用流体模型数值模拟了二次电子磁化效应的等离子体鞘层特性.得到二次电子磁化鞘层的玻姆判据.讨论了不同的磁场强度和方向、二次电子发射系数以及不同种类等离子体推进器的鞘层结构.结果表明:随器壁二次电子发射系数的增大,鞘层中粒子密度增加,器壁电势升高,鞘层厚度减小;鞘层电势及粒子密度随着磁场强度和方位角的增加而增加;而对于不同种类的等离子体,壁面电势和鞘层厚度也不同.这为霍尔推进器的磁安特性实验提供了理论解释. 关键词： 霍尔推进器 磁鞘 二次电子     

Sheath motion in a capacitively coupled radio frequency discharge

The sheath motion in a capacitively coupled RF discharge is highly nonlinear. The voltage waveform on a cylindrical probe placed in the sheath region is measured as a function of position and time. A circuit model of the probe-discharge system relates the observed probe voltage to the sheath motion. The equations derived from this circuit model are solved numerically with varying nonlinear sheath motions; the resulting waveforms are compared with the experimental observations to determine the actual sheath motion. The time-varying plasma potential is also determined, indirectly, from the comparison. The authors also report observation of oscillations related to the plasma frequency, whose peak harmonic component can be calculated from a single plasma model. These oscillations can be a useful plasma diagnostic for determining plasma density. The presence of these high-frequency oscillations may significantly enhance the rate of stochastic heating of electrons     

Li-like镁等离子体X射线光谱模拟

利用碰撞辐射平衡和离化平衡下的等离子体X射线发射谱理论,对镁元素等离子体在中低电子密度、中高电子温度范围内,以碰撞激发、共振激发、离化、辐射复合和双电子复合等动力学过程为主的Mg10 、Mg9 和Mg8 三离子体系X射线谱进行了理论模拟,得到其X射线光谱,反映出X射线辐射谱波长、强度同电子温度之间的关系,得到了等离子体诊断所关心的电子温度的定性关系.     

Sheath thickness evaluation for collisionless or weakly collisionalbounded plasmas

The widely used Child-Langmuir law for sheath thickness evaluation in semi-infinite collisionless plasmas makes the assumptions of quasi-neutrality (n_e=n_i) and zero electric field intensity E=0 at the sheath edge, as well as applying the Bohm criterion for ions entering the sheath. However, through a whole region fluid model, Poisson's equation has been solved numerically for the steady-state solution through the sheath and presheath without these assumptions. With the sheath edge defined, as in the Child-Langmuir law, at the place where the ion velocity is equal to the Bohm velocity, the sheath thickness of a bounded collisionless or weakly collisional plasma has been found with this model in some cases to be much larger than that obtained with the Child-Langmuir Law. The sheath thickness discrepancy is significant under conditions found in low pressure high density plasma (HDP) tools for plasma processing. Results presented indicate that the sheath thickness is very sensitive to the electric field and space charge density at the sheath edge. The electric field and space charge density can be successfully estimated by an intermediate scale matching method, and are used to derive a modified expression for the potential in the sheath that can be solved numerically for sheath thickness. With these results, the matching problem, arising when sheath and plasma are modeled separately, can be overcome     

负离子对等离子体鞘层特性影响的数值研究

构造强电负性等离子体中平板电极处鞘层形成的模型,使用流体动力学方程组研究等离子体鞘层.得到空间电势、空间静电荷分布和鞘层宽度作为距离函数的数值结果.结果表明,强电负性鞘层中,在等离子体区和鞘层区之间几乎没有过渡的预鞘区,在靠近极板附近鞘层里的电子、负离子和正离子的分布形成一个纯正离子鞘区.在靠近鞘边附近,空间静电荷密度分布有一个很尖的峰.发现同电正性情形相比,强电负性鞘层的宽度要窄很多,空间电势下降得快得多.     

栅网与偏压对CHF3电子回旋共振放电等离子体特性的影响

研究了电子回旋共振等离子体增强化学气相沉积系统中栅网的增加和栅网上施加+60V和-60V偏压对CHF₃放电等离子体特性的影响.发现在低微波功率下栅网与偏压对等离子 体中基团分布的影响较大，而高微波功率下的影响逐渐减小.这是由于低微波功率下等离子体中电子温度较低，基团的分布同时受栅网鞘电场和电子碰撞分解的共同作用；而高微波功率下电子温度较高，栅网鞘电场的作用减弱，基团分布主要取决于电子碰撞分解作用. 关键词： 电子回旋共振放电 3分解')" href="#">CHF₃分解 栅网 偏压     

两种温度电子对稳态等离子体鞘层的影响

建立包含冷热电子的无碰撞等离子体鞘层的流体模型,利用数值模拟研究含有两种温度电子时等离子体鞘层的产生.结果表明:对于含有两种不同温度电子的稳态等离子体,冷电子的温度越低或者冷电子的含量越多,鞘边离子的马赫数临界值就越小,鞘层的宽度就变得越窄,沉积器壁的离子动能流也就越少.此外,研究不同种类的等离子体(Ar、Ke、Xe),鞘层厚度和离子沉积器壁动能流受冷电子的影响.     

