Influence of the Self-Heated Cathode Geometry on the Parameters of a Low-Pressure Discharge in Crossed <Emphasis Type="Italic">E×H</Emphasis> Fields

The parameters of a self-maintained low-pressure discharge in crossed E×H fields are compared for the Penning discharge with a self-heated disk cathode and the combined discharge with an axial self-heated rod electrode. Based on the continuity equation for the electron flux and the energy balance on the hot cathode, the parameters of the discharge in crossed E×H fields with the self-heated cathode are calculated for a wide range of variations of geometrical sizes of the discharge cell and work functions of cathode material. It is demonstrated that the examined systems differ significantly by radial distributions of fast electron concentration (uniform for the Penning cell and highly nonuniform for the combined discharge cell) and currents and that the volt-ampere characteristics of the examined cells are described by decreasing voltage-current dependences.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 73–80, January, 2005.     

Simulation of the process of generation of atomic hydrogen in a gas-discharge cell with a self-heated cathode

This work is devoted to a theoretical simulation of the process of generation of atomic hydrogen in a source depending for its operation on a low-pressure self-sustained discharge in crossed E×H fields (reflected magnetron discharge) with an additional self-heated (hot) cathode made as a rod and located in line with the system. The distribution of the atomic hydrogen density over the volume of the discharge cell and the density atomic hydrogen flux onto the disk cathode have been analytically calculated for the discharge operating pressure and current, the pressure of the working gas, and the induction of the external magnetic field varied over wide ranges. The results of the simulation allow comprehensive analysis of the operation of actual atomic hydrogen sources based on this type of discharge. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 76–82, December, 2005.     

Discharge characteristics of the DUHOCAMIS with a high magnetic bottle-shaped field

For the purpose of producing high intensity, multiply charged metal ion beams, the dual hollow cathode ion source for metal ions (DUHOCAMIS) was derived from the hot cathode Penning ion source combined with the hollow cathode sputtering experiments in 2007. To investigate the behavior of this discharge geometry in a stronger magnetic bottle-shaped field, a new test bench for DUHOCAMIS with a high magnetic bottle-shaped field up to 0.6 T has been set up at the Peking University. The experiments with magnetic fields from 0.13 T to 0.52 T have indicated that the discharge behavior is very sensitive to the magnetic flux densities. The slope of discharge curves in a very wide range can be controlled by changing the magnetic field as well as regulated by adjusting the cathode heating power; the production of metallic ions would be much greater than gas ions with the increased magnetic flux density; and the magnetic field has a much higher influence on the DHCD mode than on the PIG mode.     

An Observation of Energetic Electron Beams in Low-Pressure Linear Discharges

Experimental observations of energetic axial electron beams in a linear Z pinch operating in the pseudospark mode are presented. The device is driven from a fast Marx generator and allows reproducible production of electron beams over a wide pressure range. Evidence of the importance of electrons generated in the cathode recess in the formation of the beams is presented. An electron beam of high energy which is not associated with formation of the discharge is identified. A second beam of high current density and lower energy associated with gas breakdown is also observed.     

新型二次电子倍增阴极构型设计与动力学过程

提出了一种可由脉冲功率驱动的新型二次电子倍增阴极构型,并对其进行了动力学过程的初步理论研究。首先,针对该二次电子倍增阴极,建立了动力学模型,获得了二次电子的位移和速度方程,讨论了电子初始出射速度对其轨迹、渡越时间和碰撞能量的影响,理论给出了渡越时间和碰撞能量的近似解析表达式。其次,通过动力学方程与Vaughan二次电子产额经验公式的耦合求解,获得了该二次电子倍增阴极的工作区间,并对其进行了细致讨论。结果表明:该新型二次电子倍增阴极二极管概念上是可行的,在涂敷高二次电子产额系数材料的圆柱形介质上施加合适的轴向和径向静电场(MV/m量级)以及轴向静磁场(T量级),可以达到电子沿阴极表面螺旋行进过程中实现二次电子倍增并最终获得电流沿轴向放大的设计目标。另外,讨论了正电荷沉积引发的二次电子倍增饱和现象,并对阴极发射电流密度进行了理论粗估,结果表明:阴极发射电流密度可达kA/cm2水平,具备强流发射特性;增加外加径向场强幅值可有效提升阴极发射电流密度。     

