Electron collision cross-section data for plasma modeling

An overview of the current status in electron collision cross section databases for plasma modeling is presented. Emphasis is on dissociation, ionization, and dissociative ionization. The effect of the operating conditions on the relative importance of different collision mechanisms is discussed. For high density plasmas with low gas pressure, a second collision with electrons becomes likely and metastable states may play a role in the dissociation and ionization processes. A number of outstanding theoretical and experimental issues in the cross-section data are discussed. For the total ionization cross section, a four-parameter fitting formula is found to represent the numerical data well and the parameters for some molecules used in plasma modeling are presented     

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.     

Radiation transport in a spherically symmetric hot plasma

An analytical solution is obtained for the problem of radiation transport in a spherically symmetric plasma. The ions are assumed to be in a complete steady state with constant ion density and electron temperature. The radiation density is assumed to be small so that the rate of the radiative processes in the plasma is small relative to that of the collisional processes, but not negligible. The effect of the plasma on the radiation density, as well as the influence of the radiation on the population probabilities, are properly accounted for. Under these conditions explicit expressions are given, valid to the first order in the plasma dimensions, for the radiation density and the population probability of the ionic states.     

普朗克谱分布的辐射场对束缚电子布居的影响

研究了外加普朗克辐射场对不同温度和密度下的等离子体的主量子壳层束缚电子的布居数的分布以及随时间的演化规律的调制作用.结果表明：当具有普朗克谱分布的辐射场的辐射温度接近于等离子体的电子温度,且辐射场的强度等于等效温度下的黑体谱辐射强度时,随着等离子体的时间演化,等离子体中主壳层束缚电子布居数分布由non-LTE分布过渡到LTE分布. 关键词： 流体动力学 激光等离子体 布居数 普朗克辐射场     

A collisional-radiative model for helium and its application to a discharge plasma

For the collisional-radiative model applied to helium, two alternative formulations are presented in which non-linear process such as ionization due to metastable-metastable collisions may be included. The atomic data, including dielectronic,recombination, employed in the calculations are summarized. The collisional-radiative calculation is applied to a positive-column plasma for which both the nonlinear processes and radiation trapping are taken into account. The calculated population densities of excited levels for a low-pressure and low-current discharge plasma show good agreement with a measurement made using the hook method. The population density distribution among excited levels is interpreted in terms of excitation-ionization mechanisms in the plasma. This leads to a grouping of the excited levels into two phases for the discharge plasma considered: the corona phase applies to the lower-lying levels and the quasi-saturation phase to the higher-lying levels. The importance of the ladder-like excitation-ionization mechanism is noted for this class of plasmas.     

Potential formation in a bounded two-electron temperature plasma system with floating collector that emits electrons

Formation of the plasma potential in a plasma that contains energetic electrons and is bounded by a floating collector that emits electrons is studied theoretically. The problem is treated by a static. kinetic plasma-sheath model of Schwager and Birdsall [Phys. Fluids B2 (1990) 1057], which we have extended in order to include additional energetic electron population. The distribution of these electrons is assumed to be a high-temperature Maxwellian. They are called hot electrons. In the paper we study effects of the density and temperature of the hot electrons on the formation of the plasma potential. The model shows that for certain densities and temperatures of the hot electron population plasmas with two different plasma potentials can coexist in the system. These two plasmas are separated spatially by a double layer. For the case when there is no emission of electrons from the collector, results of the model are compared with computer simulation and very good agreement between the model and the simulation is found. The simulation also confirms existence of two plasmas with two different potentials separated by a double layer.     

Bifurcation and Relaxation Oscillations in Divertor Plasmas

This paper presents a contribution to bifurcation phenomena in scrape-off layer modelling. The bifurcations are caused by the non-monotonic temperature behaviour of the radiation characteristics of plasma impurities. Low-frequency relaxation oscillations observed in divertor plasmas of the ASDEX Upgrade tokamak are analyzed and computed using a simple model for the dynamics of the impurity density and the electron temperature. The corresponding model equations are a special case of a general system of transport equations whose stability is analyzed. Bifurcations are calculated by solving autonomous and parametrically driven model equations.     

