Laboratory studies of back-discharge in fly ash

The back-discharge is a type of discharge that takes place in the presence of corona discharge and occurs at an electrode covered with a dielectric layer of resistivity higher than about 10⁸ Ω m. Back-discharge can be observed in electrostatic precipitators when dust covering the collection electrode has low conductivity. In this paper, the studies of back-discharge generated in ambient air, in point-to-plane geometry with the plate electrode covered with fly ash are presented. The discharge is characterised in terms of its visual forms, current–voltage characteristics, and light emission spectra. Three forms of back-discharge were investigated: glow discharge, streamers, and low-current back-arc discharge. The current of the back-arc discharge was only a few milliamps. The discharge was stabilised by a high series resistance. It was noted that the voltage of ignition of the back-discharge for negative polarity is lower than for a positive one. Spectroscopic measurements of emission spectra provided information on elements present in the discharge column. The elements present in the fly ash, including toxic metals, can be re-entrained into the gas as particles or can be emitted as ions or neutrals during the discharge, and can decrease the collection efficiency of electrostatic precipitators. These elements were detected in the emission spectra. The effect of the discharge on the fly ash layer was also discussed. It was observed that sinter-like leftovers remain in the dust layer after a back-arc discharge.     

An influence of the electrode emission properties and the gas temperature on the discharge ignition voltage in the mixture of argon with mercury vapor

Kinetics of electrons, ions and metastable excited atoms in the interelectrode gap of a low-current discharge in the argon-mercury mixture is simulated, and the discharge ignition voltage as a function of the temperature and the cathode ion-electron secondary emission rate is calculated. The mixture temperature dependence of the minimum value of the ion-electron secondary emission rate ensuring a given discharge ignition voltage is found.     

Combustion of micron-sized particles of titanium and zirconium

Particles of titanium and zirconium in the size range of 2–25 μm are ignited while passing through a CO₂ laser beam and their combustion is monitored optically. Prior to ignition, particles pass through a low-power auxiliary laser beam so that the diameter of each ignited particle is measured in situ based on the amplitude of the scattered light pulse. The particles of both Ti and Zr are observed to exhibit micro explosions, similar to those observed for larger size particles of these metals. Particle emission traces are recorded, and a data processing routine is established for discounting emission signals produced by unignited particles and particles partially combusted within the CO₂ laser beam. Burn times and combustion temperatures are measured and compared to earlier measurements for coarser particles of the same metals. For both metals, average combustion temperatures implied by the emission spectra are very close to their respective adiabatic flame temperatures. For both metals, for the particle size range considered, particle combustion temperatures do not depend on the particle size. The particle burn times were found to be only weak functions of the particle size; burn times for Zr are shorter and temperatures are higher compared to the similarly sized Ti particles.     

等离子体中活性粒子分析及化学动力学机理

利用发射光谱测量技术分析了介质阻挡放电等离子体激励空气产生的主要活性粒子,利用零维等离子体动力学模型模拟了甲烷/空气中放电阶段主要活性粒子的演化规律,并通过敏感性与化学路径分析研究了O原子影响甲烷点火过程的化学动力学机理。研究表明:空气中介质阻挡放电等离子体主要产生N2和O2的激发态粒子,激发态粒子的数密度随着电压的增加而增大;激发态粒子经过一系列物理化学反应最终转化成若干自由基,其中O原子的摩尔分数最大;O原子缩短甲烷点火延迟时间一个量级,原因在于添加O原子后甲基(CH3)的氧化途径由自点火过程中的经O2直接氧化为CH3O和CH2O转变为经HO2和O原子氧化为CH3O和CH2O,由于后者的基元反应速率快,因而明显缩短了点火延迟时间。     

Initiation of Gas Discharges in Plasma Displays Assisted by Electron Emission of Thin Insulators

At present many efforts are done for the development of colour plasma displays (PD). One main problem is to secure a reliable ignition, provided by a suitable concentration of starting particles in the discharge cells (priming). In this paper a new priming method is proposed using the exoelectron emission (EEE) of thin insulators. The influence of the EEE on the ignition behaviour of ac PD cells is investigated and compared with direct measurements of the afteremission of MgO, Al₂O₃ and MgAl₂O₄. Auger electron spectroscopy (AES), residual gas mass spectra and Ar⁺ etching are additionally used methods.     