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.     

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)     

The Double Sheath at the Plasma‐Wall Boundary

The relation between plasma parameters determining a steady‐state charged sheath at a plasma‐wall boundary was derived and analyzed using a fluid model, which accounted for inertia and partial pressure of both plasma components. The relation generalizes the well‐known Bohm criterion and, in particular, shows that a steadystate sheath may be formed in some cases where the Bohm criterion is not satisfied. Conditions allowing formation of a double sheath structure were formulated and analyzed. It was shown that the structure may be formed only if a) the generalized Bohm criterion is satisfied, and b) the Sagdeev potential has a minimum. The double sheath structure consists of two sub‐sheath of different polarity, which depends on the wall potential polarity and the relation between the plasma component temperatures and inertia. For positive wall potential, the sub‐sheath adjacent to the plasma (the sheath edge) is enriched by negative particles, and the sub‐sheath adjacent to the wall is enriched by positive particles, while the sub‐sheath polarities are reversed for negative wall potential. An analytical theory was formulated and illustrated by numerical solutions of the Poisson equation for special cases. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

用有限元法解龟磁波穿透非均匀有耗等离子体鞘套时的损耗

本文用有限无法求解了电磁波穿透具有任意密度剖面的非均匀等离子体鞘套时的反射和透射问題。鞘套的剖面参数选自典型再入飞行器的粘性激波层平衡流场数据^[1]。利用虚功原理和线性插值函数从Helmholtz方程导出有限元方程。鞘套按对数剖分为31个单元。然后,分别对频率f=400MHZ,4000MHZ和10GHZ时,计算了鞘套内的场值及反射,透射和吸收系数。并估算了信号的总功耗。误差分析表明,计算结果是令人满意的。与其它方法相比较,本文所给出的有限无法是简捷而精确的,所以该方程适于计算波穿透具有任意的而且变化剧烈的剖面的等离子体鞘套时的损耗     

Levitation and dynamics of a collection of dust particles in afully ionized plasma sheath

Examines the dynamics of a collection of charged dust particles in the plasma sheath above a large body in a fully ionized space plasma when the radius of the large body is much larger than the sheath thickness. The dust particles are charged by the plasma, and the forces on the dust particles are assumed to be from the electric field in the sheath and from gravitation only. These forces will often act in opposite directions and may balance, making dust suspension and collection possible. The dust particles are supplied by injection or by electrostatic levitation. The ability of the sheath to collect dust particles, will be optimal for a certain combination of gravitation and plasma and dust particle parameters. In a dense dust sheath, the charges on the dust particles contribute significantly to the total space charge, and collective effects become important. These effects will reduce the magnitude of the sheath electric field strength and the charge on the dust particles. As dust particles are collected, the dust sheath is stretched and the largest dust particles may drop out, because the sheath is no longer able to suspend them. In a tenuous dust sheath, the inner layer, from the surface and about one Debye length thick, will be unstable for dust particle motion, and dust will not collect there. In a dense dust sheath, collective effects will decrease the thickness of this inner dust-free layer, making dust collection closer to the surface possible. By linearization of the force and current equations, the necessary and sufficient conditions for a stable dust sheath are found. The authors consider conditions which resemble those of planetary system bodies, but the results may also be of relevance to some laboratory plasmas     

Plasma-sheath transition in the magnetized plasma-wall problem for collisionless ions

A model for the collisionless plasma-wall problem under the action of an applied magnetic field is developed. The behavior of its solution is examined and found to be qualitatively consistent with experiment. The plasma and the sheath are then modeled separately to obtain the position of the quasi-neutral plasma boundary and the position of the edge of the electron-free sheath. It is shown that the plasma boundary can be specified as the point where the component of the ion velocity normal to the wall reaches the ion sound speed (Bohm criterion), and the sheath edge is specified as the point corresponding to Godyak's condition for the electric field. Studying the behavior near the plasma boundary and the sheath edge, the plasma solution and the solution of the space charge region are patched together to approximate the solution of the plasma-wall problem.     

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