Wide-aperture high-current electron beam in a discharge with cathode plasma at elevated pressure

This work pursues investigations into the discharge with a cathode plasma in a cavity one wall of which is an insulating plate with a hole D in diameter (the cavity is 0.5 or 1.5 mm wide). This discharge is thoroughly analyzed in comparison with the high-voltage hollow-cathode discharge. Owing to the reduced emission of electrons from the cathode plasma, the discharge becomes more stable against transition to the low-voltage form, as a result of which an electron beam can be generated under higher gas pressures. Such a beam formed at the entrance to the cavity is used as an auxiliary one that propagates over the remaining (flat) surface of the cathode and adds to the gas ionization. Accordingly, the beam current from the main discharge to the anode rises (high-current regime). Wide-aperture (D = 22 mm) ≈1-μs-long pulsed beams with a current an order of magnitude higher than the total current of the equivalent anomalous discharge are obtained. Experiments are carried out at a helium pressure to 20 Torr and a voltage from 1 to 20 kV.     

微空心阴极放电时空特性

为了揭示微空心阴极放电的放电机理,利用流体模型研究了矩形微空心阴极放电的时间和空间分布特性。在氩气环境下计算得到了压强为1.3×10~4Pa时电流、电势、电场、电子和离子密度等随时间的发展变化。结果表明,整个放电过程分为四个阶段,即预放电阶段、电场由轴向向径向转换阶段、电流缓慢增长向空心阴极效应过渡阶段和稳态放电阶段。稳态放电时出现明显的空心阴极效应,阴极位降区存在很高的径向电场和较高的电子平均能量,而负辉区径向电场很弱,电子平均能量较低,电子和离子密度峰值出现在负辉区,二者数值基本相等,而在阴极位降区离子密度远高于电子密度。     

Simulation of transition from Townsend mode to glow discharge mode in a helium dielectric barrier discharge at atmospheric pressure

The dielectric barrier discharge characteristics in helium at atmospheric pressure are simulated based on a one-dimensional fluid model. Under some discharge conditions, the results show that one discharge pulse per half voltage cycle usually appears when the amplitude of external voltage is low, while a glow-like discharge occurs at high voltage. For the one discharge pulse per half voltage cycle, the maximum of electron density appears near the anode at the beginning of the discharge, which corresponds to a Townsend discharge mode. The maxima of the electron density and the intensity of electric field appear in the vicinity of the cathode when the discharge current increases to some extent, which indicates the formation of a cathode-fall region. Therefore, the discharge has a transition from the Townsend mode to the glow discharge mode during one discharge pulse, which is consistent with previous experimental results.     

空气中空心阴极不同放电模式下的发光特性

为了进一步揭示空心阴极放电中放电模式的转换机制,特别是空心阴极放电过程中自脉冲的形成机理,利用柱型空心阴极放电结构,在空气环境下研究了放电处于不同模式时的发光特性。测量得到了不同放电模式下的伏安特性曲线、放电发光图像、自脉冲阶段的脉冲波形等。实验结果表明随着放电电流的增加放电分为汤生放电模式、自脉冲放电模式、正常辉光放电模式和反常辉光放电模式。虽然所用电源为直流电源,但在自脉冲放电阶段电流和电压随时间呈周期性变化。实验结果表明在不同的放电模式下具有不同的发光特性。在由汤生放电转换为自脉冲放电模式和由自脉冲模式转换为正常辉光放电模式过程中,放电腔的径向中心处和轴向孔口附近均存在光强的突变。实验同时在200～700 nm范围内测量得到了不同电流时的发射光谱。结果表明发射光谱主要集中在330～450 nm,主要包括氮分子的第二正带系(C3Π_u→B3Π_g )和氮分子离子的第一负带系（B2Σ+_u→X2Σ+_g）。其中氮分子离子第一负带系具有较强的发射光谱。由于B2Σ+_u激发电位较高,因此该谱带较强发射光谱的存在表明空心阴极放电较其他放电形式更容易获得高激发态粒子和高能量电子。在650～700 nm附近存在一弱的发光谱带,主要为氮分子的第一正带发射谱（B3Π_g→A3Σ+_u）。在此基础上根据双原子光谱发射理论,结合氮分子第二正带系的三组顺序组带：Δν=-1,-2和-3,利用玻尔兹曼斜率法计算得到了不同放电模式下氮气的分子振动温度。结果表明在实验电流范围内分子振动温度在3 300 K左右,随着电流的增加而升高,并且在自脉冲消失时存在一突变迅速增强。由于电子能量、电子密度与分子振动温度密切相关,因此该结果也表明随着放电电流的增加电子平均能量和电子密度不断增加,当脉冲消失时,电子平均能量和电子密度出现跃变升高。最后,对空心阴极放电中自脉冲的形成机理进行了讨论,结果表明自脉冲放电源于放电模式的转换。     