Areal homogeneity verification of plasma generated by diffuse coplanar surface barrier discharge in ambient air at atmospheric pressure

This paper deals with an optical emission spectroscopy study of a diffuse coplanar surface barrier discharge (DCSBD ) in air at atmospheric pressure. The main aim of this study was to verify the areal homogeneity of the generated plasma, which is important for many applications like treatment of nonwoven fabrics, glass, metals, polymers, foils, and so on. Optical emission spectra of DCSBD plasma in air were measured for three different frequencies (15, 30, and 50 kHz ) of the applied voltage. Comparison of the calculated rotational and vibrational temperatures was carried out, and areal homogeneity of plasma was proved. Electrical parameters of discharge such as the plasma power using the area of the Lissajous figures and energy transfer efficiency to the discharge were also investigated. The effective thickness of plasma layer as a function of the input power was measured.     

利用火焰发射光谱来研究汽油机的燃烧过程

本文用一套精密的光电转换系统，采集了一台汽油机燃烧过程中火焰辐射在可见光到近紫外波段内的光谱，探测到了燃烧中间产物ＣＨ、ＣＮ、Ｃ２、Ｈ２Ｏ等的特征光谱，并分析了这些产物在燃烧过程中的变化规律，以及随过量空气系数，缸内压力的变化．实验结果表明，汽油机三个不同的燃烧阶段具有不同的燃烧光谱特征：着火过程中，存在着大量的处于激发态的分子、原子、离子、自由基等活化中心的束缚态光谱，随着燃烧发展，ＣＨ、Ｃ２自由基的光谱强度明显加强；当减小过量空气系数时，光谱强度变弱并且着火延迟期增长；自由基特征光谱的光强变化曲线可以反映它们在燃烧过程中的浓度变化．所以火焰发射光谱是实时检测燃烧中间产物，特别是ＣＨ、Ｃ２等有害排放物变化规律的有效手段，可以为分析、模拟燃烧过程，控制排放提供有用的实验数据．     

Über den Gleichspannungsdurchschlag im Ultrahochvakuum

DC-breakdowns with arc-conditioned electrodes in vapour-free vacuum have been investigated with high temporal resolution. The breakdowns started always with a cathode flare, the ignition voltage of which was strongly correlated with prebreakdown field emission. The temporal development of gap plasma, discharge current and gap voltage during the breakdowns could be satisfactorily explained by the models of explosive electron emission and space-charge limited diode conduction. No change of mechanism was observed with gap distances up to 6 mm and voltages up to 120 kV. Generally the resistance of the discharge channel became low enough for a transition to the arc stage. However, in a few cases a self-limitation of the discharge occurred due to starvation of the material transfer from the cathode. At higher gap distances the surrounding walls had a distinct influence on the ignition of the discharges.     

正庚烷燃烧反应中间自由基的光谱测量

采用ICCD瞬态光谱探测系统和化学激波管,在点火温度1 408K,点火压力2.0atm,燃料摩尔分数1.0%,当量比1.0的条件下,拍摄了正庚烷燃烧过程中不同时刻的瞬态发射光谱,光谱曝光时间6μs,拍谱范围200～850nm。确认了在所拍光谱范围内主要是OH,CH和C2自由基的特征辐射光谱,表明小自由基OH,CH和C2是正庚烷燃烧过程中重要的反应中间产物。所拍时间分辨光谱显示,在正庚烷燃烧反应中,OH,CH和C2自由基一出现很快就达到其浓度峰值,但CH和C2自由基随着反应的进行迅速减少至消失,OH自由基持续的时间却长很多。实验结果为了解正庚烷燃烧反应微观过程和验证其燃烧反应机理提供了实验依据。     

The voltage-pulsing effects in AC plasma display panel

The effect of the voltage pulsing is simulated in AC plasma display panel using various two-dimensional simulation codes. The volume-averaged density increases rapidly because of the large electric field in the AC-plasma display panel cell as the peak voltage is raised. The ignition time and the decaying time of the discharge are related to the external voltage. The space and time variations of the charged particle densities and the potential profile are described for various voltage forms. The ion energy distribution near the cathode-side dielectric which plays an important role in the MgO lifetime and the secondary electron emission coefficient is concentrated to much lower energies than the applied voltage     

直流弧光放电化学气相沉积(CVD)法制备金刚石薄膜及其等离子体的光发射谱原位测量

应用直流弧光放电分解CH₄和H₂混合气体成功地实现了高速生长多晶金刚石膜,应用扫描电子显微镜、X射线衍射仪、Raman谱仪对所得样品进行检测和分析,为了弄清楚金刚石膜的生长机理,在实际生长环境下“原位”测量了不同条件下直流弧光等离子体的光发射谱,结果发现:高速生长金刚石膜的关键是该气相反应中有大量的原子H存在。     

