Generation of homogeneous plasma in a low-pressure glow discharge

The possibility of producing homogeneous plasma in a low-pressure discharge with the use of a hollow anode or hollow cathode is analyzed. It is shown that, in contrast to the high-pressure discharge, where uniform ionization is needed to produce homogeneous plasma, in the low-pressure discharge, nearly uniform radial distribution of the plasma parameters can be achieved under nonuniform ionization conditions by increasing the ionization probability at the system periphery and reducing it near the system axis. It is shown that the magnetic field can facilitate generation of the homogeneous plasma instead of interfering with it.     

脉冲金属离子等离子体推进器的等离子体生成和传播特性

综述了不同阳极结构脉冲金属离子等离子体推进器的放电特性、等离子体生成及传播特性。首先,讨论了一种带有绝缘套筒的裸阳极推进器结构。对比分析了无、有绝缘套筒的裸阳极推进器的等离子体生成及传播特性的区别。结果表明,绝缘套筒阻碍了阴极近旁带电粒子的径向运动,提高了沿绝缘套筒轴向喷射出去的等离子体的喷射性能。此外,发现采用裸阳极推进器结构放电过程中会有大量带电粒子进入阳极。其次,讨论了一种绝缘阳极推进器结构。结果表明,采用绝缘阳极结构进一步提高了沿绝缘套筒轴向喷射出去的等离子体密度。但是,与裸阳极推进器结构相比,等离子体的生成量减少。再次,讨论了一种微孔绝缘阳极推进器结构。结果表明,与裸阳极推进器结构相比,采用微孔绝缘阳极推进器结构生成的等离子体的密度峰值和传播速度峰值分别提高了12.6倍、3.9倍。最后,分别讨论了一种螺旋阳极推进器结构和一种多阳极推进器结构。结果表明,这两种推进器结构分别利用放电过程中形成的自磁场及电场有效提高了等离子体羽流的定向喷射性能。本研究可以为金属等离子体喷射性能的提高以及脉冲金属离子等离子体推进器的设计提供支持。     

Formation of an out-of-electrode plasma in a high-voltage gas discharge

The formation of an out-of-electrode plasma in a high-voltage gas discharge is studied. The occurrence and self-maintenance of a gas discharge and its associated plasma fluxes on the straight portions of electrical field lines are predicted theoretically and confirmed experimentally. It is shown that the focusing of the gas discharge and plasma fluxes is provided by increasing the length of the field line straight portions toward the symmetry axis of a hole in the anode. It is found that, when the discharge power (more specifically, an accelerating voltage applied to the electrodes of the gas-discharge tube) rises, the straight portions of the field lines elongate and concentrate near the symmetry axis of the hole in the anode. Recommendations are given on using the out-of-electrode plasma in surface micro- and nanostructuring.     

Operational characteristics of a plasma cathode with a grid stabilization in a two-stage ion source

The discharge characteristics and the parameters of the cathode plasma in a two-stage ion source with a grid plasma cathode and a magnetic trap in the anode region are investigated. It is shown that an increase in the gas pressure and the accompanying increase in the reverse ion current in the bipolar diode between the cathode and anode plasmas lead to an increase in the cathode plasma potential and a transition of the cathode into the regime of electron emission from the open plasma boundary. The dependence of the ion current extracted from the anode plasma on the area of the exit aperture of the hollow cathode and the mesh size of the grid plasma cathode is explained. The conditions at which the ion emission current from the anode plasma is maximum are determined. The potential difference at the bipolar diode is measured by using the probe method. It is shown that, when the gas pressures reaches a critical value determined by the mesh size of the grid plasma cathode, the discharge passes into a contracted operating mode, in which the ion current extracted from the anode plasma decreases severalfold.     

