Nichtgleichgewichtsdichten inhomogener Plasmen mit Photoabsorption

The electron and population densities in nonequilibrium plasmas are computed. For these computations rate equations dependent on geometrical dimensions were used. Such rate equations result if the unknown radiation intensity is eliminated. This is achived by using the radiative transport equation. Our density computations are valid for a quasi neutral plasma. The total pressure or the density of the heavy particles and the distribution of the electron energy have to be assumed. For homogeneous plasmas the inelastic collision processes and the radiation processes are considered. For inhomogeneous plasmas the diffusion processes of the excited atoms are considered in addition to the collision and radiation processes. Using this method of calculation the electron and population densities of a cesium plasma are computed. We assumed maxwellian distributions and a total pressure of 1 Torr.     

Z箍缩动态黑腔形成过程和关键影响因素数值模拟研究

在Z箍缩动态黑腔研究中, 认识黑腔形成物理过程及主要特征, 明确优化黑腔辐射的关键参数, 是实验物理设计的重要基础. 本文针对W丝阵填充CH泡沫转换体的负载构型, 利用一维辐射磁流体程序, 在8 MA驱动电流条件下开展了动态黑腔形成过程和关键影响因素的数值模拟研究. 结果表明, 丝阵等离子体与泡沫转换体相互作用产生局部高压力区是驱动冲击波传播和形成动态黑腔的关键物理过程. 由于辐射超声速传播及其与冲击波波阵面的空间分离, 产生了辐射温度较高而物质未受明显压缩的动态黑腔区域. 丝阵等离子体碰撞泡沫转换体前的状态分布决定了动态黑腔辐射场的主要特征, 可以通过改变负载参数调整并优化黑腔辐射波形. 综合考虑黑腔峰值辐射温度和有效维持时间两个参数, 选择匹配质量的丝阵和泡沫, 使丝阵质量略小于泡沫, 可以获得相对优化的动态黑腔辐射波形. 同时, 合适的丝阵/泡沫初始半径比也是优化动态黑腔辐射的重要影响因素.     

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.     

Absorption of electromagnetic energy by slow electrons under scattering from Coulomb centers

Simple analytical expressions are obtained for the rate of the inverse stimulated bremsstrahlung absorption under electron scattering from a Coulomb center with charge Z in the presence of the electromagnetic field. The initial and final values of electron energy are assumed to be small compared to the Rydberg energy Z ² (atomic units are used throughout). Single-photon processes of absorption and induced radiation of photon by electron are treated. It is assumed that the electromagnetic field frequency ω is rather low, so that the condition Zω/p ³ ? 1, where p is the electron momentum, and the condition ?ω ? p ² are valid. However, this frequency is assumed to be fairly high compared to the electron-Coulomb center collision frequency: ω ? v _nei. The dependences of the rates of photon absorption and induced radiation on the angle θ between the direction of incident electron and the electromagnetic field polarization vector (assumed to be linearly polarized) are obtained. It is demonstrated that, for any angles θ, the rate of photon absorption is higher than the rate of induced radiation and, therefore, the Marcuse effect for slow electrons (electromagnetic field amplification) is absent. It is further demonstrated that a slow electron on the average absorbs double ponderomotive energy per collision with an ion (Coulomb center) in Maxwellian plasma. This agrees both with the known results calculation for fast electrons and with the known results of the calculation based on the classical Boltzmann kinetic equation for plasma.     

A resonance effect of the atomic populations kinetics induced by a fluctuating plasma

The effect of fluctuations of the electron temperature on the population kinetics of a reduced atomic system is investigated in the case of a low density plasma, where the coronal approximation can be used. An exponential correlation function is assumed for the electron temperature fluctuations. A resonance of the average population as a function of the correlation time is observed and analyzed.     

A study of the radiation of a Nd:YAG laser-MIG hybrid plasma

To study the coupling of a Nd:YAG laser beam and a MIG arc with the spectrum of the plasma, a new hollow probe method is used to collect radiation from specific points within the arc plasma. With the probe, the spatial distributions of the MIG arc radiation with and without the Nd:YAG laser hybrid plasma are collected and analyzed by a fiber spectrometer. Through analysis of the spectrum, radiation in the ultraviolet (Fe II dominated), visible (Fe I dominated) and infra (Ar I dominated) spectral zones are acquired for comparison of the difference between the MIG and Laser-MIG hybrid welding processes. The electronic temperature is calculated using the Boltzmann plot method and the electronic density is calculated using the Stark broadening method. The results show that the Laser-MIG hybrid welding processes cause the plasma energy to focus on the center of the welding arc and approach the welding pool. An ionizing duct abundant with Fe ions will be formed near the position of the impinging laser beam, which makes the welding arc stable. The electronic temperature of the Laser-MIG hybrid welding arc is a little higher than that of the MIG. The electronic density of Laser-MIG hybrid welding is higher than that of the MIG.     