Modelling of the Positive Column in an Argon-Hexamethyl-disiloxane dc Glow Discharge

A model is presented of the positive column of a dc glow discharge in argon with small admixtures of hexamethyldisiloxane (HMDS). The axial electric field, the ion production rates for direct-, stepwise-, pair-, and Penning ionization, the densities of metastable Ar atoms and of electrons, and the wall current of HMDS ions are calculated in dependence on HMDS admixture and discharge current density. For the calculations particle balance equations were used for a diffusion determined plasma in a mixture of two gaseous components. The reaction rates for the electron collision processes were determined applying the electron distribution function calculated for pure argon. Taking into account PENNING ionization of HMDS molecules by metastable argon atoms the decrease of electric field for increasing HMDS admixtures is according to the experimen-tally measured values. Also ion wall currents and electron densities are compared with experimen-tal values for thin film formation rate and results of probe measurements.     

Properties of the Collision Operators of the Electron Boltzmann Equation for a Weakly Ionized Plasma. I. Representation and General Properties

In the kinetic theory a great variety of physical systems is investigated by means of Boltzmann-like equations. This approach is used for neutral gases, neutron as well as radiation transport, plasmas etc. For many problems the knowledge of the properties of the collision operators is of great importance, especially if eigenvalue problems occur. The paper presents an investigation of the properties of the collision operators of the Boltzmann equation covering elastic, exciting and deexciting processes in a weakly ionized plasma. First, a short survey of the importance of eigenfunctions and eigenvalues in the kinetic theory of various systems is given. Then, properties of the outscattering operator as dependent on the course of the differential cross section are considered. Finally, for the inscattering operator such properties as selfadjointness and rotational invariance are investigated in detail. These considerations provide the basis for the proof of compactness and for first conclusions on the spectral properties of the collision operators in the second part of this paper.     

Electron and ion mobilities in argon plasma with fluorine as additive

Results of the numerical evaluation of the electron and ion mobilities in plasmas from mixtures of argon and fluorine are given. The temperature dependence of the number densities of the components and their mobilities as a function of temperature for low-pressure (from 0.1 kPa to 1.0 kPa) and low-temperature (from 500 K to 5000 K) argon plasmas with 20% and 30% of added fluorine are evaluated. It is assumed that the system is kept under constant pressure and that a corresponding state of local themodynamical equilibrium is attained in it. The previously derived expression for the modified Debye radius, offering the possibility to treat the plasmas as weakly non-ideal in the whole temperature range, is used and the cut-off at the Landau length rather than at the smallest ionic radius is introduced. This alteration in the evaluation procedure gives different changes in the final numerical results for all the relevant quantities. It was found that the equilibrium plasma composition depends considerably on the presence of fluorine in the whole temperature range for both the pressures; the addition of fluorine significantly enhances the number densities of all relevant charged constituents. This fact influences the electron and ion collision frequencies as well as their mobilities.     

辐射输运实验中的Al等离子体发射光谱研究

SGII装置上用八路激光注入金腔产生高温辐射源,然后利用该辐射源来加热输运腔中的掺Al泡沫样品.从测量得到的Al类氦离子发射光谱可以观察到a—d,qr伴线的强度明显高于jkl伴线的强度,由此可以确定出光子激发和电离过程对输运腔中等离子体的形成不可忽略.除此之外,还观察到较强的互组合线发射,这表明存在一个低密度区域.最后考虑到实验测量得到的Al类氦离子发射光谱是时间、空间积分,利用两个状态等离子体的发射光谱合成再现了实验光谱. 关键词： 发射光谱 辐射加热 光谱诊断     

