Electron kinetics in a discharge plasma produced by a focused microwave beam in free space

A study is made of the electron kinetics in a discharge plasma produced by a high-power beam of electromagnetic radiation in the centimeter-wave region under conditions approaching free space, when the dimensions of the chamber are much greater than the wavelength of the microwave radiation. Two regimes of discharge production are investigated: the regime of short microsecond pulses at a repetition rate of 200 Hz, and a single millisecond pulse regime. It is shown that at threshold values of the microwave energy flux density the electron density in the initial stages of discharge formation reaches the critical value, and that the average energy of the electrons is of the order of 1.5–3 eV. Zh. Tekh. Fiz. 67, 10–14 (June 1997)     

On the factors determining the pyrophoric stability of tungsten nanopowder obtained by plasma-chemical pyrolysis of W(CO)6

Nonpyrophoric tungsten powders with an average particle size of about 30 nm were obtained by pyrolysis of tungsten hexacarbonyl in a flow of microwave discharge nitrogen plasma. It is found that these powders are stable in air up to 300°C. The reason for such stability is that the structure of powder particles is of the core-double shell type, in which the metal core is covered with an oxide film approximately 1 nm in thickness, coated in turn with roentgenoamorphous layer consisting of carbon, oxygen, and nitrogen atoms. It is also established that the powders under investigation mainly release carbon oxides (CO and CO₂) and water into the gas phase upon heating in vacuum. Among the molecules present in the gas phase in small concentrations, nitrogen monoxide (NO) and formaldehyde (H₂CO) are worth mentioning apart from C1–C3 hydrocarbons.     

Numerical modeling of ozone production in a pulsed homogeneousdischarge: a parameter study

The pulsed volume discharge is an alternative for the efficient generation of ozone in compact systems. This paper presents a parameter study of the reactions in this kind of homogeneous discharge by using a numerical model which solves plasma chemical kinetic rate and energy equations. Simulations are performed for 10^-9-10^-5 s single pulses and oxygen gas density in the range 10<300 K and normalized electric field of 1000 decreased. The maximum concentration is 3% at 10 amagat and 100 K. The results on ozone accumulation in multiple pulse discharges are presented. In contrast to the single pulse case, higher efficiency is achieved at lower gas density. This scaling can be explained by losses due to ion currents     

Plasma parameters of a nonself-sustained microwave discharge created by a programmed pulse

The kinetics of the electrons in the plasma of a nonself-sustained discharge formed at the focus of a microwave beam when the gas is exposed to electromagnetic radiation with an energy flux density that varies with time in a programmed-pulse mode are investigated. It is shown that the temperature of the electrons in the plasma of the localized microwave discharge is of the order of 1 eV and varies weakly during the pump pulse and as the air pressure is varied, while the electron density is an order of magnitude or more lower than the critical density and depends on the level of the pump generator power. It is shown that the degree of ionization of the plasma can be regulated by altering the programmed-pulse mode. Zh. Tekh. Fiz. 67, 19–23 (July 1997)     

Time-resolved processes in a pulsed electrical discharge in argon bubbles in water

A phenomenological picture of a pulsed electrical discharge in gas bubbles in water is produced by combining electrical, spectroscopic, and imaging characterization methods. The discharge is generated by applying 1 m\mu s pulses of 5 to 20 kV between a needle and a disk electrode submerged in water. An Ar gas bubble surrounds the tip of the needle electrode. Imaging, electrical characteristics, and time-resolved optical emission spectroscopic data suggest a fast streamer propagation mechanism and the formation of a plasma channel in the bubble. Comparing the electrical and imaging data for consecutive pulses applied to the bubble at a frequency of 1 Hz indicates that each discharge proceeds as an entirely new process with no memory of the previous discharge aside from the presence of long-lived chemical species, such as ozone and oxygen. Imaging and electrical data show the presence of two discharge events during each applied voltage pulse, a forward discharge near the beginning of the applied pulse depositing charge on the surface of the bubble and a reverse discharge removing the accumulated charge from the water/gas interface when the applied voltage is turned off. The pd value of ~ 300–500 torr cm, the 1 μs long pulse duration, low repetition rate, and unidirectional character of the applied voltage pulses make the discharge process here unique compared to the traditional corona or dielectric barrier discharges.     

