Spatial and temporal electron and ion density variations within the focal region of an intense laser beam in a tenuous plasma

The recent availability of focussed laser beams with flux densities well in excess of 10¹⁵ watts/cm² at 1.06 μm has led to experimental investigations of direct interaction between intense electromagnetic fields and charged particles. In this paper the relevant hydrodynamic two-fluid mode equations for a singly ionized tenuous plasma eveloped by an intense spatially varying electromagnetic field at the focus of a pulsed laser beam is solved by computer simulations for a cylindrically symmetric geometry. Radial, axial and temporal variations of electron and ion densities are given for different laser intensities and particle number densities. Fluctuations in electron density are found due to the bulk oscillation of electrons relative to what is essentially a static array of massive ions. The possibility of generating intense electromagnetic fields devoid of charged particles is discussed.     

氖原子对2000 eV X射线激光透明的产生机制

X射线自由电子激光与物质相互作用时出现了X射线透明现象,研究X射线透明现象的产生机制对理解X射线自由电子激光与物质相互作用过程具有重要参考价值.本文基于FAC(Flexible Atomic Code)程序计算的氖原子(离子)对2000 eV X射线的光电离截面和俄歇衰变速率,确定了不同通量密度下氖原子的主要电离方式;通过建立和解速率方程,得到了确定的电离方式中氖元素各电子组态数目比例随时间变化的公式,计算了氖原子在通量密度为2000和10000?~(-2)·fs~(-1)、持续时间为20 fs、光子能量为2000 eV的X射线激光照射下,任意时刻各主要电子组态的原子数目比例和总的氖原子平均光电离截面.建立了价电子完整的空心原子的数目比例随通量密度和曝光时间变化的公式.发现裸核和空心原子都会导致X射线透明,且选择恰当的通量密度和脉冲持续时间可以使价电子完整的空心原子数目比例达到极高.     

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.     

Linear propagation of dust acoustic modes in the presence of an electron beam

The physical and optical properties of plasmas are depended on dynamics of species in the discharge volume. Then, the presence of an electron beam, as a separate component, in a dusty plasma can modify the plasma structures through altering the discharge parameters. In this report, the linear propagation of acoustic modes in a collisionless dusty plasma contains electrons, ions and charged dust grains is investigated in the presence of an electron beam. Our analysis indicates that the electron beam can modify the dispersion relations of dust acoustic modes which resulted different data transportation in dusty plasmas. The obtained results are also examined for negative and positive charged dust grains with different number densities. The charge of dust grains represents an important role in the dynamics of the low frequency waves. Additionally, our findings reveal that the propagation of acoustic waves in dusty plasmas can be controlled by adjusting the electron number density of the beam and the cathode potential. Lastly, we obtian the destabilizing effects, originated from dust charge fluctuation, by reconsidering the dispersion relations of both dust acoustic modes.     

A theoretical study of the influence of ambipolar diffusion on the population distribution of excited states in a plasma

The effect of charged particle diffusion on excitation temperature was investigated theoretically, assuming that the diffusion processes are ambipolar and that the charged particles return to the plasma in the form of ground state atoms, after recombination at the plasma boundary. An approximate expression was obtained by solving a set of balance equations for the excited state atom densities. It shows that, with increase of the plasma pressure, the excitation temperature rises and approaches the electron temperature. It was found also that, as the electron density increases, the diffusion effect on the excitation temperature diminishes. A criterion was obtained which should be satisfied so that the excitation temperature may be in agreement with the electron temperature. The theoretical results of this paper are in good agreement with experimental results.     

Low energy emission bands in a small molecular fluorene derivative for organic light-emitting diodes

Quantum similarity is investigated for neutral atoms and singly charged ions by means of their one-particle densities in both position and momentum spaces. As recently observed for neutral atoms, the analysis involving singly charged ions in momentum space provides relevant information concerning structure and periodicity properties. Momentum-space similarity is also revealed to be strongly related to the kind of ionization process the system suffers, to the structural characteristics of the momentum density as well as to the ionization potential.     