金属目标表面气体放电单元放电过程的PIC-MCC模拟

 对目标表面的浮地导体边界的存在及其对单元气体放电过程所产生的影响进行了研究,针对导体边界条件,应用高斯定理和电荷守恒定律推导出这种边界条件的数值处理方法,得到了有界等离子体空间电势的数值分布。电场的数值计算表明,浮地导体的电势随着内部场的变化而变化,其大小介于两个电极的电势之间,对放电区域的电场分布产生较大的影响。 对金属目标表面放电单元的放电过程的PIC-MCC模拟结果表明, 浮地导体的存在能够改变放电空间的电场结构,形成不均匀场,有利于气体的电离和等离子体区域的形成,同时将使虚阳极所形成的电势平台在边界附近下陷,导致等离子体壳层的厚度变小。     

大气压管板结构纳秒脉冲放电中时域X射线研究

纳秒脉冲放电能在大气压下产生高电子能量、高功率密度的低温等离子体,由于经典放电理论无法很好地解释纳秒脉冲放电中的现象,近年来以高能逃逸电子为基础的纳秒脉冲气体放电理论受到广泛关注.纳秒脉冲放电会产生高能逃逸电子,伴随产生X射线,研究X射线的特性可以间接反映高能逃逸电子的特性.本文利用纳秒脉冲电源在大气压下激励空气放电,通过金刚石光导探测器测量放电产生的X射线,研究不同电极间隙、阳极厚度下和空间不同位置测量的X射线特性.实验结果表明,在大气压下纳秒脉冲放电能产生上升沿约1 ns,脉宽约2 ns的X射线脉冲,其产生时间与纳秒脉冲电压峰值对应,经计算探测到的X射线能量约为2.3×10-3J.当增大电极间隙时,探测到的X射线能量减弱,因为增大电极间隙会减小电场强度和逃逸电子数,从而减少阳极的轫致辐射.电极间距大于50 mm后加速减弱,同时放电模式从弥散过渡到电晕.随着阳极厚度增加,阳极后方和放电腔侧面观察窗测得的X射线能量均有所减弱,在阳极后面探测的X射线能量减弱趋势更加明显,这说明X射线主要产生在阳极内表面,因此增加阳极厚度会使穿透阳极薄膜的X射线能量减少.     

介质阻挡放电点燃大气压直流均匀放电的光谱研究

由于大气压均匀放电等离子体在工业领域具有广泛的应用前景,为了获得大尺寸的大气压均匀等离子体,采用氩气作为工作气体,在大气压空气环境中利用同轴介质阻挡放电点燃了针-板电极间的大气隙(气隙宽度达到5 cm)直流均匀放电。研究发现,同轴介质阻挡放电能够有效降低针-板电极间的击穿电压。该均匀放电由等离子体柱、等离子体羽、阴极暗区和阴极辉区组成。其中等离子体柱和阴极辉区都是连续放电。而等离子体羽不同位置的放电是不同时的。事实上,等离子体羽放电是由从阴极向着等离子体柱移动的发光光层(即等离子体子弹)叠加而成。利用电学方法测量了放电的伏安特性曲线,发现其与低气压正常辉光放电类似,均具有负斜率。采集了放电的发射光谱,发现存在N₂第二正带系、氩原子和氧原子谱线。通过Boltzmann plot方法对放电等离子体电子激发温度进行了空间分辨测量,发现等离子体柱的电子激发温度比等离子体羽的电子激发温度低。通过分析放电机制,对以上现象进行了定性解释。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。     

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)     

Levitation of charged macroparticles in the anode region of a glow discharge

Levitation of dust particles in the anode region of a dc glow discharge was observed for the first time. A dust cloud of several tens of particles formed at a distance of several millimeters above the central part of the anode. When the discharge parameters were varied, the shape of the cloud and its position above the anode varied. An analysis of the experimental conditions revealed that these particles are positively charged in contrast to other experiments on the levitation of dust particles in a gas-discharge plasma. An estimate of the particle charge taking into account processes of electron emission from their surface is consistent with results of measurements of the electric field strength.     

Structure of an extended plasma jet in a vacuum arc in an axial magnetic field

The motion, heating, and ionization of a plasma in a ring anode vacuum arc in an axial magnetic field are studied using a quasi-one-dimensional MHD model. The region between the cathode and anode (a current-carrying plasma jet), as well as the region behind the anode (a current-free plasma jet), is considered. It is shown that, over a long portion of a current-free plasma jet, the electron density and temperature remain high and the ion charge increases substantially due to electron-impact ionization.     

