Scaling characteristics of plasma parameters for low-pressureoxygen RF discharge plasmas

For a low-pressure (1-100 mtorr) oxygen RF discharge plasma, the scaling laws for the densities of charged species such as positive ion, negative ion, and electron are estimated in terms of external and internal plasma parameters for the ion-flux-loss-dominated region based on the global balance equations. The scaling formulas are compared with Langmuir probe measurement results performed on a planar inductively coupled oxygen plasma. The transition point from the ion-flux-loss-dominated region to the recombination-loss-dominated region moves to a lower pressure region as the absorbed power increases     

Numerical studies of atmospheric pressure glow discharge controlled by a dielectric barrier between two coaxial electrodes

The glow discharge in pure helium at atmospheric pressure, controlled by a dielectric barrier between coaxial electrodes, is investigated based on a one-dimensional self-consistent fluid model. By solving the continuity equations for electrons, ions, and excited atoms, with the current conservation equation and the electric field profile, the time evolution of the discharge current, gas voltage and the surface density of charged particles on the dielectric barrier are calculated. The simulation results show that the peak values of the discharge current, gas voltage and electric field in the first half period are asymmetric to the second half. When the current reaches its positive or negative maximum, the electric field profile, and the electron and ion densities represent similar properties to the typical glow discharge at low pressures. Obviously there exist a cathode fall, a negative glow region, and a positive column. Effects of the barrier position in between the two coaxial electrodes and the discharge gap width on discharge current characteristics are also analysed. The result indicates that, in the case when the dielectric covering the outer electrode only, the gas is punctured earlier during the former half period and later during the latter half period than other cases, also the current peak value is higher, and the difference of pulse width between the two half periods is more obvious. On reducing the gap width, the multiple current pulse discharge happens.     

Influence of the Emission of Charged Particles on the Characteristics of Glow Discharges with Oscillating Electrons

The influence of the extraction of charged particles on the conditions of sustaining and the characteristics of a glow discharge with oscillating electrons is considered. It is shown that there is some pressure-dependent optimum level for the extraction of ions at which the energy efficiency of the ion source reaches a maximum. Experimentally, it has been established that the sustaining of a discharge is adversely affected by the run-off of fast ionizing electrons from the discharge, whereas the emission of slow plasma electrons can facilitate the sustaining of the high-current variety of discharge and even lead to a stabilization of an unstable gas-discharge structure. It has been shown that due to the different character of the spatial distributions of fast and slow particles in discharges with electrons oscillating in a magnetic field it is possible to realize highly efficient electron emission without loss of discharge stability by extracting electrons from the near-anode region.     

Electron-forbidden energy gap of hydrogen in a wide pressure interval

A simple model is used for estimating the bottom energy of the electron conduction band and the electron-forbidden gap energy. It is shown that electrons in liquid hydrogen are localized not in electron bubbles, as was considered previously, but in molecular negative ions surrounded by voids about 0.5 nm in radius. The conductivity of fluid hydrogen at not very high pressures is connected to transfer of positively charged clusters and negatively charged bubbles. As the pressure and density increase, molecular dissociation occurs and electron localization on atoms becomes more favorable, also with the creation of a void around atomic negative ions. At a sufficiently high concentration of atoms, the probability of tunnel transition of an electron from one atom to another becomes close to unity, the energy level of the negative ion degenerates in the band, and the conductivity is caused by the transfer of these quasifree electrons. It is supposed that this charge transfer mechanism may play an important role in the region of fluid hydrogen metallization.     

Theory for formation of a low-pressure, current-free double layer

A diffusion-controlled theory is developed for the formation of a low-pressure, current-free double layer just inside an upstream insulating source chamber connected to a larger diameter, downstream chamber. The double layer is described using four groups of charged particles: thermal ions, monoenergetic accelerated ions flowing downstream, accelerated electrons flowing upstream, and thermal electrons. The condition of particle balance upstream is found to determine the double layer potential. The double layer disappears at very low pressures due to loss of ionization balance upstream and due to energy relaxation processes for ionizing electrons at higher pressures, in good agreement with experiments.     

