Electron emission from sharp points in liquid argon

Summary Electron charge injection from a point electrode in gaseous and in liquid argon has been observed and its characteristics studied. In liquid argon, as we have observed, the charge dependence on applied voltage is governed by space charge limitation. The emission was found to take place (with the electrode in liquid phase) not continuously but as a pulsed injection. The pulsing rate has been studied as a function of the voltage, time and electrode separation. The measurements done so far have been performed in a closed environment with an impurity content of argon of the order of some tenth of p.p.b. (oxygen, CO₂, etc.). Possible applications in liquid-argon purification and particle detection is presented. The authors of this paper have agreed to not receive the proofs for correction.     

大气压脉冲调制射频氩气放电等离子体特性的数值研究

基于等离子体流体理论,建立了大气压脉冲调制射频氩气放电的一维流体模型。通过数值模拟的方 法研究了放电参数(放电电极间距、射频频率)对氩等离子体特性的影响。研究结果表明：当电压固定时,随着电 极间距的增加,等离子体区逐渐增大,最大电子密度也增加,在 0.20cm 达到最大值后略有降低;放电电流密度 与输入功率密度随着电极间距的增加而增加;鞘层区电子温度随着电极间距的增加而降低;在脉冲开启前期,等 离子体区电子温度随着电极间距的增加而增加,但当脉冲开启后期,电极间距对等离子体区电子温度影响较小。 不同射频频率下最大电子密度随电极间隙的增加而减小,也具有一个最优值。      

Townsend coefficient,escape curve,and efficiency of runaway-electron beam formation in argon

The ionization and drift characteristics of electrons in argon are simulated by the method of multi-particle dynamics. It is shown that, in argon (as well as in other gases studied earlier), the Townsend regime of ionization sets in even in strong fields if the electrode distance is much larger than the reciprocal Townsend coefficient. The dependences of the basic ionization and drift characteristics on the reduced field intensity are obtained, and an escape curve is constructed separating the region of effective electron multiplication from the region where the electrons leave the discharge gap having no time to multiply. The formation efficiency of a runaway-electron beam is calculated. It is shown that the dependence of the electrode voltage generating a given fraction of runaway electrons on the product of the pressure by the electrode distance has a form that qualitatively agrees with the runaway curve. When the efficiency is not too high (≤20%), the runaway curves virtually coincide with isoefficiency curves.     

介质阻挡放电中超四边斑图沿面放电的光谱研究

利用水电极介质阻挡放电装置,在氩气和空气的混合气体中,首次观察到了超四边斑图沿面放电,它是由中心点和暗点组成的。通过观察普通相机的斑图照片,可以发现中心点位于周围四个暗点的中心处。利用高速录像机对斑图进行短曝光拍摄,观察发现中心点对应体放电,暗点对应沿面放电,暗点由这些沿面放电形成。中心点和暗点的亮度有所不同,这说明中心点和暗点的等离子体状态可能不同。采用发射光谱法,研究了超四边斑图沿面放电的的中心点和暗点的等离子体参量随氩气含量的变化趋势。利用氮分子第二正带系(C3Π_u→B3Π_g)发射谱线,计算得出了中心点和暗点的分子振动温度; 然后通过氩原子696.57 nm (2P₂→1S₅)谱线的展宽,研究了中心点和暗点的电子密度。实验结果表明： 在相同氩气含量下,暗点的分子振动温度和电子密度均高于中心点的相应等离子体参量; 在其他实验条件不变的情况下,随着氩气含量从90%增大到99.9%,中心点和暗点的分子振动温度和电子密度均逐渐增大。结果表明中心点和暗点的等离子状态不同,说明二者的放电机制可能不同。     

Long‐Time Evolution of a Low Pressure Laboratory Plasma after Application of Transient high Voltage Positive Pulses

The long‐time evolution of weakly‐collisional plasma with application of high voltage positive pulses to an electrode immersed in plasma, with pulse widths less than as well as more than ion plasma periods, is studied. The plasma is produced by electron impact ionization of argon or helium gas, where electrons are coming out from dc biased hot thoriated tungsten filaments. It is observed that during the temporal evolution of argon plasma, a beam component exists along with temporal bulk electrons giving rise to a double hump profile of transient Electron Distribution Function (EDF). However, in the case of temporal evolution of helium plasma, only a bulk electron population is present. The obtained results are explained by understanding the role played by thermionically emitted electrons during the plasma evolution, the role of the difference of ionization rates of helium and argon, and the higher temporal plasma potential. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Breakdown mechanism of an atmospheric-pressure radio-frequency capacitive argon discharge in parallel-plate bare metal electrodes

The breakdown behavior of an atmospheric-pressure radio-frequency capacitively coupled argon plasma discharge at 13.56 MHz is investigated in order to produce a stable and homogeneous atmospheric-pressure argon glow discharge. It is found that the electrode separation distance plays a pivotal role in exciting argon discharge whether in mobility-controlled electric breakdown or in diffusion-controlled breakdown and the discharge mode (arc discharge, α-mode glow discharge, or γ-mode glow discharge) immediately following breakdown is closely related to the breakdown mechanism.     