Self-sustained self-sputtering: a possible mechanism for thesuperdense glow phase of a pseudospark

Self-sustained self-sputtering occurring during high current pseudospark operation (≈10⁴ A/cm², I>10³ A) is shown to be a possible mechanism for the superdense glow. The mean-free-path for ionization of cathode material sputtered in the low-current hollow-cathode phase can be shorter than the cathode-anode gap distance, and ionized atoms can return to the cathode surface, self-sputtering with a yield greater than one. The self-sputtered cathode atoms become ionized in the beam of electrons accelerated in the cathode sheath. A large fraction of the discharge current at the cathode surface can be carried uniformly over the surface by ions and a very high electron emission density is not required to maintain the high current     

Thermionic cathode parameters versus pressure in a sustained argon arc

Experimental data on the influence of the argon pressure on the performance of a rodlike thoriated-tungsten cathode in a sustained arc discharge are reported for the first time. The integral parameters of the cathode, namely, the electron work function, temperature, current density, and specific surface area of erosion at currents of 20–150 A in the pressure range 2.66 × 10³?1.00 × 10⁵ Pa, are measured. Taken together, these parameters provide comprehensive information about the operating conditions of the activated cathode. A significant influence of the pressure on the specific erosion of the cathode is noted.     

Generation of a pulsed high-current low-energy beam in a plasma electron source with a self-heated cathode

The transition of a low-current discharge with a self-heated hollow cathode to a high-current discharge is studied, and stability conditions for the latter in the pulsed–periodic mode with a current of 0.1–1.0 kA, pulse width of 0.1–1.0 ms, and a pulse repetition rate of 0.1–1.0 kHz are determined. The thermal conditions of the hollow cathode are analyzed, and the conclusion is drawn that the emission current high density is due to pulsed self-heating of the cathode’s surface layer. Conditions for stable emission from a plasma cathode with a grid acting as a plasma boundary using such a discharge are found at low accelerating voltage (100–200 eV) and a gas pressure of 0.1–0.4 Pa. The density of the ion current from a plasma generated by a pulsed beam with a current of 100 A is found to reach 0.1 A/cm². Probe diagnostics data for the emitting and beam plasmas in the electron source are presented, and a mechanism behind the instability of electron emission from the plasma is suggested on their basis.     

Simulation of radio-frequency atmospheric pressure glow discharge in γ mode

The existence of two diffe1：ent discharge modes has been verified in an rf （radio-frequency） atmospheric pressure glow discharge （APGD） by Shi [J. Appl. Phys. 97, 023306 （2005）]. In the first mode, referred to as a mode, the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion. In the second mode, termed γ mode, the discharge current density is relatively high, the secondary electrons emitted by cathodc under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. In this paper, a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in the mode in helium discharge between two parallel metallic planar electrodes. The results show that as the applied voltage increases, the discharge current becomes greater and the plasma density correspondingly increases, consequentially the discharge transits from the a mode into the γ mode. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region, and the electron Joule heating and the electron collisional energy loss reach their maxima in the region. The validity of the simulation is checked with the available experimental and numerical data.     

Study of a dc magnetron discharge by using a movable grid anode

A method is proposed for determining the density of the ionizing electron component in a dc magnetron discharge in the region where the magnetic field is strong and highly nonuniform. The method is based on the analysis of the measured dependence of the discharge current on the distance between the cathode and the grid anode. The method was used to study the parameters of a magnetron discharge in a process magnetron operating in the regime with a narrow discharge region localized close to the cathode surface.     