Experimental Measurements on the Current Balance at the Cathode of a Cylindrical Hollow Cathode Glow Discharge

The contribution of positive ions, photons and metastable particles to the secondary emission of electrons from the cathode of an argon glow discharge with cylindrical cathode is measured by sampling the plasma species through a small orifice in the cathode wall and analyzing them in a high vacuum region. Additional experiments are made with a second apparatus which enables the sampling fo plasma species from the negative glow and the dark space. The paper describes the experimental techniques used in both cases and presents estimates on the validity of the results obtained. The positive ions are found to be the dominant contribution to the discharge current at the cathode. The secondary emission of electrons is — in decreasing order of importance — due to impact fo singly charged ions, doubly charged ions and metastable atoms.     

The Calculations of the Concentrations,of the Radial Distributions,and of the Radial Particle Currents of Excited and of Doubly Charged Ions in Low Pressure Discharges

A system of differential equations describing the radial profiles of the number densities and of the radial drift velocities of the ions and of the electrons in positive columns at low pressure containing several species of ions is derived. Excited ions and doubly charged ions, generated in two-step processes by electron impacts, the inertia of the ions and space charge effects are taken into account. For the excited ions de-excitation processes by electron collisions and by spontaneous emission are regarded. A set of nonlinear equations to determine the population densities and the initial values of the differential equations and corresponding boundary conditions are put up. Numerical solutions are given for discharges in argon under free-fall conditions similiar to argon ion lasers. One notices that without stepwise processes via excited ion levels the concentration of double charged ions remains small. In some cases the radial drift of the ions considerably reduces the population of the metastable ion levels. The radial density profiles of the double charged ions and of long-living excited ions considerably deviate from the squared radial profile of the electron density. In addition, for low degrees of ionization the theory of the free-fall column given by Tonks and Langmuir is extended to plasmas containing two species of ions.     

Sheath characteristics in plasma carrying finite mass negative ions and ionization at low frequency

The sheath characteristics are investigated in a collisional environment by considering three electronegative plasmas, namely CF₄, Oxygen, and C₆₀ plasmas. Specifically, positive ion velocity, electrostatic potential, charged species densities and net space charged density are examined inside the sheath region with the variation of collisional parameter, mass ratio of negative to positive ions, temperature ratios of electrons to positive ions and electrons to negative ions, non-neutrality parameter and negative ion density. Collisional cross-section is assumed to have a power-law dependence, and both the cases of constant mobility and constant collisional cross-section are discussed and compared. In order to introduce the concept of finite mass of negative ions, both kinds of ions are considered to be governed by their fluid equations, where contribution of ionization, attachment, and detachment is also taken through the continuity equations.     

Neutralization of multiply charged ions at a surface

We report on emission processes induced by particle-solid interaction involving ions with a large potential (i.e., high ion charge state) and low kinetic energy. After an introduction into existing neutralization models for ion scattering at a metal surface a detailed discussion on the electron emission processes is presented.The number of electrons emitted per incident ion is shown to be proportional to the potential energy only within a restricted parameter field involving charge state and ion velocity. The kinetic energy distribution of emitted electrons is dominated by low-energetic electrons (30 eV), while inner shell holes of the projectile ion can initiate high-energetic characteristic Auger electrons. The presence of inner shell holes is also of importance for the charge state of highly charged ions being scattered at surfaces whereas normally the charge state distribution of scattered ions depends on the impact parameter only.The influence of the primary ion charge state on the sputtering yield of insulating surfaces is seen for the charge state of sputtered particles, whereas the total sputtering yield seems to be insensitive. This question is still subject to controversy, however.Photon emission dependent on the charge state of the impinging ion has been observed up to now only for extremely highly charged ions as hydrogenlike Ar or Kr.     