Electromagnetic Plasma Modes at ?pe

The problem of the electromagnetic plasma radiation near the electron plasma frequency ?pe is studied in the absence of an external magnetic field. Maxwell's equations together with the fluid equations, which include thermal effects, are solved for an infinite geometry as well as for a finite geometry, and the problem of the eigenmodes at a plasma frequency ? ?pe is studied. The excitation of these modes by a small density relativistic beam is discussed. It is shown that the presence of a small density beam with finite ?o/c (where ?o is the beam velocity and c the velocity of light in a vacuum) in a plasma of finite thermal velocity, can couple linearly the plasma oscillations excited by the beam with the electromagnetic plasma mode at ?pe. It is also shown that surface waves at a frequency ? ?pe can be excited by the beam.     

In situ diagnostics in plasmas of electronic-ground-state hydrogen molecules in high vibrational and rotational states by laser-induced fluorescence with vacuum-ultraviolet radiation

Detection of excited electronic-ground-state hydrogen molecules with v(") up to 13 in a magnetic multipole plasma source was performed for the first time by laser-induced fluorescence with vacuum-ultraviolet radiation. The measurements are taken after fast shutoff of the discharge current. The rovibrationally excited molecules live longer than the plasma background light so that the fluorescence light can be detected with good signal-to-noise ratio. Absolute level populations are measured as well as decay times. The theoretically predicted suprathermal population of the vibrational distribution is clearly identified. The H- density is calculated on the basis of the measured populations and the measured electron energy distribution function. It is in excellent agreement with the H- density measured by photodetachment.     

喷气Z箍缩内爆动力学过程的数值模拟研究

在分析了喷气Z箍缩内爆等离子体物理过程的基础上，作了必要的假定和简化，给出了物理模型和相应的数学方程组.研制了一维三温辐射磁流体动力学的数值模拟程序，对氖喷气Z箍缩内爆产生高温高密度等离子体过程进行了总体数值模拟，得到了等离子体各参量在内爆过程中的时空分布，再现了该内爆动力学整体过程，并与GAMBLE-Ⅱ装置的实验结果进行了比较.计算结果表明，在对热传导系数和等离子体电阻率作适当调整后，得到的计算结果是自洽的，其中内爆到心时刻、x射线辐射脉冲的脉宽和总能量等宏观量与实验结果比较接近.同时对喷气Z箍缩内爆过 关键词： 喷气Z箍缩 x射线辐射 辐射磁流体动力学     

Self-focusing of laser beams in a plasma by ponderomotive forces

Self-focusing of a laser beam in a plasma is treated in terms of the ponderomotive acceleration due to the gradient of the light intensity. The focusing of radiation within the first minima of diffraction sets a lower limit to the laser power which is of the order of 1 MW for the usual lasers if cut-off density and a plasma temperature of about 10 eV are assumed.     

等离子体中氖输运及辐射特性模拟

通过假定一定的温度和密度分布,数值求解了一维等离子体连续性方程,获得了杂质氖电离态分布及辐射功率随空间位置和时间的变化.计算结果显示,在杂质注入时间较短时,由于离子输运及各种损失机制,总杂质密度在空间分布尚未达到平衡,电离态离子主要分布在等离子体周边,完全电离离子所占份额很小.当时间达到0.2s时,氖在等离子体中完全达到平衡状态,体积辐射功率趋于一个稳定的数值.达到平衡状态后,芯部一个很大范围内氖杂质主要以Ne 10离子型态存在,辐射功率以轫致辐射为主,因此辐射功率较小.辐射功率在空间的分布随时间变化较小,主要分布在等离子体周边及边界层一个狭小的辐射带内,说明氖引起的辐射主要由低电离态离子引起.     

Formation of a plane layer of plasma under irradiation by a soft X-ray source

We investigate theoretically the formation of a plasma in a plane layer of polymer foam (density ρ = 0.002 g/cm³ and thickness 800 μm) under the action of an external source of soft X-ray radiation under the conditions of PHELIX experiments. The incident flux is assumed to have a Planck’s distribution over the spectrum with T _rad = 20–40 eV. In numerical calculations, the flux of incident X-ray radiation and the spectral constants of the target substance are varied. The action of an external X-ray radiation source on a low-density foam substance with a density of 2 mg/cm³ causes a plasma to be formed with relatively homogeneous profiles of density and temperature T = 15–35 eV. Absorption of externalradiation energy is distributed in the volume. The plasma temperature increases with increase in the external energy, and the energy passed through the plasma also increases. The results prove to be sensitive to the values of optical constants used in numeral simulation. The spectral flux of external radiation passed through the plasma is chosen as a criterion of correctness of the optical constants used in the calculations. In future experiments using the PHELIX facility, we plan to investigate the slowing-down of an ion beam in a plasma formed as a result of indirect heating of low-density polymer triacetate cellulose (TAC) foam with densities ρ = 0.001–0.01 g/cm³ under the action of a pulse of X-ray radiation, into which the laser radiation is preliminarily transformed.     