Effect of catalysts on dc corona discharge poisoning

The processes of ozone generation in non-thermal plasma produced by an electrical discharge in air at atmospheric pressure are burdened by the presence of nitrogen oxides, which on the one hand contribute to ozone generation and on the other hand are responsible for unpleasant discharge poisoning. The term discharge poisoning refers to the situation when the discharge ozone formation completely breaks down. Discharge poisoning can be affected by placing a catalyst in the discharge chamber. For the dc hollow needle to mesh corona discharge enhanced by the flow of air through the needle electrode we studied the effect of titanium dioxide TiO₂, ZSM-5 zeolite or Cu⁺⁺ZSM-5 zeolite on discharge poisoning by monitoring the ozone, nitrogen monoxide and nitrogen dioxide discharge production. We found that placing globules of any of these catalysts on the mesh decreases the energy density of the onset of discharge poisoning, and this energy density is smallest for a discharge with globules of a TiO₂ on the mesh.     

Relaxing Phenomena in Negative Corona Discharge: New Aspects

Two theories, explaining the time dependence of the negative corona discharge current in air, are confronted with new experimental results. The influence of the ozone concentration on the discharge current was experimentally confirmed in dry air and in mixtures of nitrogen with oxygen. Assuming that only the dissociative electron attachment to ozone molecule is a process being responsible for a reduction of the electron component in the total mean discharge current, the mean value of the electron attachment rate constant k = (3 - 5.5) × 10^?9 cm^?3 s^?1 was derived from the measured dependence of the discharge current on the ozone concentration. The calculated value of the rate constant k corresponds to the dissociative attachment of electron to ozone molecule via process e + O₃ → products (O^? or O^?₂ negative ions).     

Production of nitric monoxide using pulsed discharges for a medicalapplication

Nitric monoxide (NO) is widely used in medical treatment of acute respiratory distress syndrome (ARDS). The production of NO is of interest to the medical community. In the present work, NO is generated by pulsed discharges between two rod electrodes in a mixture of nitrogen and oxygen. An arc discharge having a temperature of about 10000 K was produced, which was sufficient to generate NO. Some of the important parameters affecting the production of NO have been investigated. These include the percentage of O₂ (6-94%) in the mixture of Na and O₂, the energy of the discharge (0.5-12 J/pulse), the pulse repetition rate (0.54.5 pps) and the flow rate (1.35-5.4 l/min) of the gas mixture. NO₂ produced in the discharge was successfully changed to NO using a heated molybdenum tube, NO₂ must be extracted from the gas before clinical inhalation. The concentration of ozone was completely eliminated by bubbling the gas mixture through water. A maximum of NO and a minimum of NO₂ concentrations were generated when the proportion of O₂ in the gas mixture was in the range of 20-27%. The concentrations of NO and NO₂ increased with increasing pulse repetition rate and with decreasing flow rate of the mixture. In all cases, No₂ was effectively removed using a heated molybdenum tube     

吸收光谱法测量大气压空气介质阻挡放电的臭氧浓度

臭氧作为一种强氧化剂和杀菌剂,在污染物降解,食品加工,杀菌消毒,医疗卫生等方面有着非常广泛的应用。大气压介质阻挡放电是一种极为高效的产生臭氧的方法,利用平行平板介质阻挡放电装置,采用交流高压激励,产生了大气压空气非平衡态等离子体。通过测量其电压和发光信号,发现在电压的正、负半周期均存在着许多随机的放电脉冲,并且其脉宽均在几十到几百纳秒之间,这表明其机制是流光放电。放电的光学发射谱包含氮分子的第二正带系(C 3Π-B 3Π)和第一正带系(B 3П-A 3П),氮分子离子的第一负带系(B 2Σ-X 2Σ),以及氧原子谱线(715.7和799.5 nm)。由于流光放电在紫外区域(200~300 nm)没有明显的发射谱线,但臭氧在此区域存在吸收峰,因此可以利用此区域的紫外吸收光谱测量放电产生的臭氧浓度。吸收光谱法可以有效的监测其臭氧浓度的变化情况,其优势在于操作简单,对实验环境要求低,可在放电条件下使用,并且可以连续监测臭氧浓度变化。基于此,通过Beer-Lambert定律计算了臭氧浓度随实验参数的变化,结果发现随外加电压幅值和驱动频率的增加,臭氧浓度升高。这些结果对于大气压介质阻挡放电的工业应用具有重要价值。     

