Efficiency of arc production of fullerenes versus discharge chamber geometry. II: Double-sided gas delivery and 3D geometry

Based on the model suggested in part I of this work and analysis of a chamber for arc production of fullerenes, two ways of gas delivery to the chamber, one-sided pumping of the gas (from the side of the anode) and double-sided pumping (from both the cathodic and anodic flanges), are considered. In the latter case, the efficiency of fullerene extraction from the discharge chamber rises substantially. The axisymmetric 2D case considered earlier is extended for the 3D geometry of the chamber, where gas outlet is accomplished through the side wall of the chamber.     

Rise of the fullerene yield in an arc discharge under the action of gas flow in a hollow electrode

When a turbulent gas flow and a hollow electrode are combined in a discharge chamber, a synergetic effect is observed: the weight of the deposit decreases by five to six times and fullerene-containing soot dissolves completely (without precipitation) in benzene. The mass spectra of the synthesis products contain the lines of C₆₀, C₇₀, and C_{74 + 2n} (n = 0–40) fullerenes. This indicates a 100% yield of the fullerene mixture. As a result, the output of the fullerene synthesis process increases by a factor of more than 10.     

Analysis of a hollow-cathode reflex discharge in a hydrocarbon flow

Variation of parameters of a reflex discharge with a hollow cathode operating continuously in propane with a flow rate of 1.3–5.6 (m³ mPa)/s and a discharge current of 0.1–0.4 A is analyzed. It is shown that for a hydrocarbon flow rate of 2.4 (m³ mPa)/s and higher, an increase in the discharge voltage takes place after a time interval depending on the discharge current and gas pressure; this is explained by the formation of coating of the dissociation products of hydrocarbon molecules on the electrodes of the discharge chamber. An increase in the thickness of the carbon coating of the cathodes with time and their charging with ions lead to electric breakdown of coatings and the formation of cathode spots. The oscillograms of the discharge current and voltage indicate a short-term transformation of the glow discharge into the arc discharge. The energy spectra of ions emerging from the discharge are measured, and the effect of the discharge current and the gas flow rate on the energy spread of ions is analyzed. The operation time of the discharge in hydrocarbon after which the cleaning of the discharge chamber is required is determined. The possibility of using an ion source based on the reflex discharge with a hollow cathode for technological purposes is established.     

离子推力器放电腔数值模拟

 为更好地理解放电腔内等离子体物理机制,对Kaufman型离子推力器放电腔进行了数值研究,其中初始电子采用粒子模拟的方法处理,二次电子和离子采用漂移-扩散流体近似描述。模拟结果与已有实验测量数据进行对比表明：所采用计算方法适用于放电腔内等离子体流动规律的数值研究;模拟得到的稳态下等离子体分布及变化规律与实验测量数据相吻合;磁场的设计对初始电子起到显著的约束作用,有效地提高了其与工质气体的电离碰撞几率;二次电子的精确描述还需在流体方程中耦合磁场效应。     

Cover Picture: Contrib. Plasma Phys. 9/2019

Typical discharge images of multi‐arcs in the arc chamber (for every arc, gas flow rate: 10 slm, discharge power: 450 W). Note that timing starts from forming three independent arcs in the arc chamber. Fig. 3 of the paper by Q.F. Lin et al.     

Gas flow characteristics of argon inductively coupled plasma and advections of plasma species under incompressible and compressible flows

In this work, incompressible and compressible flows of background gas are characterized in argon inductively coupled plasma by using a fluid model, and the respective influence of the two flows on the plasma properties is specified. In the incompressible flow, only the velocity variable is calculated, while in the compressible flow, both the velocity and density variables are calculated. The compressible flow is more realistic; nevertheless, a comparison of the two types of flow is convenient for people to investigate the respective role of velocity and density variables. The peripheral symmetric profile of metastable density near the chamber sidewall is broken in the incompressible flow. At the compressible flow, the electron density increases and the electron temperature decreases. Meanwhile, the metastable density peak shifts to the dielectric window from the discharge center, besides for the peripheral density profile distortion, similar to the incompressible flow.The velocity profile at incompressible flow is not altered when changing the inlet velocity, whereas clear peak shift of velocity profile from the inlet to the outlet at compressible flow is observed as increasing the gas flow rate. The shift of velocity peak is more obvious at low pressures for it is easy to compress the rarefied gas. The velocity profile variations at compressible flow show people the concrete residing processes of background molecule and plasma species in the chamber at different flow rates. Of more significance is it implied that in the usual linear method that people use to calculate the residence time, one important parameter in the gas flow dynamics, needs to be rectified. The spatial profile of pressure simulated exhibits obvious spatial gradient. This is helpful for experimentalists to understand their gas pressure measurements that are always taken at the chamber outlet. At the end, the work specification and limitations are listed.     

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设计建立了同轴圆柱介质阻挡放电线状射流装置,并利用其实现了宽度为50mm的大尺度线状射流,并对射流长度随放电参数的变化进行了研究.研究发现,随气压、流量的增加,射流长度呈先增加后达到饱和;随激励电压的增加,射流长度增加.对氮气射流等离子体进行发射光谱诊断,表明氮气等离子体中主要是氮分子和少量氮原子,并利用光谱拟合得出射流的温度范围为290～350K.     

