Gas dynamics and thermal-ionization instability of the cathode region of a glow discharge. Part II

The influence of the gas flow structure in the cathode sheath of a glow discharge on the discharge stability is studied numerically. The electric parameters are calculated in a diffusion-drift model that consistently takes into account associative dissociation as an additional electron source. The model also includes equations describing both the thermal mode of the cathode and the nonequilibrium physicochemical gas dynamics of a moderately rarefied gas. It is shown that, in a pulsed discharge, the increasing branch of the current-voltage characteristic, which is associated with the gas rarefaction behind the cathode shock wave, can change to a descending branch associated with the intensification of associative ionization. This gives rise to cathode sheath instability. The results of calculations agree well with experiments.     

Cathode Sheath Instability at Frequencies Near the Ion Plasma Frequency

We consider steady-state and nonstationary processes in a near-cathode region. Equations describing the plasma dynamics near a cathode at frequencies close to the ion plasma frequency are derived, and solutions of these equations for various zones of a discharge gap are found. A piecewise-uniform model of a cathode sheath is developed, which points out the possibility of an instability at a frequency slightly less than the near-cathode ion plasma frequency. The gas pressure effect on the instability threshold with respect to the discharge current is considered. The obtained results are in good agreement with the data of experimental studies of the cathode sheath in a hollow-cathode discharge.     

Calculation of the electrode surface temperature in the cathode spot of normal glow discharge at atmospheric pressure

The system of equations relating the electrode surface temperature in the cathode spot of normal glow discharge at atmospheric pressure to the discharge cathode sheath parameters is formulated. An analytical expression for the cathode surface temperature in the spot has been obtained, and the parameters of the spot and discharge cathode sheath in helium have been calculated.     

Calculation of time dependence of the electrode surface temperature in the cathode spot of atmospheric pressure normal glow discharge

Non-stationary heat transfer in the volume of the electrode with the cathode spot of atmospheric pressure normal glow discharge on its surface is calculated in case the spot radius is much less than the electrode dimensions. A system of equations describing variation of the temperature in the spot and the discharge cathode sheath parameters after discharge ignition is formulated, and the time dependence of the electrode temperature in the spot for discharge in helium is found. It is shown that within a short time fast heating of the electrode surface in the spot proceeds and then its temperature is slowly approaching the equilibrium value.     

Sheath formation study in Ne–Xe DBD discharge

The sheath is formed near the cathode immediately after applying the voltage across the electrodes. This formation is mainly due to the depopulation of this region by electrons. Using the one-dimensional model of the sheath region and the kinetic model coupled to the dielectric barrier discharge discharge electric circuit, we studied the formation of the cathode sheath and its evolution during the first pulse in Ne–Xe mixture for 10% and 20% of xenon at a total gas pressure of 400 Torr and for an applied voltage of 3 kV. The results illustrate the discharge behaviors as well as the evolution of the electric field and the charged particles in the cathode region. The effect of the xenon concentration is also studied and discussed.     

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采用蒙特卡罗模型对氮空心阴极放电等离子体鞘层离子(N2 、N )的输运过程进行了模拟研究,计算了阴极鞘层中氮离子(N2 、N )的能量及角分布的空间变化和粒子密度及平均能量随放电参数的变化规律。研究结果表明:空心阴极放电产生的氮离子,在鞘层输运过程中,N2 是密度几乎不变的低能粒子;N 是密度逐渐减少的高能粒子。随着电压增加,N 密度减小,平均能量增加;N2 密度和平均能量变化不明显。能量及入射角的相对分布规律与平板电极氮直流辉光放电基本类似,但圆筒空心阴极放电更有利于氮离子的产生。     

Light emission during cathode sheath formation in preionizedhigh-pressure glow discharges

The cathode sheath formation in a transient spatially small X-ray preionized high-pressure glow discharge with parameters typical for neon buffered XeCl lasers is investigated experimentally. Pictures of the light emission are taken with high temporal (≈1 ns) and spatial (≈25 μm) resolution using a gated CCD-camera as well as a streak camera. Quantitative data on the spatial and temporal development of the sheath structure are given. The most prominent feature is an extremely fast formation of a thin and very bright cathode sheath at a time which could be time correlated with an accuracy of ±0.2 ns with the discharge current. The results can be used to check numerical model calculations of the sheath formation     

Numerical simulation of super-short pulsed discharge in Helium with particle-in-cell Monte--Carlo collisions technique

This paper reports that a simulation of glow discharge in pure helium gas at the pressure of 1.333×10³ Pa under a high-voltage nanosecond pulse is performed by using a one-dimensional particle-in-cell Monte Carlo collisions (PIC--MCC) model. Numerical modelling results show that the cathode sheath is much thicker than that of anode during the pulse discharge, and that there exists the phenomenon of field reversal at relative high pressures near the end of the pulse, which results from the cumulative positive charges due to their finite mobility during the cathode sheath expansion. Moreover, electron energy distribution function (EEDF) and ion energy distribution function (IEDF) have been also observed. In the early stage of the pulse, a large amount of electrons can be accelerated above the ionization threshold energy. However, in the second half of the pulse, as the field in bulk plasma decreases and thereafter the reverse field forms due to the excessive charges in cathode sheath, although the plasma density grows, the high energy part of EEDF decreases. It concludes that the large volume non-equilibrium plasmas can be obtained with high-voltage nanosecond pulse discharges.     