Influence of gas flow rate and reactor length on NO removal usingpulsed power

A short duration of 100-ns pulsed power has been used to remove nitric oxide (NO) in a mixture of nitrogen, oxygen, water vapor, and NO, simulating flue gases from a power station. The effects of the gas flow rate, the reactor length, and the pulse repetition rate on the percentage of NO removal and its energy efficiency are reported. The percentage of NO removal at a fixed gas flow rate increased with increasing pulse repetition rate due to the increased energy into the discharge. At a fixed pulse rate, the removal of NO increased with decreasing gas flow rate due to the increased residence time of the gas in the discharge reactor, thus facilitating the creation of increased radicals of O and N which then decreased NO. The energy removal efficiency of NO (in mol/kWh) decreased with increasing gas flow rate and increasing removal ratio of NO. The removal of NO increased with increasing energy density (J/I), input into the discharge at different reactor length     

Barrier Discharge The Transferred Charge and Ozone Synthesis

We have undertaken an experimental investigation of the influence of the conditions of barrier discharge implementation such as: the discharge gap value, the type of gas, and the polarity and dielectric permittivity of the dielectric electrode on the value of charge transferred in a micro-discharge. It is shown that the increase in the specific capacitance of the electrodes leads to proportional increase in the transferred charge value, reaching 100–200 nC in a discharge gap 1 mm, in air. In this case the amplitude and duration of a current pulse in the microdischarge reach, respectively, 10 to 15 A and 40 ns. It is also demonstrated that in air with increase in the discharge gap value one can observe a decrease in the efficiency of the ozone synthesis whereas in oxygen there exists a more complicated dependence: the maximum of efficiency is observed at a discharge gap value of 0.7 to 1.0 mm.     

A compact high-voltage pulse generator using a rotary airhole sparkgap

A pulsed-corona-discharge nonthermal plasma technique offers the advantages of energy efficiency and capability for the simultaneous removal of coexisting pollutants. The key to success in a nonthermal plasma approach is to produce a corona discharge in which the bulk of the electrical energy goes into the production of energetic electrons, rather than into gas molecules heating. By driving the nonthermal plasma reactor with very short pulses of high-voltage, short-lived corona discharge plasmas are created that consist of energetic electrons, which in turn produce the radicals and ozone responsible for the decomposition of the toxic molecules. Because of the short lifetime of the pulsed corona discharge, little electrical power is dissipated in movement of heavy molecule ions in the processing region, thus avoiding heating the gas and providing good electrical energy efficiency.A high-voltage pulse generator with a rotary airhole sparkgap, instead of a conventional rotary ball sparkgap, has been proposed. Its rise and fall time characteristics to be used as a smart pulse generator for pulsed corona discharge-type nonthermal plasma reactor were investigated. Parametric studies showed that the proposed airhole sparkgap had a very fast rise time, very short fall time, very low-and-stable breakdown voltage, and reliable pulse repetition characteristics, compared with the conventional rotary ball sparkgap.     

Frequency dependence of amplifying parameters of a copper vapor laser using air as a buffer gas

We use a pair of copper vapor lasers in an oscillator–amplifier configuration to investigate amplifying parameters such as the small signal gain and the saturation intensity versus the pulse repetition frequency when two different types of buffer gases are employed. We show that the values of these parameters are not the same if different gas mixtures are used in the gain medium. We show that the values of the parameters are estimated to be higher if a He–Ne buffer gas is used than in the case of air. The laser output power is relatively high and has fairly good stability at some special pulse repetition frequencies when air is used as a buffer gas.     

Schwingungsrelaxation und Gasaufheizung bei periodischem Betrieb der Impulslaser-Gasentladung in Stickstoff

For a typical pulse laser gas discharge in nitrogen without gas streaming, the time variation of the vibration temperature between two pulses is calculated based on a generalized Treanor distribution of the vibration term population. As processes destroying vibration energy, the wall desactivation, VV- and VT-processes are considered. As results, values of the vibration temperature at the beginning of the pulse discharge and data for the gas temperature difference between tube wall and axis are presented in dependence on the repetition frequency. Finally is discussed, in which scale the elevated vibration temperature and the gas temperature may influence the power of the nitrogen pulse laser.     

