Peculiarities of interaction of the cathode stream with anodevapors in a vacuum arc

A one-dimensional model of the quasi-stationary vacuum arc plasma zone is considered in terms of a three-liquid hydrodynamics approximation. It is supposed that atoms evaporated from the anode are ionized in a narrow near-anode zone, slow ions are rapidly maxwellized in the result of Coulomb collisions, ion braking length of the cathode stream may be comparable with a gap size, i.e., the cathode stream breaking is important. Outside the narrow near-electrode zones slow anode and rapid cathode ion flows are maintained, the electron thermal conductivity equalizes the electron temperature     

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.     

The Influence of the Self-Magnetic Field on the Steady-State Current Distribution in an Axially Flowing Conducting Medium

The steady-state electric current distribution in a multicathode-spot vacuum arc was determined by a solution of the magnetic transport equation subject to various boundary conditions. The inter-electrode region of the arc is modeled as a uniform plasma flowing from the cathode to the anode. Dimensional analysis shows that three parameters determine the magnetic field, and hence the current density which is derived from it: AR-the ratio of the electrode separation to the electrode radius, Rmm-magnetic Reynolds number of the axial material flow, and Rme-magnetic Reynolds number of the axial electron flow. While the anode side of the conducting medium is described as an equipotential surface, the following three cases of boundary conditions for the cathode side are examined: 1) a known current density distribution is assumed over the entire cathode side of the plasma surface; 2) the cathode side is an equipotential surface; and 3) the current is allowed to cross the cathode surface only through a finite number of ring shaped regions. Numerical solutions of the nonlinear magnetic transport equation show a constriction of the current at the anode side for all boundary conditions mentioned. On the other hand, the current moves to the perimeter of the cathode for boundary condition 2). When AR, Rmm, and Rme equal 0.72,-0.16, and 1.73, respectively, and a uniform current density flows at the cathode side, the on-axis current density at the anode is six times larger than its value at the cathode.     

Cathodic-arc plasma density, radial profile, and total plasmainventory as measured by microwave interferometer and Faraday cup

The total amount of plasma, peak plasma density, and plasma density radial profile are derived from a series of measurements of ion current density (with a Faraday cup) and integrated electron line density (with a microwave interferometer) for a cathodic arc derived plasma. Comparisons are made between cathode material (erbium and titanium), arc current, background gas pressure, and the presence or absence of a series magnetic solenoid around the coaxial anode and cathode     

Charge Flow and Plasma Behavior in Intense Ion Beam Diodes

Intense ion beams are produced in high-power vacuum diodes of various configurations and are believed to be useful for applications in inertial confinement fusion and plasma confinement. Using magnetically insulated diodes, we investigated spatial nonuniformities of the diode plasmas, plasma expansion, ion transverse velocities in the diode gap, electron flow to the anode, and the charge distribution in the gap. Various time-dependent diagnostic techniques including recently developed spectroscopic methods have been used. We observed rapid closure of the diode gap, resulting from fast expansion of the electric field-excluding anode plasma early in the pulse. This contributes significantly to the measured ion current density enhancement. The electron cloud in the gap was seen to spread towards the anode beyond the region of the theoretical electron sheath. This is consistent with observed ion current densities being larger than the values calculated using the actual diode gap. The ion angular spread was found to increase locally due to nonuniform expansion of the cathode plasma for one class of phenomena and of the anode plasma for the other two classes. Part of these phenomena were associated with electron flow to the anode. The ion divergence angle in the gap was observed to be independent of the ion mass and to be significantly smaller than angles previously observed outside the diode.     

Application of vacuum arc cathode spot model to graphite cathode

A model of near-electrode processes is applied here to describe the behavior of cathode spots on graphite cathode in vacuum arc. The physical model is based on a kinetic treatment of cathode evaporation, electron emission from the cathode, and plasma production. The model consists of physical assumptions and a system of equations that are formulated in the paper. Spot parameters, such as cathode erosion rate, cathode potential drop, cathode surface temperature, current density, electric field, and plasma density, temperature, and velocity in the near-electrode region are calculated numerically. The calculation includes the dependence of spot parameters on spot current and spot lifetime. The variation of spot parameters as a function of spot lifetime are very strong at lifetimes shorter than 10 μs. The calculations indicate that Joule heating in the cathode body is significant, and may exceed cathode heating by the ion heat flux. Calculated spot parameters are compared with the corresponding experimental data for relatively low arc currents (<100 A) and their agreement is discussed     