Effect of the gas dynamic structure of a flow on the parameters of a self-sustained discharge. II

The limiting current of a self-sustained glow discharge is calculated. Two-dimensional equations for the flow of a viscous, vibrationally nonequilibrium gas and a model of the cathode sheath are used. The validity of the approximations which form the basis of the cathode sheath model was tested with experimental data for anomalous and normal currents. The effects of laminar and turbulent gas flow and of the geometric dimensions of the channel on the limiting discharge current are examined. Zh. Tekh. Fiz. 69, 49–55 (November 1999)     

Measurements of the Cathode Fall Characteristics in an Electron Beam Ionized Discharge

Measurements of cathode fall characteristics and of the influence of the cathode fall on the discharge properties of an electron beam ionized discharge (EBD) are reported. Gases investigated included methane, nitrogen, and argon. The cathode fall voltage was found to be a function of discharge current, voltage, gas type and pressure, and the rate of external ionization. At low discharge voltages, the cathode sheath region was found to require microseconds to get established. Added attaching gases had little influence on the cathode fall. Extrapolating to high rates of external ionization, the cathode fall voltage still can be expected to be hundreds of volts for the gases investigated.     

Simulation of radio-frequency atmospheric pressure glow discharge in γ mode

The existence of two diffe1：ent discharge modes has been verified in an rf （radio-frequency） atmospheric pressure glow discharge （APGD） by Shi [J. Appl. Phys. 97, 023306 （2005）]. In the first mode, referred to as a mode, the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion. In the second mode, termed γ mode, the discharge current density is relatively high, the secondary electrons emitted by cathodc under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. In this paper, a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in the mode in helium discharge between two parallel metallic planar electrodes. The results show that as the applied voltage increases, the discharge current becomes greater and the plasma density correspondingly increases, consequentially the discharge transits from the a mode into the γ mode. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region, and the electron Joule heating and the electron collisional energy loss reach their maxima in the region. The validity of the simulation is checked with the available experimental and numerical data.     

直流空心阴极放电中鞘层区电子的输运过程

采用蒙特－卡洛方法对空心阴极放电中鞘层区电子的输运过程进行了研究，电子在鞘层区被非均匀电场加速，两次碰撞之间的步长是由电子与中性粒子的碰撞频率确定。模型中三种碰撞截面积是由实验和理论数据拟合而来。研究了电子平均能量、电子密度和电离系数在径向的分布。电子能量的空间分布结果与实验很好地符合。 关键词：     

Simulation of the glow-to-arc discharge transition due to the cathode heating by ion bombardment

A model of gas-discharge transition from glow to diffuse arc, involving the equations of the discharge cathode sheath and the cathode heat balance equation, has been built. Time dependences of the cathode temperature and voltage drop from the moment of glow discharge ignition to the transition from glow to arc have been calculated.     

用发射光谱法测量氮气直流辉光放电的转动温度

本文报道了氮气气压分别为10和20Pa时，对直流辉光放电的发射光谱进行测量和分析的结果。选择的研究对象为N2放电中形成的N2^ B^2∑u^ →X^2∑g^ 跃迁的Δv=v′-v″=0谱带系中v′=0→v″=0谱带的R支。在阴极背面辉光区、阴极鞘层区、正柱区以及阳极辉光区中分别选择一点进行了转动分辨的发射光谱的测量。利用自己编写的光谱拟合程序，获得了相应的实验条件下N2^ 的转动温度，给出了转动温度随放电电压的变化趋势，其结果可以用直流放电的帕邢定律得到很好的解释。在10和20Pa气压下，放电的阴极鞘层区、正柱区、阳极辉光区中的转动温度都随放电电压呈现出了不同的变化趋势，甚至是完全相反的变化趋势。我们认为这是由于气压不同时，放电状态不同所致：气压为10Pa时的放电是正常辉光放电，而气压为时20Pa的放电为反常辉光放电。     

Gas dynamics and thermal-ionization instability of the cathode region of a glow discharge. Part I

