Measurement of population density of excited helium atoms by the hook method

The hook method has been applied to the positive-column discharge plasma of helium and the population density of the lowest-lying excited levels, 2^1,3S and 2^1,3P, was measured. The results are compared with data obtained by use of the line absorption method, and discrepancies between these data are discussed. The result is also compared with a calculation based on application of the collisional-radiative model.     

Population inversion in recombining hydrogen plasma

The collisional-radiative model is applied to a recombining hydrogen plasma in order to investigate plasma conditions in which a population inversion between the energy levels of hydrogen can be generated. Population inversion is expected in plasmas for which three-body recombination makes a large contribution to the recombining processes and the effective recombination rate is larger than a critical value for a given electron density and temperature. Calculated results are presented in figures and tables.     

Lack of partial thermal equilibrium in pinch discharges

Measurements of the deuterium Balmer line intensities in the early phase of a theta pinch discharge show strong deviations from steady state population of levels considerably above the collision limit, even if criteria for the relaxation times of these levels are well satisfied. Solutions of time dependent rate equations for a hydrogen plasma confirm these observations. In cases of rising electron temperatures and rising degree of ionization the population of the ground level deviates by orders of magnitude from the steady state population of the momentary electron temperature and density. The population rates from the highly overpopulated ground level into levels above the collision limit then successfully compete with population rates from other levels and the continuum. Under such circumstances the relaxation time of excited levels above the collision limit into thermal equilibrium is determined by the relaxation time of the ground level into its steady state population.     

Dynamical description of transient x-ray lasers seeded with high-order harmonic radiation through Maxwell-Bloch numerical simulations

The Maxwell-Bloch code COLAX has been upgraded to use detailed hydrodynamical and collisional-radiative simulations of a soft x-ray laser plasma with traveling-wave pumping. The seeding of short pulses of high-order harmonics of the pump laser into the x-ray laser medium has been simulated. The amplification is shown to be a dynamic, two-stage process: the atomic dipoles of the lasing ions are first coherently excited by the short pulse, and subsequently generate a radiation wake which is amplified along its path through the plasma, with consequences on the experimentally recorded spectra.     

Lowering of the Populations of Excited Hydrogen and Helium Atoms in Plasmas Irradiated by a Laser Beam

The populations of excited hydrogen and helium atoms in non-L.T.E. plasmas have been calculated for two different physical situations, namely (i) no external radiation field present, and (ii) strong superposed radiation originating from a CO₂, Ruby or Argon Ion Laser. The radiation field intervenes in the collisional-radiative model via two new terms: Photoionization and stimulated recombination induced by the Laser radiation field. The solutions of the rate equations yield a lowering of the populations when a plasma is irradiated by a Laser beam. The lowering is different for cold recombining and hot diffusion-dominated plasmas. At high radiation densities a saturation effect occurs, since the photoionization rate becomes equal to the rate of stimulated recombination. Measurements of the populations of excited H and He atoms in a glow discharge irradiated by a CO₂ Laser beam of power density 10⁵ W/cm² are in broad agreement with the theoretical predictions.     

低气压闭式等离子体放电实验及光谱诊断

闭式等离子体可以克服等离子体隐身技术在开放环境中等离子体难以维持及能耗过大的问题。针对等离子体隐身应用,设计了一种封闭式的等离子体发生装置,选用微秒脉冲电源,以氩气为工质气体,在低气压环境下进行了放电实验。采用发射光谱法,测量了密闭腔体内部厚度方向上的Ar谱线强度,并将碰撞-辐射模型用于分析等离子体参数的分布规律。当放电参数确定时,给定电子温度和电子密度,可通过碰撞-辐射模型计算得到2p能级上的布居分布比值,将其与从光谱数据中得到的布居分布比值进行比较,当差异值最小时,即可确定相应的等离子体参数。通过对电子温度在1～5 eV范围内的2p9和2p1能级布居分布比值进行计算,分析了碰撞-辐射模型计算可能存在的误差。实验结果表明,在厚度方向上,封闭式腔体中的等离子体电子密度达到1011 cm-3量级且呈一定的梯度分布,但变化幅度不大,其分布情况有利于等离子体隐身技术的应用。     

Vibrational population densities of the A3Σu+, B3Πg, W3Δu, B'3Σu-, and C3Πu states in nitrogen plasmas

Population densities of vibrational levels of triplet excited states have been calculated using a collisional-radiative model applicable to nitrogen gas discharges with a Maxwellian electron energy distribution. Relative contributions of various electron impact and radiative processes, including intrasystem cascade, to the total cross sections of the triplet excited states are investigated.     