Quantum plasma effects in the classical regime

For quantum effects to be significant in plasmas it is often assumed that the temperature over density ratio must be small. In this paper we challenge this assumption by considering the contribution to the dynamics from the electron spin properties. As a starting point we consider a multicomponent plasma model, where electrons with spin-up and spin-down are regarded as different fluids. By studying the propagation of Alfvén wave solitons we demonstrate that quantum effects can survive in a relatively high-temperature plasma. The consequences of our results are discussed.     

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     

Influence of electron energy distribution on helium recombining plasma diagnostics using line emissions

The influence of electron energy distribution on helium recombining plasma diagnostics is investigated using a helium collisional‐radiative model. The population densities of excited helium atoms are calculated for Maxwellian and non‐Maxwellian distribution plasma cases. In the case of the Maxwellian distribution plasma, the electron temperature and electron density determined by the Boltzmann plot method agree well with the input plasma parameters. On the other hand, it is indicated that the electron temperature and electron density are significantly underestimated in the bi‐Maxwellian distribution plasma case, even though the density of the hot electron components is three orders smaller than that of the bulk electrons. This result indicates that in a non‐Maxwellian helium recombining plasma, evaluation of the particle balance based on line emissions from excited helium atoms would be difficult because the reaction rate of atomic and molecular processes is strongly dependent on the electron temperature and density.     

Nonlinear deformations of a radiating shell moving with acceleration

The two-dimensional nonstationary motion of a cold dense shell accelerated under the action of rarefied hot gas pressure is modeled. The influence of radiative processes on manifestations of cumulative effects predicted by the inertia model is analyzed. It is assumed that the major medium component is hydrogen with small admixtures of oxygen, nitrogen, carbon, silicon, and iron. Radiative energy loss caused by photorecombinations and excitation followed by impurity ion metastable level deexcitation is taken into account. An approximation to the cooling function is suggested and implemented. This approximation can be used to perform calculations over wide ionization degree and gas temperature ranges. The formation of a shell during gas expansion ionized and heated by a source of ultraviolet radiation is studied. The characteristic time of shell appearance and its gas dynamic parameters are determined. The distribution of plasma temperature is shown to be nonmonotonic and have a maximum close to the ionization front. An increase in small perturbations of the velocity of a shell is shown to cause the formation of radial fibers and the concentration of gas mass and momentum in them. The structure of condensates formed is, however, much more complex than that predicted by the model of a thin layer of incompressible matter. In particular, condensation includes “fingerlike” thickening and a more extended region with a lower density. It also follows from the calculations that radiative cooling contributes to shell expansion in the radial direction but does not change the integral characteristics of condensations substantially.     

Electromagnetic radiation from filamentary sources in the presence of axially magnetized cylindrical plasma scatterers

Electromagnetic radiation from filamentary electric-dipole and magnetic-current sources of infinite length in the presence of gyrotropic cylindrical scatterers in the surrounding free space is studied. The scatterers are assumed to be infinitely long, axially magnetized circular plasma columns parallel to the axis of the filamentary source. The field and the radiation pattern of each source are calculated in the case where the source frequency is equal to one of the surface plasmon resonance frequencies of the cylindrical scatterers. It is shown that the presence of even a single resonant magnetized plasma scatterer of small electrical radius or a few such scatterers significantly affects the total fields of the filamentary sources, so that their radiation patterns become essentially different from those in the absence of scatterers or the presence of isotropic scatterers of the same shape and size. It is concluded that the radiation characteristics of the considered sources can efficiently be controlled using their resonance interaction with the neighboring gyrotropic scatterers.     

Ecton mechanism of the vacuum arc cathode spot

A new mechanism for the operation of a cathode spot in a vacuum arc, based on ecton processes, is proposed. An ecton is formed by the explosion of the tip of a jet of molten metal as it interacts with plasma. The time of ecton operation is assumed to be limited by the thermal conductivity of the liquid metal. For copper electrodes, the theoretical expressions are derived for the specific mass removal, ion erosion characteristics, current density, and the diameters of craters. The results agree well with the experimental data available     

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.