Acoustic wave amplification in a plasma of a molecular gaseous discharge

From the experimental investigation of sound propagation in a gaseous discharge, it is established that, in a stationary diffusive discharge in nitrogen, sound amplification takes place according to the linear theory, whereas a sharp transverse constriction of the discharge leads to a sharp, jump-like increase in the amplification factor because of the steep dependence of the relaxation constant of a vibrationally excited molecule on the gas temperature. This result agrees well with the nonlinear theory of sound amplification in a nonequilibrium molecular gas. It is shown that the introduction of oxygen in the nitrogen discharge leads to an increase in the sound amplification factor in the plasma, because the constant of the vibrational relaxation of an excited nitrogen molecule by oxygen is two orders of magnitude greater than the constant of relaxation of nitrogen by nitrogen.     

Optical characteristics of barrier discharge plasma based on mixtures of cadmium diiodide vapors with gases

The spectral, integrated, and working life characteristics of the radiation from atmospheric pressure gas discharge plasma based on multicomponent mixtures (cadmium diiodide with helium and small admixtures of molecular nitrogen and xenon) are analyzed. A pulsed barrier discharge (pulse repetition rates 5000, 5500, and 6000 Hz; pulse duration ～150 ns) is used both to produce the gas discharge plasma and to excite the components of the working mixture. Visible radiation is detected from excimer molecules of cadmium monoiodide and cadmium, xenon, and krypton atoms. Regular features are found in the variations of optical plasma characteristics with pumping pulse repetition rate and with component and quantitative mixture composition.     

发射光谱研究氩气/空气混合气体介质阻挡放电能量传递过程

使用水电极介质阻挡放电装置,对比氩气与氩气/少量空气的混合气体以及空气与空气/少量氩气的混合气体放电的发射光谱,研究了氩气与空气相混合时气体放电中的能量传递过程。实验发现, 当氩气中加入少量的空气时,氩原子谱线均变弱,说明空气中的氮分子对氩原子的各激发态具有猝灭作用。并且随着空气含量的增加,各谱线变弱的速率不同。越是与氮分子的激发电位接近的氩原子的激发态被猝灭的作用越明显。另一方面,当空气中加入少量氩气时,发现氮分子第二正带系和氮分子离子第一负带系的谱线均被增强。说明在空气/少量氩气放电中,氮分子的激发由于亚稳态氩原子的潘宁激发传能而增强。因此在氩气/空气混合气体放电中,气体成分及比例影响放电的发光特性和能量传输特性。     

Macrokinetic study on ozone boundary concentration. Effect of temperature

The process of ozone production in pure oxygen using the tubular, high voltage pulse supplied ozonizer was studied. The unusual methodology of conducting kinetics measurements of the ozone synthesis process was presented. It was shown how the process rate changes along the discharge gap. The effect of power density and gas residence time in the discharge gap on the process rate was analysed. The temperature influence on the course of the process, particularly on the ozone boundary concentration and ozone decomposition rate constant, was discussed.     

Ozone production in supersonic nozzles

Ozone production during the efflux of oxygen from a supersonic nozzle is studied theoretically and experimentally. The formation kinetics of atomic oxygen in an electrical discharge in the nozzle is analyzed. An experiment is set up using an optimized nozzle with an electrical discharge in its supersonic section. It is shown that the highest ozone content at the nozzle output is attained when the excess concentration of oxygen atoms is produced in the supersonic section of the nozzle. Zh. Tekh. Fiz. 68, 20–26 (November 1998)     

Compact 1000 pps high-voltage nanosecond pulse generator

A compact high-voltage nanosecond generator is described with pulse repetition rate of up to 1000 pps. The generator includes a 30-Ω coaxial forming line charged by a built-in Tesla transformer with high coupling coefficient, and a high voltage (N₂) gas gap switch with gas circulating between the electrodes. The maximum forming line charge voltage is 450 kV, the pulse duration is ~4 ns, and its amplitude for a matched load is up to 200 kV. The generator has been applied to create powerful sources of ultrawide-band electromagnetic radiation and nanosecond microwave pulses