Excitation of He+ in a hollow cathode arc discharge

Population densities of the excited He(II) levels i=4 to i=10 in hollow cathode arc discharge have been determined and compared with those calculated from a collisional-radiative model. In the experiment the electron temperature was about 2x10⁵K and the electron density about 3x10¹⁹ m^-3. The observed population densities agree within 50% with those calculated by Drawin and Emard, but differ by a factor 4–6 from the ones found by McWhirter and Hearn. The results of the present experiment suggest that near the threshold energies the cross sections for electronic excitation of He⁺ levels from the He⁺ ground state are half those expected from the calculations of Burgess. We conclude that Drawin and Emard's calculations should preferably be employed for the determination of the electron density and the electron temperature from observed population densities.     

A kinetic model for alternate oscillation of self-terminating and recombination lasers in strontium ions

A self-consistent model has been developed to simulate the kinetics of alternate oscillation of self-terminating and recombination lasers in univalent ions of strontium excited by the high-repetition-rate (HRR) pulsed discharge. Photoelectric pulse waveforms predicted by the model are in good agreement with the experimental measurement. The model also gives temporal behaviors of long-life particles, the population densities of upper and lower laser levels and the electron temperature (Te) in He-Sr discharge plasma. The emission process of the two laser alternate oscillations, the characteristic of the laser pulse width and the population inversion mechanics are also analyzed.     

Excitation transfer in the plasma of a barrier discharge in a krypton-xenon mixture

Processes of excitation transfer in the plasma of a barrier discharge in a krypton-xenon mixture are studied. A kinetic diagram of energy transfer is proposed, and a system of coupled kinetic equations describing the dynamics of processes in the plasma of a barrier discharge of mixtures in pure xenon as well as in a krypton-xenon mixture is composed and solved. The effect of excitation transfer on the population distribution over vibrational levels of krypton excimers excited by a barrier discharge in a krypton-xenon mixture is studied. The deviation from the equilibrium distribution over vibrational levels is found. It can be interpreted as an indication of the activation of the mechanism of depopulation of the lowest vibrational levels of excimer states of krypton.     

Collisional radiative model of helium including response to resonant radiation

A collisional-radiative model of helium has been developed that incorporates the effect of optical pumping with light that is resonant with an atomic transition. A steady-state solution of the collisional- radiative model with no pumping is used as an initial condition. The pumping radiation is switched on, and the population densities of the atomic levels are calculated as a function of time. Results are given for a wide range of plasma parameters and various pumping conditions.     

Spatial-Resolved Measurement and Analysis of Extreme-Ultraviolet Emission Spectra from Laser-Produced Al Plasmas

Extreme ultraviolet emission from laser-produced A1 plasma is experimentally and theoretically investigated.Spatial-evolution emission spectra are measured by using the spatio-temporally resolved laser produced plasma technique.Based on the assumptions of a normalized Boltzmann distribution among the excited states and a steady-state collisional-radiative model,we succeed in reproducing the spectra at different detection positions,which are in good agreement with experiments.The decay curves about the electron temperature and electron density,as well as the fractions of individual A1 ions and average ionization stage with increasing the detection distance are obtained by comparison with the experimental measurements.These parameters are critical points for deeply understanding the expanding and cooling of laser produced plasmas in vacuum.     

Excited atoms in argon gas discharge plasma

General principles are discussed for a gas discharge plasma involving excited atoms where electron-atom collision processes dominate. It is shown that an optimal kinetic model of this plasma at not large electric field strengths can be based on the rate constants of quenching excited atom states by electron impact. The self-consistent character of atom excitation in gas discharge plasma is important and results in the tail of the energy distribution function of electrons being affected by the excitation process, which in turn influences the excitation rate. These principles are applied to an argon gas discharge plasma where excitation and ionization processes have a stepwise character and proceed via formation of argon atom states with the electron shell 3p ⁵4s.     