General intense electron beams by means of a contracted arc discharge

Two types of contracted arc discharge are investigated with a view to generating intense electron beams over a wide pressure range (1–10^–3 Pa). For an arc discharge with a hollow cathode and anode, an electron beam corresponding to a current of up to 300 A and a pulse length of 25 µsec is obtained at a pressure of 1–10^–1 Pa in the accelerating gap with an accelerating voltage of up to 15 kV. At pressures of 10^–2–10^–3 Pa, emitting plasma is created by a low-pressure arc discharge on the basis of a Penning cell. Three discharge systems operating in parallel are used to increase the working life of the cathode and improve the current density distribution of the beam. An electron beam of diameter 200 mm with a current of up to 125 A and a pulse length of 50 µsec is obtained.Institute of High-Power Electronics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshkikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 76–82, March, 1994.     

Glow Discharge with an Extended Hollow Cathode

Russian Physics Journal - A low-voltage low-pressure glow discharge with a hollow cathode is discussed. The discharge is ignited in a reflective discharge system with two symmetric peripheral...     

The Role of High-Velocity Electrons in a Hollow-Cathode Discharge

In this paper, the features of a low-pressure discharge in a hollow cathode are considered. It has been shown that a free mode can be realized with a double electric layer at the cathode and high density of the current of high-velocity electrons in the discharge bulk, considerably exceeding the current density at the cathode. In this case, the rate of ionization by high-velocity electrons considerably exceeds the rate of ionization by plasma electrons.     

Phenomenological model of the unstable stage of a vacuum spark discharge

A model of the unstable stage of a spark discharge in vacuum is proposed, which describes all typical manifestations of this stage, including current spikes in the diode, an increase in the potential at the cathode flame front, collective acceleration of ions in vacuum and plasma diodes, change in the cathode erosion mechanism, and the emergence of electron microbeams with a high current density at the anode. It is shown that these processes are associated with the formation of a charged electron layer of a spatially inhomogeneous plasma at the cathode flame boundary at the unstable stage of the spark discharge in vacuum. The emergence of this layer is associated with a limited emissive ability of the plasma at the cathode flame front during its expansion in vacuum. This leads to disruption of the plasma (field-induced emission of electron from the boundary region of the flame) and the formation of a short-lived charged plasma, viz., high-density ion cluster at the cathode flame boundary. The estimates obtained using this model are in good agreement with the experimental data on physical processes at the unstable stage of a vacuum spark discharge.     

Nonself-sustained arc discharge in anode material vapors

The basic characteristics of the nonself-sustained arc discharge in a vapor of the anode material are studied. The influence of thermoemission parameters of the cathode on volt-ampere characteristics of the discharge is described. It is established that in a free mode of the discharge cathode operation, when the discharge current I_D is lower than the current of the thermoelectron emission from the discharge cathode I_C, the deposition rate of the films q is directly proportional to the discharge current I_D. In the compelled mode of the cathode operation, when I_D>I _C, q~W_D², where W_D=I_D U_D with U_D being the discharge voltage. It is shown that the magnetic field increases the plasma density and changes the density profile from n(x)-1/x² to n(x)-1/x with x being a distance along the flow. The motion of created plasma flow is shown to have a noncollisional character with constant electron temperature of 5-7 eV along the flow. The values of plasma potential and electric field in the flow are determined; the values of cathodic and anodic potential drops in the discharge are evaluated. The angular distributions of ion and neutral fluxes in the created plasma flow are described. It is shown that the plasma flows parameters depend substantially on the working material. With use of crossed electric and magnetic fields, the flow ionization coefficient was enhanced up to 85% for the discharge in Ti vapors, and 35% for the discharge in Cu vapors     

爆炸电子发射边界的粒子模拟实现

 介绍了自行研制的全电磁柱坐标粒子模拟程序的电流分配方法和金属爆炸电子发射边界的模拟实现，该电流分配方法满足电荷电流连续性方程，避免了繁琐的泊松修正，适用于复杂边界物理问题的模拟研究。基于此电流分配方法的基础上，给出了建立在高斯定理基础上的简单且易于程序实现的阴极发射边界算法。利用该程序对平面二极管电子发射现象的模拟结果证明了算法的正确性。