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     

A model for a uniform steady-state vacuum arc with a hot anode

A model is formulated and evaluated for a Uniform electrical discharge sustained in vapor evaporated from an arc-heated anode. The plasma potential is positive with respect to both the cathode and anode. For a Cu anode, the anodic vapor dominates the plasma for current densities exceeding 8 kA/m². The anode heating potential is approximately 6.5 V, and the dominant cooling mechanism is evaporation for current densities exceeding 20 kA/m². Over the range 10 to 10000 kA/m², the electron density increases from 8×10¹⁷ to 5×10²³ m^-3, while the ionization fraction rises from 0.3% to 4%. At the lower end of this current range the electrical resistivity of 4 mΩ-m is determined primarily by electron-neutral collisions, while with increasing current the resistivity decreases to 0.7 mΩ-m, with electron-ion collisions contributing an equal share. This hot-anode vacuum arc may have potential for industrial application as a macroparticle-free high-deposition-rate coating source     

真空电弧等离子体发射光谱诊断

真空断路器的开断容量限制其在高压大电流开断领域的应用,获取燃弧过程中的等离子体参数对于提高真空断路器的开断容量至关重要。利用发射光谱法对真空电弧内的等离子体参数进行了诊断,研究了在不同电流幅值条件下真空电弧内电子温度、电子密度、谱线强度的轴向分布规律,结合真空电弧高速图片对真空电弧内不同粒子的扩散过程与弧柱直径之间的关系进行了分析。得到的电子温度在8000～10 000 K量级,电子密度在1019～1020 m?3量级,电子温度与电子密度从阴极向阳极逐渐下降,同时铜原子谱线强度主要集中在两极而一价铜离子谱线强度由阴极向阳极逐渐升高。铜原子谱线强度的径向分布呈现类平顶波分布、一价铜离子谱线强度的径向分布呈现类高斯分布的特点,且铜原子的谱线范围略大于弧柱直径,一价铜离子的谱线范围略小于弧柱直径,两种粒子的扩散速度存在差异。     

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.     

纵磁作用下真空电弧单阴极斑点等离子体射流三维混合模拟

真空电弧的特性直接受到从阴极斑点喷射出的等离子体射流的影响,对等离子体射流进行数值仿真有助于我们深入了解真空电弧的内部物理机制.然而,磁流体动力学和粒子云网格仿真方法受限于计算精度和计算效率的原因,无法有效地应用于真空电弧等离子体射流仿真模拟.本文开发了一套三维等离子体混合模拟算法,并在此基础上建立了真空电弧单阴极斑点射流仿真模型,模型中将离子作宏粒子考虑,而电子作无质量流体处理,仿真计算了自生电磁场与外施纵向磁场作用下等离子体的分布运动状态.仿真结果表明,单个阴极斑点情况下真空等离子体射流在离开阴极斑点后扩散至极板间,其整体几何形状为圆锥形,离子密度从阴极到阳极快速下降.外施纵向磁场会压缩等离子体,使得等离子体射流径向的扩散减少并且轴线上的离子密度升高.随着外施纵向磁场的增大,其对等离子体射流的压缩效应增强,表现为等离子体射流的扩散角度逐渐减小.此外,外施纵向磁场对等离子体射流的影响也受到电弧电流大小的影响,压缩效应随电弧电流的增加而逐渐减弱.     

A physical model of the low-current-density vacuum arc

A one-dimensional (1-D) physical model of the low-current-density steady-state vacuum arc is proposed. The model is based on the continuity equations for ions and electrons and the energy balance for the discharge system; the electric potential distribution in the discharge gap is assumed to be nonmonotonic. It is supposed that the ion current at the cathode is generated within the cathode potential fall region due to the ionization of the evaporated atoms by the plasma thermal electrons having Boltzmann's energy distribution. The model offers a satisfactory explanation for the principal regularities of a hot-cathode vacuum arc with diffuse attachment of the current. The applicability of the model proposed to the explanation of some processes occurring in a vacuum arc, such as the flow of fast ions toward the anode, the current cutoffs and voltage bursts, and the backward motion of a cathode spot in a transverse magnetic field is discussed     

Investigation of ion behavior associated with anode-spot modetransition in vacuum arcs

The ion behavior phenomenon associated with transitions of the anode discharge mode to the anode-spot mode is studied by measuring the wall ion current and by spectroscopic observation in vacuum arcs. The anode mode transfers when the wall ion current attains a certain magnitude that is independent of the cathode, but dependent on the anode. The ion-current function to the arc current increases when the arc current increases in the diffuse arc. Spectral-line intensity of Cu III emitted from the plasma in the anode region increases with an instantaneous arc current of a 5-kA peak (kAp) sinusoidal half-wave. These findings suggest an idea for the mode transition, that an ion generation region appears, and that an increase in the ion density produces a positive potential hump near the anode, which results in the negative anode voltage drop triggering the mode transition. After the mode transition, an arc current is found to reduce the ion current near the crest of a sinusoidal current in a copper arc. This appears to be significant for the arc on a small anode. The decrease in the ion current is attributed to the recombination of ions decelerated by anode vapor with electrons emitted from the hot spot on the anode     