Evolution of the density profiles and flows of charged particles during the diffusive decay of an electronegative gas plasma

Different scenarios of the spatiotemporal evolution of the parameters of the diffusive decay of a pulsed electronegative gas plasma in the absence of plasma chemical processes are studied. It is shown that nonlinear diffusion in a plasma with negative ions occurs in several stages. The rate of electron density decay increases with time and, in the beginning of the second stage, almost all the electrons escape from the discharge volume. On the other hand, the ion density profile is smoothed out due to ion-ion ambipolar diffusion and the flow of negative ions toward the wall is absent in the first stage of decay. In the second stage, the main diffusion mode is first established and then the ion-ion (electronless) plasma decays exponentially with a characteristic time determined by ion-ion ambipolar diffusion.     

Interaction between glow discharge plasma and dust particles

The effect of dust particle concentration on gas discharge plasma parameters was studied through development of a self-consistent kinetic model which is based on solving the Boltzmann equation for the electron distribution function. It was shown that an increase in the Havnes parameter causes an increase in the average electric field and ion density, as well as a decrease in the charge of dust particles and electron density in a dust particle cloud. Self-consistent simulations for a wide range of plasma and dust particle parameters produced several scaling laws: these are laws for dust particle potential and electric field as a function of dust particle concentration and radius, and the discharge current density. The simulation results demonstrate that the process of self-consistent accommodation of parameters of dust particles and plasma in condition of particle concentration growth causes a growth in the number of high-energy electrons in plasma, but not to depletion of electron distribution function.     

Numerical study of penetration and distribution of charged particles in the magnetosphere during southward IMF case

The energetic charged particles penetration in the plasmasphere are carried out using the updated version of 3D Stanford PIC code. We considered slow and fast wind streams to know the penetration of the energetic charged particles (electrons and ions) having different velocities into four regions i.e. cusp, plasmasphere, sunward, and tailward sides. It is observed that the ion penetrations are higher than electrons for solar slow wind streams in the plasmasphere, while it is reverse for the solar fast streams. Also, the results show that the percentage of penetration of the energetic charged particles (both electrons and ions) are the same into the cusp and subsolar point reconnection region. It is different for the higher speed of fast streams; so that the penetrated electrons reached about 10–20 times than penetrated ions. The results show that for the tailward reconnection region, the penetration of ions is 2–3 times higher than the penetration of electrons, but it is the same for the case of higher solar fast speed.     

电子回旋共振放电的电离特性PIC/MCC模拟(Ⅱ)——数值模拟与结果讨论

采用粒子模拟与蒙特卡罗相结合(PIC/MCC)的方法，应用电磁模型，编写了准三维的电子回旋共振(ECR)放电电离过程的模拟程序，得到了ECR放电过程中电子与离子的相空间分布、电磁场分布.通过对这些分布随时间演化的分析，得出ECR加热发生在ω≈ω_c0且垂直于轴向的区域；ECR区域，微波能量几乎全部耦合给电子，获得能量的电子通过与中性粒子的电离碰撞产生了大量的带电粒子；随着放电的进行，大量带电粒子通过频繁的碰撞，分布由各向异性逐渐趋于各向同性. 关键词： 电子回旋共振放电 粒子模拟 蒙特卡罗 电离     

电子和负离子的反射运动对碰撞电负性磁鞘的影响

研究了鞘层中电子和负离子的反射运动对碰撞电负性磁鞘玻姆判据和鞘层结构的影响.通过理论推导得到了考虑鞘层中电子和负离子的反射运动时鞘层玻姆判据表达式,并通过数值模拟得到了电子和负离子采用玻尔兹曼模型和反射运动模型时离子马赫数的下限随参数的变化曲线以及鞘层中带电粒子密度的分布曲线.结果表明,电子和负离子的反射运动模型和玻尔兹曼模型离子马赫数的上限完全相同,下限表达式不同,反射运动模型中下限还与基板电势有关,且随着基板电势值的增加而增大,达到与玻尔兹曼分布中相同值后保持不变,随着鞘边负离子浓度和温度的不同达到最大值的速度不同;离子马赫数的下限在玻尔兹曼和反射运动模型中都随鞘边负离子浓度的增加和温度的降低而减小,只是在反射运动模型中的最大值要小;两种模型中离子马赫数的下限都随鞘边电场的增加而增加,但在玻尔兹曼模型中增加得更快最终值更大;两种模型离子马赫数的下限都随碰撞参数或磁场角度的增加而降低,但在玻尔兹曼模型中降低更快,随着碰撞参数或者磁场角度的增加两种模型中离子马赫数的下限趋于一致;当基板电势值较小时,电子和负离子的反射运动对鞘层结构影响较大,当基板电势值较大时电子和负离子反射运动对鞘层中带电粒子密度分布的影响很小.     