大功率电弧加热器起弧过程流场特性研究

电弧等离子体是通过电极之间击穿放电,产生热电弧,实现对冷态介质加热,目前大功率的电弧等离子体发生器在航空航天领域有重要的应用,是国内外开展飞行器防热材料筛选和考核研究最重要的地面模拟试验设备.本研究基于发展的高焓气流非接触式光谱诊断方法,开展对10 MW量级大功率长分段电弧加热器起弧过程流场特性的定量、定性研究,在线测量等离子气流的辐射光谱,并获得了等离子体气流电子温度和电极烧蚀铜原子摩尔组分浓度的测量结果.研究结果表明:起弧开始阶段,纯氩气通入,等离子体辐射光谱以分立的氩原子谱线为主;过渡阶段,随着空气的通入出现了N₂和N₂+的连续分子谱和Ar、N、O原子谱,等离子体电子温度随之降低;正式运行阶段,纯空气介质运行,辐射光谱以N₂和N₂+连续分子谱和N、O原子谱为主.整个电弧加热器起弧过程伴随持续的电极烧蚀,等离子体辐射光谱中铜原子谱线一直存在.氩气起弧时,等离子体气流电子温度稳定在11 000 K±300 K,显示出电弧加热器稳定的起弧特性.同时,电弧等离子体气流中铜原子摩尔组分浓度在（1~25）×10-6之间周期性变化,显示弧根旋转过程中不规则的电极烧蚀变化.发展的发射光谱诊断方法为研究电弧加热器真空氩气起弧特性提供量化手段,可以为真空-常压氩气起弧试验技术的发展和电极优化提供直接量化依据,为大功率常压叠片电弧加热试验平台发展奠定基础.      

氩气/甲烷/空气介质阻挡放电等离子体的发射光谱诊断

为了获得可燃气体的放电及等离子体发射光谱特性,进一步揭示等离子体助燃作用下燃料在稀燃状态的点火与燃烧特性,在常压下以氩气作为载气对预混的甲烷和空气进行放电研究。实验基于平行板电极射频(13.56 MHz)介质阻挡放电的等离子体发生装置,首先在常压下对体积分数为90%氩气/10%空气的混合气体开展放电研究;再在90%氩气含量不变的情况下,调节空气含量并加入与之能形成燃烧化学当量比Φ＝1的甲烷,氩气/甲烷/空气的混合气体同样能实现稳定而均匀的放电;最后分别在90%氩气含量不变,甲烷和空气在当量比为Φ＝0.4～1.9六种情况下进行放电实验。由光谱仪记录不同放电工况下的发射光谱信息,诊断反应产物类型,利用观测到的氮分子第二正带系(0-2)380.4 nm和(1-3)375.4 nm处的发射谱线,与自编程序计算的模拟谱线拟合,得出分子转动温度(即气体温度)。研究结果表明：通过拟合模拟光谱与实验所测发射光谱的方法推测分子转动温度,进而获得气体的平动温度,氩气/空气放电的气体温度可达到1 150 K,氩气/甲烷/空气Φ＝1时放电气体温度升高到1 390 K;甲烷与空气形成不同当量比时,所测等离子体气体温度相对于90%氩气/10%空气混合气体温度的温升在70～240 K范围变化;由光谱信息观测到CH,H,OH和CH₂O等活性粒子的存在以及气体温度的升高,表明可燃成分混合气在射频电场放电作用下发生等离子体燃烧化学反应并释放出化学热。     

氮气/氨气的高压快脉冲放电光谱研究

本文测量了氮气、氨气及其混合气体的快脉冲放电等离子体的光谱,用光谱分析方法研究了等离子体成分及其与气压和配比的关系,并讨论了在制备氮化物时使用氮、氨混合气体的有利之处。     

脉冲金属氢化物真空弧放电等离子体发射光谱特性

利用发射光谱方法对真空弧离子源放电等离子体特性进行了诊断。同时,基于局域热力学平衡等离子体的发射光谱理论,建立了等离子体的发射光谱拟合模型,对真空弧放电等离子体光谱进行了分析。针对TiH真空弧离子源,分别对330~340nm与498~503nm范围内Ti+离子与Ti原子的发射光谱进行了对比拟合,获得了较好的符合度,解决了传统Boltzmann斜率法计算等离子体温度需要孤立的不受附近谱线干扰的线状光谱的困难。最后,利用该方法计算了真空弧离子源在不同放电条件下的等离子体发射光谱、等离子体密度与温度参数。结果表明,TiH真空弧放电等离子体温度在1eV左右,同时,放电所产生的氢原子要远远大于金属原子,并且随着真空弧离子源馈入功率的增加,TiH电极中解吸附出来的氢比蒸发出来的金属增加得更多,这有利于TiH离子源在中子发生器方面的应用。     