Characteristics of a large gap uniform discharge excited by DC voltage at atmospheric pressure

A large-gap uniform discharge is ignited by a coaxial dielectric barrier discharge and burns between a needle anode and a plate cathode under a low sustaining voltage by feeding with flowing argon. The basic aspects of the large-gap uniform discharge are investigated by optical and spectroscopic methods. From the discharge images, it can be found that this discharge has similar regions with glow discharge at low pressure except a plasma plume region. Light emission signals from the discharge indicate that the plasma column is invariant with time, while there are some stochastic pulses in the plasma plume region. The optical emission spectra scanning from 300 nm to 800 nm are used to calculate the excited electron temperature and vibrational temperature of the large-gap uniform discharge. It has been found that the excited electron temperature almost keeps constant and the vibrational temperature increases with increasing discharge current.Both of them decreases with increasing gas flow rate.     

Ion emission from arc discharge plasmas

Results are presented from studies of the ion-emission properties of the anode plasmas of low-pressure contracted arc discharges and vacuum arcs. It is shown that creating a longitudinal magnetic field in the anode region of a discharge changes the plasma parameters significantly and facilitates a large increase in the ion current. Space charge limited ion current in a vacuum arc leads to a reduction in the noise level of the total ion current and of its components with charges of up to +3, while creating Penning discharge conditions ensures that ions of different gases can be generated in this discharge system at fractions as high as 90%, depending on the type of plasma forming gas.Institute of High-Current Electronics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vuzov, Fizika, No. 3, pp. 24–33, March, 1994.     

Behavior of the spatial phase of modulated ionization waves in a glow discharge

Work is described in which it was shown experimentally that a modulated glow discharge with striations acquires an additional characteristic which can be called spatial phase of the oscillations. In fact, this characteristic reflects the spatial distribution of the maxima of the time-averaged integrated plasma luminescence on the tube axis which arises due to the influence of the current modulation on traveling striations. It was discovered that for fixed discharge conditions such a distribution remains constant when the discharge is switched off and on, i.e., it is not random. An experimental investigation was made of the influence of a magnetic field on the spatial phase of the modulated ionization waves. It was found that in a low-pressure glow discharge with small currents the striations are initiated in the cathode region. The anode region does not directly initiate the striations but it influences their spatial phase characteristics through a feedback mechanism. State University, St. Petersburg. Ukhtinskii Industrial Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 108–113, January, 1997.     

Generation of a submillisecond electron beam with a high-density current in a plasma-emitter diode under the conditions of open plasma boundary emission

The generation of a 250-μs-wide electron beam in a plasma-emitter diode is studied experimentally. A plasma was produced by a pulsed arc discharge in hydrogen. The electron beam is extracted from a circular emission hole 3.8 mm in diameter under open plasma boundary conditions. The beam accelerated in the diode gap enters into a drift space in the absence of an external magnetic field through a hole 4.1 mm in diameter made in the anode. The influence of electron current deposition at the edge of the anode hole on the beam’s maximum attainable current, above which the diode gap breaks down, is studied for different accelerating voltages and diode gaps. The role of processes occurring on the surface of the electrodes is shown. For an accelerating voltage of 32 kV, a mean emission current density of 130 A/cm² is achieved. The respective mean strength of the electric field in the acceleration gap is 140 kV/cm. Using the POISSON-2 software package, the numerical simulation of the diode performance is carried out and the shape of steady plasma emission boundaries in the cathode and anode holes is calculated. The influence of the density of the ion current from the anode plasma surface on the maximum attainable current of the electron beam is demonstrated.     

Self-modulation processes in a low-current rare-gas discharge at low pressures

Experimental investigations were made of self-modulation processes in a striated rare-gas discharge at low pressures and currents. It was found that interrelated self-modulation oscillations of the discharge current and of the voltage across the tube can develop in the discharge circuit. Depending on the resistance in the external circuit, either current or voltage self-modulation occurred. An important factor is that the transition from one form of self-modulation to the other does not produce quantitative changes in the spatial modulation of the time-averaged plasma luminescence on the discharge axis. It is shown that a qualitative interpretation of the observed phenomena can be given in terms of an equivalent striation voltage source in the anode region which exists because of interaction between traveling kinetic ionization waves and the discharge region near the anode, and also in terms of processes of a capacitative nature near the electrodes. State University, St. Petersburg. Ukhtinskii Industrial Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 103–108, January, 1997.     