A model of the cathode sheath of a glow discharge is developed. The model includes the equations for calculating the non-steady-state nonequilibrium physicochemical gas dynamics, cathode temperature, and electric field. The model applies to describing the flow of a viscous, heat-conducting, moderately rarefied gas at Knudsen numbers of about Kn ∼0.03. The electric field and gas density distributions are determined consistently by renormalizing the values obtained by the Engel-Steenbeck theory. A formula for calculating the time during which a homogeneous volume discharge phase exists is proposed. The formula is based on the relation between the rates of electron production via associative ionization (A+B → AB ⁺+e) and impact ionization (A2+e → A ₂ ⁺ +e+e). Calculations are carried out for nitrogen and air. It is shown that, at high current densities, due to the dissociation and strong heating of the gas, the rate of thermal ionization becomes as high as that of electrical ionization. The calculated ionization time is in reasonable agreement with the measured duration of a uniform anomalous cathode sheath.     

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基于OOPIC软件,对平面直流磁控溅射放电等离子体进行了二维自洽粒子模拟,重点研究了磁场、阴极电势和气压等工作参数对磁控放电特性的影响。模拟发现,在一定的工作参数范围内,随着磁场的增强,鞘层厚度变窄,鞘层电势降减小,阴极离子密度增大,但是分布变窄;随着阴极电势的增加,鞘层厚度稍微变窄,鞘层电势降增大,阴极离子密度增大,分布变宽;随着气压的升高,鞘层厚度基本不变,鞘层电势降会增大,阴极离子密度先增大后减小,分布略微变宽。     

Modeling of the effect of a dielectric film on the electrode surface upon the discharge glow-to-arc transition

It is shown that when a dielectric film is on the cathode, the current density in the discharge sheath is raised due to the field emission of electrons from the cathode metal substrate under the effect of the electric field generated in the film by the surface charge that accumulates on it. This results in more intense cathode heating and faster glow-arc transitions.     

Emission from a sulfur-hexafluoride volume discharge induced by attachment instability

Characteristics of a low-pressure volume discharge in SF₆ are investigated. The discharge with a spherical anode-plane cathode electrode system was induced by attachment instability. It is found that applying a dc voltage (U _ch≤1.3 kV) to the anode in the absence of a confining dielectric wall results in the ignition of a repetitive discharge (f=0.1–120 kHz). The spatial, frequency, and current-voltage characteristics of the volume discharge; plasma emission in the spectral range 200–700 nm; and the waveforms of the discharge voltage, current, and plasma emission are investigated. It is shown that the plasma under study exists in the form of a domain (autosoliton) and that the volume discharge is self-induced because, during attachment instability, the plasma itself acts as a switch enabling repetitive pulsed operation. The results obtained can be applied to studying the physics and technology of electric-discharge chemical HF (DF) lasers based on nonchain reactions and to developing high-aperture low-pressure repetitive Ar-, Kr-, and Xe-fluoride lamps with low-corrosive working media (low-pressure mixtures of Ar, Kr, and Xe with SF₆).     

Effect of Cathode and Anode Voltage on an Ion Sheath Thickness in a Magnetically Confined Diffusion Plasma

This article reports about the ion sheath thickness variation occurring in front of a negatively biased plate immersed in the target plasma region of a double plasma device. The target plasma is produced due to the local ionization of neutral gas by the high energetic electrons coming from the source region (main discharge region). It is observed that for an increase in cathode voltage (filament bias voltage) in the source region, the ion flux into the plate increases. As a result, the sheath at the plate contracts. Again, for an increase in source anode voltage (magnetic cage bias), the ion flux to the plate decreases. As a result, the sheath expands at the plate. The ion sheath formed at the separation grid of the device is found to expand for an increase in cathode voltage and it contracts for an increase in the anode voltage of the main discharge region. One important observation is that the applied anode bias can control the Bohm speed of the ions towards the separation grid. Furthermore, it is observed that the ion current collected by the separation grid is independent of changes in plasma density in the diffusion region but is highly dependent on the source plasma parameters. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of Dissociation Mechanism of CO_2 in a Micro-Hollow Cathode Discharge

Considering the feature of distributions of parameters within the micro-hollow cathode discharge,we use a simple method to separate the sheath region characterized by drastic changes of plasma parameters and the bulk piasma region characterized by smooth changes of plasma parameters.A zero-dimensional chemical kinetic model is used to analyze the dissociation mechanism of CO_2 in the bulk plasma region of a micro-hollow cathode discharge and is validated by comparisons with previous modeling and experimental results.The analysis of the chemical kinetic processes has shown that the electron impact dissociation and heavy species impact dissociation are dominant in different stages of the micro-hollow cathode discharge process for a given applied voltage.The analysis of energy consumption distributions under different applied voltages reveals that the main reason of the conversion improvement with the increase of the applied voltage is that more input energy is distributed to the heavy species impact dissociation.