On the role of dust in the cometary plasma

The cometary coma consists of neutral gas, plasma, and dust grains. The dust grains can influence both the neutral and charged coma’s constituents. Usually, the presence of dust particles in a plasma results in additional losses of both electrons and ions due to the plasma recombination on the particle surfaces. Solar radiation makes the impact of dust even more complicated depending on the solar flux, the dust number density, the photoelectric properties of the dust particles, the dust particle composition, the distribution of the sizes, etc. We propose a simple kinetic model evaluating the role of dust particles in the coma plasma chemistry and demonstrate that this role can be crucial, resulting in a nontrivial behavior of both the electron and ion densities of the plasma. We show that a coma’s dust particles can be negatively as well as positively charged depending on their composition. These opposite charges of the grains can result in fast coagulation of dust particles, thus, forming complex aggregate shapes of cometary grains. The text was submitted by the authors in English.     

Screening of a charged particle field in rarefied ionized gas

A self-consistent field of a charged micron-size particle placed in a rarefied ionized gas is created by both free ions moving along infinite trajectories and trapped ions moving in closed orbits. The character of screening of the particle field is analyzed under dynamic conditions in a nonequilibrium plasma where the temperature (or the mean energy) of electrons greatly exceeds the ion temperature. Under these conditions, trapped ions are generated in a restricted region of the particle field where the transitions between closed ion orbits resulting from resonant charge exchange dominate. This leads to a higher number density of trapped ions compared to that of free ions. The parameters of the self-consistent field of the particle and ions are found when free or trapped ions determine the screening of the particle field, and a similarity law is established for a simultaneous variation of the number density of plasma particles and the particle size. In dusty plasmas of the Solar System, which result from the interaction of the solar wind with dust, formation of trapped ions increases the plasma number density compared to that in the solar wind.     

Neutron yield when fast deuterium ions collide with strongly charged tritium-saturated dust particles

The ultrahigh charging of dust particles in a plasma under exposure to an electron beam with an energy up to 25 keV and the formation of a flux of fast ions coming from the plasma and accelerating in the strong field of negatively charged particles are considered. Particles containing tritium or deuterium atoms are considered as targets. The calculated rates of thermonuclear fusion reactions in strongly charged particles under exposure to accelerated plasma ions are presented. The neutron generation rate in reactions with accelerated deuterium and tritium ions has been calculated for these targets. The neutron yield has been calculated when varying the plasma-forming gas pressure, the plasma density, the target diameter, and the beam electron current density. Deuterium and tritium-containing particles are shown to be the most promising plasmaforming gas–target material pair for the creation of a compact gas-discharge neutron source based on the ultrahigh charging of dust particles by beam electrons with an energy up to 25 keV.     

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.     

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.     

等离子体鞘层附近尘埃颗粒特性的数值模拟

为了研究尘埃等离子体中尘埃颗粒以及鞘层中粒子密度分布等特性,对尘埃颗粒存在条件下等离子体鞘层结构的采取数值模拟.采用稳态无碰撞的尘埃等离子体鞘层模型,对玻姆判据、尘埃颗粒的荷电性质、平板鞘层区域的电势分布及鞘层内粒子分布特性进行了系统的数值模拟研究.计算结果显示,鞘层边缘尘埃颗粒数密度的增加、尘埃温度的升高,将引起孤立尘埃颗粒对电子吸附能力的减弱,集体效应也受到一定程度的影响;二者同时对离子玻姆速度以及鞘层厚度的增加都有着极大的促进作用.鞘层电势在靠近下极区处降落迅速,主要聚集在接近阴极极板的鞘层区域,各种微粒数密度的空间分布满足准中性条件.     

温稠密铝等离子体物态方程及其电离平衡研究

温稠密物质的物性参数在惯性约束聚变能源、Z箍缩等高能量密度物理领域的实验结果分析和物理过程数值模拟等方面有着重要的应用价值.本文应用部分电离等离子体模型,在理想自由能的基础上考虑了库仑相互作用、排斥体积作用和极化作用等非理想特性,开展了温稠密等离子体物态方程和电离平衡的研究.计算了温稠密铝等离子体的压强等物态方程数据和在密度为1.0×10^-4-3.0 g/cm^3,温度为1.0×10^4-3.0×10^4 K范围内的粒子组分.计算结果显示,铝等离子体的平均电离度在临界密度区域内随着密度的增加而突然增大.根据非理想Saha方程中有效电离能这一关键参数,分析了铝等离子体平均电离度在临界密度区域内随密度迅速增大的现象.     