Operatorenmethode zur Lösung der Vlasov-Gleichung mit BGK-Stoßglied und äußerem elektrischem Feld

Irreversible processes in a classical electron plasma are treated on the basis of a linearized Vlasov equation supplemented by Bhatnagar-Gross-Krook terms describing electron-electron and electron-ion collisions correctly. The infinitely extended plasma is under the action of a space- and time-dependent external electric field. A general method of solution with projection operator techniques is given which results in a system of two coupled Volterra integral equations of the convolution type for the internal electric field and the current density. From there follows the electron distribution function, the electric field in the plasma, the electrical conductivity and a very general dispersion relation including Landau and collision damping. The method given can be generalized f. i. for multicomponent plasmas and for strong external electric fields.     

On the use of atmospheric pressure plasmas as electromagneticreflectors and absorbers

Tenuous man-made plasmas in the Earth's atmosphere from sea level to 100 km are discussed. An ionization source generates a tenuous plasma with an electron number density n_e that is high near the source and diminishes with distance from the source. After the source shuts off, n_e decreases as a function of time as electrons recombine with positive ions or attach to negative ions. The electromagnetic properties that are essential to an understanding of these plasmas, which can be modeled as cold collisional plasmas, is discussed. Gas and plasma characteristics, such as momentum-transfer collision rate, plasma lifetime, recombination kinetics, and the effect of noble gases, are presented. Typical collision rates and plasma lifetimes at atmospheric pressure are quantified. Applications for a plasma with a gradient are discussed. They include a high-altitude plasma that can reflect or absorb from HF to VHF and a broadband atmospheric pressure absorber. The generation and use of plasma, including electron impact ionization with a high-energy electron-beam source and UV photoionization of an alkali vapor or an organic vapor such as tetrakis (dimethylamino)ethylene (TMAE), is described. The power required to sustain a plasma is quantified, and properties such as maximum absorption and bandwidth are discussed. Tradeoffs among maximum absorption, absorption bandwidth, duty ratio, and applied power permit optimization of absorption primarily at VHF     

Elementary Many‐Particle Processes in Plasma Microfields

The effect of electric and magnetic plasma microfields on elementary many‐body processes in plasmas is considered. As detected first by Inglis and Teller in 1939, the electric microfield controls several elementary processes in plasmas as transitions, line shifts and line broadening. We concentrate here on the many‐particle processes ionization, recombination, and fusion and study a wide area of plasma parameters. In the first part the state of art of investigations on microfield distributions is reviewed in brief. In the second part, various types of ionization processes are discussed with respect to the influence of electric microfields. It is demonstrated that the processes of tunnel and rescattering ionization by laser fields as well as the process of electron collisional ionization may be strongly influenced by the electric microfields in the plasma. The third part is devoted to processes of microfield action on fusion processes and the effects on three‐body recombination are investigated. It is shown that there are regions of plasma densities and temperatures, where the rate of nuclear fusion is accelerated by the electric microfields. This effect may be relevant for nuclear processes in stars. Further, fusion processes in ion clusters are studied. Finally we study in this section three‐body recombination effects and show that an electric microfield influences the three‐body electron‐ion recombination via the highly excited states. In the fourth part, the distribution of the magnetic microfield is investigated for equilibrium, nonequilibrium, and non‐uniform magnetized plasmas. We show that the field distribution in a neutral point of a non‐relativistic ideal equilibrium plasma is similar to the Holtsmark distribution for the electrical microfield. Relaxation processes in nonequilibrium plasmas may lead to additional microfields. We show that in turbulent plasmas the broadening of radiative electron transitions in atoms and ions, without change of the principle quantum number, may be due to the Zeeman effect and may exceed Doppler and Stark broadening as well. Further it is shown that for optical radiation the effect of depolarization of a linearly polarized laser beams propagating through a magnetized plasma may be rather strong. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of first and second ionization on optical properties of atmospheric plasmas

In this paper, the relation between refractive index and other atmospheric plasma properties including electron, ion and molecule number densities and temperature are calculated. Considering the first and second ionization and the atmospheric air composition, the temperature dependence of the atmospheric plasma on refractive index is obtained by Saha equation. Also, in contrast to conventional models for evaluating electron number density by optical techniques, in which the refractive index of plasmas is approximately assumed equal to electron refractive index, this work is proposed for accurate and absolute measurement of electron density profile and determination of plasma region.     