Inter‐Electrode Distance and Breakdown Voltage in Low Pressure Argon Discharges

This work deals with the Paschen law in electrical breakdown of gases at pd values around the Paschen minimum. From the Townsend model, it is possible to deduce theoretical forms of the coefficients in Paschen's law, of which our calculated values are in the range of the tabulated values from the literature. These formulae show that the breakdown voltage must be influenced by the inter‐electrode distance, while the product pd remains a key parameter. This is confirmed by the Paschen curves measured in an argon discharge for inter‐electrode distances varying from 2 to 9 cm (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the increase in the limiting current of an atmospheric-pressure glow discharge in an argon flow

The initiation of an atmospheric-pressure glow discharge (APGD) is studied in the multitip cathode-planar anode electrode system through which an argon flow passes. It is shown that sectioning of the cathode and ballast resistances present at corona tips make it possible to substantially expand the current region of the discharge and considerably raise the limiting current of the APGD. The shape of the coronafree electrode is found to influence the limiting discharge current.     

KLY—1低能宽束离子源的研制

本文简介了KLY-1低能宽束离子源的结构,给出了物理设计的主要公式和调试实验的结果。对于氩离子束,在离子能量为4keV,距引出电极24cm处的5cm直径的靶上,测得最大氩离子流为45mA,束流不均匀性优于±25％。     

The effects of electrostatic, molecular drag and gravitationalforces on the behavior of particle clouds in an RF discharge

A Gaseous Electronics Conference (GEC) Reference Cell was used to investigate the behavior of graphite particles sputtered in an argon plasma. By configuring the reactor in a normal (wafer facing upward) and in an inverted (wafer facing downward) configuration, and by varying the direction and flow of the argon, the relative importance of gravity, molecular drag and electrostatic trapping could be observed. It was observed that particle clouds in specific locations adjacent to the wafer (referred to as particle traps) were identical in both configurations. Drag forces from the showerhead grounded electrode were sufficient to prevent particles from accumulating in the traps over the wafer. Drag forces from flow parallel to the wafer (⩾25 cm/s) easily emptied the traps. Gas flow from a ring source mounted above a downward facing driven electrode were observed to affect the manner in which particles were removed from the ring-shaped trap surrounding the cathode. In that case, particles were observed to flow down eight fluid streamlines toward the eight openings through which the gas flowed from the reaction chamber to the pump. Comparison of results for the normal and inverted configurations also permitted estimates of the number of electrons attached to each particle     

Variation of focus action with electrode length for an innerelectrode of negative polarity

A 2.3 kJ plasma focus (PF) device was used to investigate the variation of focus action with electrode length using argon, air, and nitrogen as filling gases. It was observed that for an inner electrode (IE) of negative polarity, the focus action improved tremendously as the electrode lengths were reduced from 80 to 40 mm. The high voltage (HV) pulse width also decreased with the decrease in electrode lengths. Third, the best focus peak of the HV pulse was shifted toward higher focus pressures, and the range for good focus action was broader. There is a tendency toward improvement of focus action toward significantly shorter IE lengths (about 40 mm) for negative polarity as compared to positive polarity, where the optimum IE length is 150 mm. This suggests that plasma contamination due to ablated IE material is minimized with shorter electrode length for negative IE. This could provide part of the solution to the polarity riddle of PF devices in relation to neutron production     

氩气/空气等离子体助燃激励器的实验研究

为了获得氩气/空气等离子体助燃激励器的特性,首先在空气中加入少量氩气的条件下,对条状、王状和网状三种不同电极形状的等离子体助燃激励器的放电特性进行对比,实验结果表明电极形状对激励器放电特性影响不大。然后对条形电极在纯空气和10%氩气/90%空气两种条件下的放电特性和发射光谱进行研究,发现加入氩气后,放电参数变化趋势与纯空气相似,但电流脉冲增多,放电均匀度增加,起始放电电压由27kV降低到24kV,并且介质阻挡放电发射光谱增强。     

Current-voltage characteristics of a non-transferred plasma spray torch

I–V characteristics of a non-transferred DC plasma spray torch operating on argon and argon + nitrogen mixtures are reported. Arc voltage is decreased with increase in arc current and increased with increase in electrode gap. Arc power is higher at higher percentage of nitrogen in argon. Nottingham co-efficients were calculated using numerical method.     