闪电放电通道等离子体成分及相关特性的研究

以无狭缝摄谱仪获得了青海和西藏地区的云对地闪电回击过程的光谱,依据谱线波长和相对强度等信息,结合等离子体相关理论,得到了放电通道温度和电子密度;在此基础上,根据Saha方程、电荷守恒方程和粒子数守恒方程计算了闪电通道主要元素各级电离的离子数密度,进而得到通道质量密度、压强及平均电离度, 并分析了不同强度闪电放电通道的电离度、粒子数密度及其分布特征。结果表明：回击通道接近于完全电离,一次电离离子占主要地位,且NⅡ离子数密度最高;不同强度的闪电放电通道中,NⅡ和OⅡ离子的相对浓度值变化不大;计算过程中考虑带电离子间库仑相互作用以后,原子电离能的计算值降低,中性原子以及一次电离离子数密度的计算值变小,高次电离离子数密度的计算值变大。     

激光等离子体二维相对论电磁粒子模拟

研制了激光等离子体二维相对论电磁粒子模拟程序(2DCIC)。追踪几万甚至百万个模拟粒子在外加激光场和自洽场中运动,自洽地计算电荷和电流密度,求解完全的Maxwel方程,电子的相对论运动方程和离子的牛顿运动方程,辅以灵活的诊断研究波-波,波-粒子相互作用的发生、发展和饱和的细节以及时间演化规律。激光可以正入射,也可以斜入射;等离子体可以是均匀密度,也可以具有密度梯度;为了节约机时,还发展了并行运算。物理模型参数可调,既适用于研究激光聚变等离子体相互作用,也适用于超短脉冲超强激光等离子体相互作用和其它等离子体问题。经过多次试算检验,对等离子体平衡态进行了计算研究,对于超短脉冲超强激光的传播也进行了初步模拟计算。     

A Probe Method for Determination of time Evolution of Metastable Atoms Density in a Flowing Afterglow Plasma

The Langmuir Probe Measurement of the electron density and electron temperature has been made in the argon flowing afterglow plasma. Furthermore the metastable and charged particle densities evolution in the afterglow plasma are investigated by means of a numerical solution of the set of balance equations. From the fitting of the results of these calculations to the experimental data obtained from Langmuir probe measurements it is possible to determine the evolution of the metastable atoms density in the flowing afterglow plasma.     

Diffraction and channeling in nanotubes

A theory of the interaction of fast charged particles and gamma rays with nanotubes with different helicity is developed. Analytical expressions are obtained for the potential and the electron density of a nanotube taking account of the anisotropic thermal vibrations of the atoms. A system of equations determining the quantum states of the transverse motion of relativistic electrons, positrons, and x-ray photons in a superlattice consisting of nanotubes is formulated, and methods for solving this system are developed. Calculations of the soft x-ray Bragg reflection coefficients of a superlattice are performed in the two-wave approximation of the dynamical theory of diffraction.     

Cylindrical and Spherical Ion-Acoustic Shock Waves in a Relativistic Degenerate Multi-Ion Plasma

A rigorous theoretical investigation has been made to study the existence and basic features of the ion-acoustic (IA) shock structures in an unmagnetized, collisionless multi-ion plasma system (containing degenerate electron fluids, inertial positively as well as negatively charged ions, and arbitrarily charged static heavy ions). This investigation is valid for both non-relativistic and ultra-relativistic limits. The reductive perturbation technique has been employed to derive the modified Burgers equation. The solution of this equation has been numerically examined to study the basic properties of shock structures. The basic features (speed, amplitude, width, etc.) of these electrostatic shock structures have been briefly discussed. The basic properties of the IA shock waves are found to be significantly modified by the effects of arbitrarily charged static heavy ions and the plasma particle number densities. The implications of our results in space and interstellar compact objects like white dwarfs, neutron stars, black holes, and so on have been briefly discussed.     

Electron emission from a cathode doped by fast particles of a working gas

The effect of fast charged and neutral particles on the emission properties of different materials is investigated. In many plasma devices, a flux of fast atoms and ions produces a specific self-sustained state of the layer near the cathode surface. In particular, this layer is saturated by working gas atoms to a depth of several monolayers. This state variously modifies different types of emission. Potential emission, taking place under the action of metastable atoms and ions, weakens because of a rise in the work function. On the contrary, kinetic emission, which is due to fast heavy particles, is enhanced, since energy losses in the modified near-surface layer increase. Photoemission resulting under the action of resonance radiation increases considerably, and its mechanism changes.