Nonlinear Behavior of Electromagnetic Waves in Ultra‐Relativistic Electron‐Positron Plasmas

A set of nonlinear equations which can self‐consistently describe the behavior of high frequency Electromagnetic (EM) waves in un‐magnetized, ultra‐relativistic electron‐positron (e‐p) plasmas is obtained on the basis of Vlasov‐Maxwell equations. Nonlinear wave‐wave, wave‐particle interactions lead to the coupling of high frequency EM waves with low frequency density perturbations which result from EM waves radiation pressure. The same as that in conventional electron‐ion (e‐i) plasmas, strong EM waves in e‐p plasmas will give rise to density depletion in which itself are trapped. But on the contrary to that in e‐i plasmas, there no longer exists electrostatic acoustic–like wave in e‐p plasmas due to the absence of mass difference. For linear polarized EM waves, a stationary EM soliton with a spiky structure will be formed. The possible relation of the localized field to pulsar radio pulse is discussed (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ISSDE: A Monte Carlo implicit simulation code based on Stratonovich SDE approach of Coulomb collision

A Monte Carlo implicit simulation program, Implicit Stratonovich Stochastic Differential Equations(ISSDE), is developed for solving stochastic differential equations(SDEs) that describe plasmas with Coulomb collision. The basic idea of the program is the stochastic equivalence between the Fokker–Planck equation and the Stratonovich SDEs. The splitting method is used to increase the numerical stability of the algorithm for dynamics of charged particles with Coulomb collision. The cases of Lorentzian plasma, Maxwellian plasma and arbitrary distribution function of background plasma have been considered. The adoption of the implicit midpoint method guarantees exactly the energy conservation for the diffusion term and thus improves the numerical stability compared with conventional Runge–Kutta methods. ISSDE is built with C++ and has standard interfaces and extensible modules. The slowing down processes of electron beams in unmagnetized plasma and relaxation process in magnetized plasma are studied using the ISSDE, which shows its correctness and reliability.     

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.     

Electron-catalyzed mutual neutralization of various anions with Ar+: evidence of a new plasma process

The mutual neutralization of anions with Ar+ has been studied by variable electron and neutral density attachment mass spectrometry. Evidence of a previously unobserved plasma loss process, electron-catalyzed mutual neutralization (ECMN), e.g., SF6-+Ar+ + e-→neutrals + e-, is reported. Results for 10 species suggest that ECMN occurs generally and significantly affects the total ion-loss rate in plasmas with electron densities exceeding 10(10) cm-3. ECMN is discussed in the context of other known three-body plasma processes, the mechanisms for which appear insufficient to explain the observed effect. A mechanism for ECMN involving an incident electron facilitating energy transfer to the internal modes of the anion is proposed.     

Zum Einfluß der Energieabhängigkeit der Elektronenstoßfrequenz auf die elektrische Leitfähigkeit schwach ionisierter Plasmen

With regard to experimental applications in plasma diagnostics numerical approximations are given for the Gurevic-type correction functions, which in the kinetic theory of weakly ionized plasmas describe the deviations from the Lorentzian electrical conductivity. The approximations are based either upon the dependance of the collision frequency on the power of the electron velocity, or on a first order Taylor expansion around the most probable electron velocity of a Maxwellian distribution. For all assumptions regarding the velocity dependance of the collision frequency the influence of temperature (and pressure) on the effective collision frequency is indicated.