Optical emission spectroscopy of point-plane corona and back-corona discharges in air

Results of spectroscopic investigations and current-voltage characteristics of corona discharge and back discharge on fly-ash layer, generated in point-plane electrode geometry in air at atmospheric pressure are presented in the paper. The characteristics of both discharges are similar but differ in the current and voltage ranges of all the discharge forms distinguished during the experiments. Three forms of back discharge, for positive and negative polarity, were investigated: glow, streamer and low-current back-arc. In order to characterize ionisation and excitation processes in back discharge, the emission spectra were measured and compared with those obtained for normal corona discharge generated in the same electrode configuration but with fly ash layer removed. The emission spectra were measured in two discharge zones: near the tip of needle electrode and near the plate. Visual forms of the discharge were recorded with digital camera and referred to current-voltage characteristics and emission spectra. The measurements have shown that spectral lines emitted by back discharge depend on the form of discharge and the discharge current. From the comparison of the spectral lines of back and normal discharges an effect of fly ash layer on the discharge morphology can be determined. The recorded emission spectra formed by ionised gas and plasma near the needle electrode and fly ash layer are different. It should be noted that in back arc emission, spectral lines of fly ash layer components can be distinguished. On the other hand, in needle zone, the emission of high intensity N₂ second positive system and NO _γ lines can be noticed. Regardless of these gaseous lines, also atomic lines of dust layer were present in the spectrum. The differences in spectra of back discharge for positive and negative polarities of the needle electrode have been explained by considering the kind of ions generated in the crater in fly ash layer. The aim of these studies is to better understand the discharge processes encountered in electrostatic precipitators.     

离子液体对氩气介质阻挡放电光谱的影响

采用自行设计的介质阻挡放电反应器,以氩气和离子液体为放电介质,实现大气压下稳定的气(等离子体)-液(离子液体)等离子体放电,并运用光谱法在线诊断氩等离子体光谱。考察了不同咪唑基离子液体以及放电参数对大气压氩气介质阻挡放电光谱的影响。结果表明,离子液体的引入降低了氩气放电光谱的强度,谱峰强度与离子液体阳离子咪唑环上的碳链长度有关,且随碳链长度增加,谱峰强度降低;同时阴离子结构对称性低的离子液体,谱峰强度较低。加入离子液体后氩谱随放电电压及放电频率变化均呈现峰值变化。     

X-ray generation in low-voltage vacuum discharges

The dynamics of x-ray emission from a low-voltage laser-induced discharge was studied with the aid of a picosecond x-ray streak camera. Directed x-ray emission in the spectral range from 100 eV to 10 keV in the form of point sources and thin layers with lifetimes ranging from 30 ps to 1 ns was observed in a low-voltage vacuum discharge (U=150 V) initiated by a picosecond laser beam. X-ray emission from a discharge was detected with a time delay (1–20 ns) relative to ignition by the laser beam in order to prevent the radiation of the laser plasma from entering the detector. Detection of directed x-ray emission in a low-voltage vacuum discharge is demonstrated. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 622–627 (10 May 1998)     

介质阻挡放电中圆圈点放电丝的分子振动温度

在氩气、空气混合气体介质阻挡放电中,首次利用光谱方法,测量了圆圈点放电丝中圆圈放电和中心点放电的振动温度,并研究了它们随空气含量的变化.振动温度的计算利用的是氮分子第二正带系(C3Ⅱu→B 3Ⅱg)的发射谱线.结果表明:圆圈放电的振动温度高于中心点放电的振动温度;二者均随空气含量的增加而增加,但圆圈放电振动温度的增加速...     

Electric strength of the accelerating gap of a plasma electron source at rough vacuum

Conditions for the electric breakdown of the accelerating gap of a plasma electron source are determined. It is shown that, depending on the gas pressure and the size of openings in the emission electrode, two types of breakdown can occur. One of them is due to the ignition of a low-voltage discharge between the electrodes of the accelerating system, and the other one is caused by switching of the main discharge from the anode to the accelerating electrode.