外磁场作用下的磁等离子体动力学过程仿真

磁等离子体动力学推力器是空间高功率电推进装置的典型代表,磁等离子体动力学过程是其核心工作机制.为深入理解外磁场对其工作特性的影响,本文采用粒子云(particle in cell,PIC)方法结合基于自相似准则的缩比模型,进行外加磁场作用下磁等离子体动力学推力器工作过程的建模仿真,通过与实验结果对比验证模型和方法的可靠性,并重点分析推力器点火启动过程的等离子特性参数分布,以及外磁场和阴极电流对推力器工作性能的影响.研究结果表明:阴阳极放电电弧构建是推力器启动和高效工作的关键步骤;外磁场强度较低工况不利于构建稳定放电电弧,等离子体束流集中于轴线附近,推力主要产生机制是自身场加速;外磁场强度较高时,阴阳极放电电弧稳定,推力产生主要机制是涡旋加速,推力、比冲随外磁场强度线性增大;推力器效率随阴极电流和外磁场强度增大而增大;放电电压随阴极电流增大而增大,但随外磁场强度的增大表现出先减小后增大的趋势.     

外磁场作用下的磁等离子体动力学过程仿真

磁等离子体动力学推力器是空间高功率电推进装置的典型代表,磁等离子体动力学过程是其核心工作机制.为深入理解外磁场对其工作特性的影响,本文采用粒子云(particle in cell,PIC)方法结合基于自相似准则的缩比模型,进行外加磁场作用下磁等离子体动力学推力器工作过程的建模仿真,通过与实验结果对比验证模型和方法的可靠性,并重点分析推力器点火启动过程的等离子特性参数分布,以及外磁场和阴极电流对推力器工作性能的影响.研究结果表明:阴阳极放电电弧构建是推力器启动和高效工作的关键步骤;外磁场强度较低工况不利于构建稳定放电电弧,等离子体束流集中于轴线附近,推力主要产生机制是自身场加速;外磁场强度较高时,阴阳极放电电弧稳定,推力产生主要机制是涡旋加速,推力、比冲随外磁场强度线性增大;推力器效率随阴极电流和外磁场强度增大而增大;放电电压随阴极电流增大而增大,但随外磁场强度的增大表现出先减小后增大的趋势.     

Formation and focusing of electron beams in an electron-optical system with a plasma emitter placed in a magnetic field

The configuration and strength of a magnetic field are calculated in the regions of electron generation, acceleration, and transport in the electron-optical system of the plasma electron source. A magnetic field necessary for discharge initiation and maintenance is generated with a permanent magnet placed in a discharge chamber. It is shown that the magnetic field strength and configuration in these regions can be considerably varied by appropriately choosing the materials of electrodes forming the magnetic circuit. It is found that the beam focusing can be significantly improved by producing a quasi-uniform magnetic field in the electron-optical system of the plasma electron source.     

Microstructure of the current channel of an atmospheric-pressure diffuse discharge in a rod-plane air gap

The multichannel structure of the current channel of an atmospheric-pressure diffuse discharge excited in a 10-cm rod-plane air gap was investigated using the imprint technique. A voltage pulse with an amplitude of 240 kV, a duration of 180 ns, and a rise time of 14 ns was applied to a 1-cm-diameter bullet-shaped cathode with a tip curvature radius of 0.2 mm; the discharge current reached 350 A. It is found that the diameter of the discharge channel in the anode plane varies in the range 2.5–9.7 mm from shot to shot. The overall imprint of the current channel is formed of 170–9500 imprints of microchannels with an average diameter of 5–20 μm. The parameters of the observed microstructure do not correlate with variations in the main electric characteristics of the discharge and the parameters of the generated X-ray pulse. It is shown that the formation of the microstructure is related to the onset of cathode-directed plasma structures developing from the anode. It is suggested that the same mechanism is responsible for both the formation of the current channels structure and the anode microstructure of diffuse nanosecond discharges developing in atmospheric-pressure air gaps with a highly nonuniform electric field.