大气压氩气射流等离子体的光谱特性研究

等离子体喷枪是一种重要的等离子体源,已成为近几年低温等离子体研究的一个重要课题。本文利用钨针-钨丝网电极制作了直流喷枪装置,在大气压空气中产生了稳定的等离子体羽,并采用发射光谱的方法,对等离子体羽的等离子体参数进行了研究。在钨针电极与钨丝网电极之间放出耀眼的白光,钨丝网电极出口的气流下游有火苗形状的等离子体羽喷出。在电压保持不变的条件下(13.5 kV),等离子体羽长度随气体流量增加而增大;在气体流量保持不变的条件下(10 L·min-1),羽长度随外加电压的增大而增大。在气体流量一定的条件下,放电电压和放电电流呈反比例关系,即电压随着电流的增大而减小,说明放电属于辉光放电。采集了该喷枪在300～800 nm范围内的放电发射光谱,通过玻尔兹曼方法对放电等离子体电子激发温度进行了测量。结果表明,电子的激发温度随外加电压的增大而降低,随着工作气体流量的减小而升高。利用放电的基本理论对上述现象做了解释。这些研究结果对大气压均匀放电等离子体源的研制和工业应用具有重要意义。     

Calculation of parameters of krypton plasma excited by electron beam with additional heating by high-frequency electric field

The electron energy-distribution functions, the rates of plasmochemical reactions, and the densities of various plasma components have been calculated for a microwave discharge (f = 1000 MHz, P = 100–700 mW) in krypton plasma (p = 0.5 atm) excited by an electron beam with an energy of 12 keV. It has been found that the heating by a microwave field shifts the peak of the electron energy-distribution function (EEDF) from 0.5 to 2 eV, which leads to higher rates of reactions with excitation thresholds lying near the EEDF peak. As a result, the population of excited levels increases by two to three times.     

Thermalization of the high pressure mercury lamp positive columnduring the warm-up phase

This work deals with the modeling of high pressure mercury lamp positive column thermalization. Because of the important pressure variation during the start-up phase, the discharge properties change fundamentally before the steady state is reached. For this reason the corresponding pressure range is split into three phases, namely the low pressure, intermediate and high pressure phases, in which interesting simplifications of the simulation can be obtained. A self-consistent collisional-radiative model is used to simulate the first phase. In the second, the model has been modified in order to include the significant reactions of this phase. Finally a LTE model describes the high pressure phase. Partial validation of the models are obtained from the study of the warm-up of a mercury lamp supplied under DC conditions. The results obtained are in good agreement with literature experimental data and, furthermore, allow the discussion of the validity limits for each model     

三种电极的大气压氩等离子体射流光学特性

采用铜片-单匝线圈电极、螺旋缠绕电极和双铜片电极3种结构的放电装置,以氩气作为工作气体,在正弦波激励下获得了大气压等离子体射流。利用电学方法测量了放电电流以及电荷量,并对放电脉冲和放电功率进行了研究;利用发射光谱法对射流的等离子体参量进行了空间分辨测量,并根据ArⅠ 763.5 nm和Ar Ⅰ 772.4 nm的光强计算了电子激发温度。结果发现：在外加电压的正负半周期内,电流脉冲的个数和幅值呈现非对称的变化趋势;随着外加电压的增加,3种结构电极的放电功率从1.7 W逐渐增加到6.0 W;在相同的外加电压情况下,电极面积越小,等离子体射流的长度越长;3种等离子体射流的电子激发温度在1 348.5~3 212.1 K之间,并且随着气体流量的增加,各位置的电子激发温度总体上呈下降趋势,而等离子体的电子密度呈上升趋势。实验结果表明：外加电压对放电功率有一定影响;射流长度与电极面积有关;气体流量对电子激发温度和电子密度的空间分布起重要作用。     

Collisional — Radiative Model of Helium Plasma. I. Description of the Model

We present a collisional-radiative model of helium plasma with explicit inclusion of metastable state influences and give results of numerical calculations of population coefficients and effective reaction rates (generalization of effective ionization and recombination coefficients for the case when metastable states are not explicitly included) for a large region of electron densities and temperatures for optically thin and thick helium plasma. The model is described in this paper.