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...     

RF discharges at atmospheric pressure

The described plasma source is based on the RF torch discharge. The powered RF electrode of the torch discharge plasma source is made from the thin metal pipe with an inner diameter of 1–2 mm and with a length of several cm. The working gas (argon pure or with an admixture of reactive components) flows through the RF electrode as the nozzle. The electrode is connected through the matching unit to the RF generator of the frequency of 13.56 MHz.The advantage of this described plasma source consists in the fact that the torch discharge remains stable up to the atmospherical pressure of the working gas even in the liquid environments. Up to now the torch discharge has been used only for treatment in liquid environment only for archaeological artefacts. For further applications it is necessary to additional study of this phenomenon. While in previous papers we presented a measurement of different temperatures from spectra emitted by plasma, there is drawn attention to equiintensity maps inside the nozzle.     

双频容性耦合等离子体相分辨发射光谱诊断

采用相分辨发射光谱法, 对双频容性耦合纯Ar和不同含O₂量的Ar-O₂混合气体放电等离子体的鞘层激发模式进行了探究. 在射频耦合电源上极板的鞘层区域处观察到两种电子激发模式: 鞘层扩张引起的电子碰撞激发模式和二次电子引起的电子碰撞激发模式; 并发现这两种激发模式均受到低频射频电源周期的调制. 在纯Ar放电等离子体中, 两种激发模式的激发轮廓相似; 而在Ar-O₂混合气放电等离子体中, 随着含O₂量的增加, 二次电子的激发轮廓变弱. 此外, 利用相分辨发射光谱法对不同含O₂量的Ar-O₂混合气放电下Ar的 750.4 nm谱线的平均低频电源周期轴向分布进行了研究, 得到了距耦合电源上极板约3.8 mm处为双频容性耦合射频等离子体的鞘层边界. 关键词： 双频容性耦合等离子体 等离子体鞘层 发射光谱     

Effect of CO2 shielding gas on metal droplet formationin arc welding

We have developed a unified arc electrode model that enables us to make predictions of the time development of molten drops from the welding wire in gas metal arc welding. The wire is taken as the positive electrode, and the effects of surface tension, magnetic pinch forces, and convection within the drop are taken into account to predict drop detachment for any given arc current. For pure argon, we have previously predicted the sharp transition that is observed experimentally at about 300 A between globular transfer at low current, when drop diameters are larger than the wire diameter, and spray transfer, for currents above 300 A, when drop diameters are smaller than the wire diameter. In this paper, we predict that addition of 25% of CO₂ to the argon leads to an increase in the transition current to more than 325 A, also in agreement with published experimental results. For pure CO₂, we find a significantly different drop behavior due to the more constricted arc. Both small and large drops are produced, with many very small drops being produced successively between each large drop     

Equilibrium characteristics of strong ion sheaths in a partiallyionized plasma

Experiments have been carried out to study the equilibrium characteristics of strong ion sheaths (eΦ/T_e≫1) in a partially ionized argon plasma. An improved method of potential measurement in the sheath is described. The potential profile shows deviations from Child's law and the reduction of sheath size with pressure. Energy analysis of ions reaching the negative biased electrode show that the ions are not monoenergetic but have a spread in energies which depends on neutral pressure     

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。     

Prediction of gas tungsten arc welding properties in mixtures ofargon and hydrogen

A theory of gas tungsten arc welding (GTAW) arcs that treats the tungsten electrode, the arc, and the workpiece as a unified system has been applied to make predictions in two dimensions of the temperature distributions in the arc, the tungsten cathode, and the workpiece, for any given arc current and gas mixture. Predictions of arc temperatures, radii, and voltages are compared for argon and mixtures of argon and hydrogen. It is found that arcs in gas mixtures containing hydrogen are more constricted and have a higher maximum temperature and arc voltage than arcs in pure argon. The addition of hydrogen also significantly increases the volume of molten material in the weld pool due to the higher thermal conductivity of argon-hydrogen mixtures at temperatures at which molecules of hydrogen dissociate. Predictions are also compared for workpieces of steel and aluminum. The volume of molten material is very much less for aluminum, despite its lower melting point, because of the higher thermal conductivity of aluminum. Predicted arc voltages as a function of current for a mixture of 10% hydrogen in argon are in good agreement with experimental results