Effect of gas flows on fullerene formation process

The effect of helium flow on the fullerene formation process is studied. It has been shown that an annular flow of helium directed into the fullerene formation zone along the discharge axis reduces the content of fullerenes in soot. If the velocity of the additional flow of helium pumped through a hole in the cathode becomes close to that of the gas-plasma jet, practically no fullerenes are formed.     

Algorithms for optical control of reactive magnetron deposition of film coatings

Algorithms for optical control of a reactive gas flow rate are considered for processes of magnetron deposition of film coatings. The algorithms are based on registration of spectral elements (lines, bands) of a cathode material and reactive/inert gases in magnetron discharge plasma spectra. The influence of instabilities in the magnetron discharge power and vacuum chamber pressure on the composition of the deposited flow and gaseous medium was studied for titanium oxide and nitride deposition that were carried out using various algorithms for optical control of the reactive gas flow rate.     

20cm离子推力器放电室流场计算模拟

为了优化20cm离子推力器放电室内部推进剂供气方式,研究了在不发生气体放电时,推力器阳极和主阴极供气接口处的流体速度和压强,并以此开展推力器放电室内部的流场计算。结果表明:采用单阳极供气管方式,阳极出口处压强为4~158Pa,气流出口速度为0.1~47m/s;阴极小孔出口处的压强约为33.1Pa,出口速度约为12m/s;考虑真空系统的返流作用时,单阳极供气管方式下放电室内部压强为0.001~0.4Pa,大部分区域Xe原子数密度为(0.2~3)×1018/m3,在靠近栅极的部分区域数密度达到9×1019/m3左右;在增加阳极组件的供气管数量后,阳极的气体出口速度为18~40m/s,放电室压强为0.03~0.1Pa,大部分区域Xe原子数密度为(0.72~2.4)×1019/m3,靠近阳极与主阴极进气端的小部分区域原子数密度约2×1017/m3,且放电室内部原子密度整体分布较为均匀。     

Simple Physical Model of Generation of the Low-Pressure Radio Frequency Supersonic Plasma Jet

The paper presents a simple theoretical model of the breakdown of the supersonic plasma jet generated by the hollow cathode discharge inside the nozzle in the low pressure RF plasma-chemical reactor. Through the nozzle which is drilled in the RF electrode the working gas flows to the reactor chamber. If at the outlet of the nozzle the gas flow is supersonic the well defined plasma jet is created inside the reactor chamber. The results of our model are in qualitative agreement with experimental data.     

Numerical investigation of the plasma processes for propellant heating in electrothermal plasma thruster for nanosatellites

Numerical investigation of the plasma processes in a cylindrical chamber with small dimensions of a novel microwave electrothermal plasma thruster for nanosatellites has been conducted. The absorbed microwave power from the electrons in the plasma column of the surface wave discharge is included in the computational model as a heat source with Gaussian distribution. The computational model takes into account the elastic and inelastic collisions of the electrons with the atoms in the ground state and two excited states (−s, −p) and the processes of recombination and deactivation of the plasma species in the volume and on the walls of the chamber. The computational model includes the flow of neutral gas and the processes in the plasma for effective heating of neutral particles by collisions not only with electrons but also with ions. Selected combinations of input power and propellant mass flow rates are used as initial parameters for the numerical investigation. The results show that at higher mass flow rates the heating of the neutral gas is more effective and at power levels of 4 W and propellant mass flow rate of 3 mg/s the electrothermal plasma thruster demonstrates effective performance and thrust levels in the order of 1 mN.     

Dissociation of molecular iodine in RF discharge for oxygen-iodine lasers

The dissociation of molecular iodine in 40 MHz-RF discharge was studied experimentally. This generation of atomic iodine is aimed at use in oxygen-iodine lasers. The discharge was ignited in a mixture of I₂ + buffer gas fast-flowing through the cylindrical chamber and the discharge products were injected into a supersonic flow of nitrogen. The atomic iodine number density was measured in a low-pressure cavity after mixing with nitrogen and the dissociation fraction was calculated related to the input I₂ flow rate. The dissociation fraction of 46.2% was achieved at 0.22 mmol/s of I₂ and 7 mmol/s of Ar and RF power of 500 W. Argon and helium were used as a buffer gas; discharge stability and dissociation efficiency were better with argon. At the I₂ flow rate corresponding to the operation of a 1 kW chemical oxygen-iodine laser, the dissociation fraction was about 20%. The dissociation efficiency (the fraction of absorbed energy used for the dissociation) significantly decreased with increasing in the specific energy. At a reasonable I₂ flow rate (0.32 mmol/s), the maximum achieved efficiency was 8.5% and the corresponding energy cost was 8.9 eV per dissociating of one I₂ molecule. The input energy of more than 3 kJ per 1 mmol of I₂ is needed for dissociating at least 50% of I₂. The obtained dependencies on the gas flow rates infer a good chance for scaling-up of the tested RF discharge generator for the intended application.     