Analysis of dusty plasma in the positive column of glow discharges

The radial distributions of ions,electrons and dust particles in the positive colum of glow discharges are investigated in a tripled-pole diffusion model.The dust particles are mainly trapped in the region around the column axis where the electrostatic potential is the highest.The presence of the dust particles results in the ion density increasing and the electron density decreasiung in the dust-trapped region.The dust-trapped region is wider for a higher dust temperature or a smaller particulate redius.The ions and electrons in the dust-free region away from the column axis are in ambipolar diffusion.     

Detailed comparison between probe density measurements of glow discharge in argon and in helium

In this article we shall look a bit more closely at some of the fundamental plasma parameters obtained by a cylindrical Langmuir probe within low-pressure electrical gas discharge plasma. The presented measurements were made in argon and in helium glow discharge plasmas. We are mainly concerned with the densities of the charged particles (electrons and ions) within the plasma and the effect of the discharge conditions upon them. The electron density is calculated from the electron current at the space potential and from the integration over the EEDF. The ion density is calculated by using the OML collisionless theory. The parameterization of Laframboise's numerical results is also used for the ion density calculation. In the range of our experimental conditions the results of plasma density, for both gases, tend to show that the ion densities measured with the OML and Laframboise theories exceeds the measured electron densities. The results also show that the plasma electron and ion densities increased with both discharge power and gas pressure.     

Specific boundary conditions for the simulation of extraction for ECRIS,LIS, and H−-sources

The necessity of a three-dimensional simulation of the extraction has been accepted for electron cyclotron resonance ion sources (ECRIS) as well as for negative ion sources. For an ECRIS, the magnetic hexapole together with the solenoidal mirror field defines a minimum B structure which confines the plasma. Depending on the magnetic flux density distribution, the plasma density in front of the extraction electrode might be non-homogeneous. In H^?-sources, magnetic filter fields are used to separate electrons with different energies or to separate electrons from negative ions. These magnetic filters influence the ions as well. Besides these asymmetry effects other quantities have to be considered, namely the correct formulation of initial conditions of all present charged particles. For ECRIS the initial conditions for ions are assumed to be in the electronvolt range, whereas it can be in the kilo electronvolt range for laser ion sources. Another quantity of interest is the electron energy and the distribution of electrons in real space and their movability if magnetic fields are present.     

Numerical modeling of low-pressure RF plasmas

A particle-in-cell simulation is used to model the plasma generated in a parallel plate RF reactor at low pressure. Nonperiodic boundary conditions are used, and the electric field and particle motion are obtained by finite-difference methods leading to the self-consistent creation of sheaths on the boundaries. Model cross sections are used to describe collisions between particles. Ionization is included, and the plasma is maintained by fast electrons generated in the RF sheaths. Most of the power dissipation is due to the acceleration of ions in the time-average sheath fields. At high applied voltage, the power dissipation is described well by the power law P∝V^5/2. Simple scaling laws for the density and plasma potential are obtained. The effect of ion mass and charge-exchange colisions on the ion energy spectrum collected by the electrodes is examined. The ion loss rate drops in the presence of charge-exchange collisions, and this leads to an increase in the density. The collisions also markedly alter the ion energy distribution function     

[Russian Text Ignored].

Radial distributions of charged particles in an oxygen discharge depending on the relative concentration of negative ions are experimentally investigated. Corresponding change of the selfconsistent field along the radius of the discharge positive column at different pressures and currents are given.