Arc discharge with a vaporizable anode: Why is the fullerene formation process affected by the kind of buffer gas?

A model for calculating an arc discharge with the vaporizable graphite anode used in production of fullerenes has been proposed. Calculations with the use of this model give a clue to the dependence of fullerene formation efficiency on the kind of buffer gas used.     

Two types of spherical stratified gas discharge

Results on experimental investigation of a volume low-pressure gas discharge between the walls of a metal chamber and the small anode in the center of the chamber are represented. The discharge current-voltage characteristics in the range of currents corresponding to Townsend and glow discharges are measured. The experiments were carried out in nitrogen, argon, helium, carbon dioxide, and argon-acetone mixture. Hysteresis of current voltage characteristics is observed for some gas media in the link area of Townsend and glow discharges. It is found that the Townsend spherical discharge plasma is always stratified. Contrary to the glow discharge, stratification in this case is observed in all gases and remains for an arbitrary length of time.     

Review of relaxation oscillations in plasma processing discharges

Relaxation oscillations due to plasma instabilities at frequencies ranging from a few Hz to tens of kHz have been observed in various types of plasma processing discharges. Relaxation oscillations have been observed in electropositive capacitive discharges between a powered anode and a metallic chamber whose periphery is grounded through a slot with dielectric spacers. The oscillations of time-varying optical emission from the main discharge chamber show, for example, a high-frequency (\sim 40~kHz) relaxation oscillation at 13.33Pa, with an absorbed power being nearly the peripheral breakdown power, and a low-frequency ( \sim 3 Hz) oscillation, with an even higher absorbed power. The high-frequency oscillation is found to ignite plasma in the slot, but usually not in the peripheral chamber. The kilohertz oscillations are modelled using an electromagnetic model of the slot impedance, coupled to a circuit analysis of the system including the matching network. The model results are in general agreement with the experimental observations, and indicate a variety of behaviours dependent on the matching conditions. In low-pressure inductive discharges, oscillations appear in the transition between low-density capacitively driven and high-density inductively driven discharges when attaching gases such as SF₆ and Ar/SF₆ mixtures are used. Oscillations of charged particles, plasma potential, and light, at frequencies ranging from a few Hz to tens of kHz, are seen for gas pressures between 0.133 Pa and 13.33 Pa and discharge powers in a range of 75--1200 W. The region of instability increases as the plasma becomes more electronegative, and the frequency of plasma oscillation increases as the power, pressure, and gas flow rate increase. A volume-averaged (global) model of the kilohertz instability has been developed; the results obtained from the model agree well with the experimental observations.     

中性束在束线中的透射

本文给出TEXTOR束线中放电室送气量和气压、中性化室送气量和气压、中性化室气体靶厚、中性化转换效率、再电离、热负荷、注入TEXTOR功率和粒子数的计算和实验结果。     

Influence of the current magnitude and working gas temperature on electrode erosion in the discharge chambers of high-power pulsed plasmatrons

Results are presented from experimental studies of the erosion of different electrode materials in the discharge chambers of high-power pulsed plasmatrons—the main component of light-gas electric-discharge accelerators of solid bodies. The specific erosion as a function of both the discharge current and the mass-average temperature of the working gas (hydrogen) is determined. The experiments were carried out at discharge currents of 300–1400 kA, average gas temperatures of 700–4500 K, electric charges of 70–700 C, initial hydrogen pressures of up to 42 MPa, and pulsed gas pressures in the chamber of 100–520 MPa.     

基于FLUENT的动态高压微射流内部孔道流场的数值模拟

 以动态高压微射流振荡反应腔内部孔道流场为研究对象,建立了反应腔内部孔道的几何模型和网格模型,选择SIMPLEC算法和RNG k-ε模型,运用FLUENT软件对流场进行数值模拟,以揭示流场内各位置上的静压和速度分布。计算结果表明：高速射流撞击增加了流场内的撞击作用力,流速的急剧增加使速度梯度迅速加大,剪切应力极大增强,保证了微射流均质机极佳的作用效果;由于反应腔内部孔道流场的静压与速度变化相反,静压的急剧变化使空穴效应及压力释放效应极大增强,空穴作用力得以强化,对微射流均质机反应腔内的材料产生“腐蚀”作用。设计反应腔内部孔道时,在保证撞击区速度的前提下,应适当减小进料速度和分流管进口拐角角度,并选用较短的出料管。     

Cathode erosion rate in high-pressure arcs: influence of swirling gas flow

Cathode erosion in a 400-A arc burning in oxygen at 4-atm pressure was measured at different gas dynamic conditions near the cathode. The flow pattern near the cathode was changed by varying the swirl of the gas entering the discharge chamber. Experiments showed that the erosion rate depends on the swirl in a very strong way. Model calculations of velocity distribution showed that the gas velocity near the cathode increases as the swirl intensifies. According to our model, it leads to an increase of the net evaporation rate (evaporation rate minus rate of previously evaporated particles returning back to the cathode).