A Model for DC Interruption in Diffuse Vacuum Arcs

A theoretical model for current interruption in a diffuse vacuum arc with dc commutation is described. Before current zero the interelectrode plasma is modeled as an ion-neutral fluid through which electrons are flowing. After current zero a positive ion sheath grows into the plasma from the former anode, driven by the transient recovery voltage. Using the basic laws of conservation, the decay of the plasma during commutation is evaluated numerically, enabling the post-arc current, the electric field at the former anode, and the power input to this electrode after current zero to be calculated. For copper electrodes, with a commutation time of 30 ?s, the ion density and velocity at current zero are 23 percent and 35 percent of their respective steady state values. The calculated post-arc currents of tens of amps are in good agreement with experimental data. The post-arc data generated with this model can be used to study reignition mechanisms and the interrupting capability of different contact materials.     

微腔等离子体放电过程的数值研究

采用2维自洽完全流体模型,数值研究了阳极为通孔的高气压微腔放电结构中等离子体参数的变化过程。模拟结果获得了当氩气压强为13.3 kPa时,放电中的电势分布、等离子体密度分布、径向电场分布和电子温度分布等重要参数的演化过程。模拟结果表明在放电过程中,阴极附近的电场由轴向电场逐步转变为径向电场,等离子体密度最大值位于放电腔中间处,并随时间推移由阳极附近向阴极附近移动,电子温度的最大值出现在阴极环形鞘层区域。     

A hydrodynamic model of vacuum arc plasmas

The properties of plasmas expanding from cathode spots of vacuum arcs are calculated with a one-dimensional two-fluid model. The system of simplified hydrodynamic equations can be solved under stationary conditions using asymptotic power series. Although necessarily only an approximation, such analytical solutions prove to be advantageous compared with numerical integrations. All the plasma parameters are functions of (I/r)^2/5 (current, I: distance, r). The three forces accelerating the ions to high kinetic energies are quantitatively calculable: the electric field, the ion pressure gradient, and the electron-ion friction. The potential is decreasing towards the anode, and the residence of the plasma is negative. The ion temperature reaches only about 35% of the electron temperature. Although only asymptotic, the solution is suited to describe the arc plasma in a sufficient manner all over the expansion region     

Anode Phenomena in Vacuum and Atmospheric Pressure Arcs

This paper summarizes recent experimental data related to anode phenomena in both vacuum and atmospheric pressure arcs. Currents in the range 10A to 3OkA are discussed, and particular emphasis is placed on the effect of plasma flow from the cathode. For vacuum arcs this plasma flow is the directed motion of metal ions from the cathode spots. These ions reduce the anode voltage drop, and maintain a diffuse anode termination. At atmospheric pressure the ion flow is impeded by gas-atom collisions. However, a plasma flow towards the anode can result from magnetic pinch forces at the constricted cathode termination. In the absence of plasma flow, the anode termination constricts to a vigorously evaporating anode spot. For a typical non-refractory electrode such as copper, the spot operates at a temperature close to the boiling point irrespective of the gas pressure. The spot temperature is dictated by the balance between electrical input power and evaporative losses. These anode phenomena are discussed in relation to vacuum switchgear, arc welding and arc furnaces.     

分段表面放电沉积效率研究

利用一种结构紧凑的分段表面放电辐射源模块，详细研究了在不同电压、电容、气压实验条件下回路等效电阻、等效电感及放电能量沉积效率的变化规律，利用四分幅相机拍摄获得了不同实验条件下的放电等离子体通道图像，分析讨论了放电等离子体运动对放电能量沉积效率的影响，提出了提高能量放电沉积效率的有效途径。     

阴阳极等离子体运动对强箍缩离子束二极管束流特性的影响

 主要研究了阴阳极等离子体运动对“闪光二号”加速器强箍缩离子束二极管束流特性的影响。给出了考虑阴阳极产生的等离子体运动对二极管间隙影响的Child-langmuir流、弱聚焦流、强聚焦流和饱和顺位流4个阶段的离子流和二极管总束流修正公式，利用这些修正公式计算的二极管总束流和离子束流强度与实测结果符合很好，在此基础上分析了提高离子束流强度和效率的方法，通过调整加速器参数，实验得到了峰值能量约500 keV，峰值电流约160 kA的高功率离子束。     

Theory of the cathode sheath in a vacuum arc

A previous theory of the plasma sheath transition starting from the charge exchange model for ion collisions is extended to account for ionization and recombination. It is applied to the quasi-neutral boundary layer (presheath) in front of the cathode sheath of a vacuum arc. An essential potential and density difference between the sheath edge and cathodic plasma ball is found. This difference is accounted for in a unified theory of the arc cathode based on G. Ecker's (1971) existence diagram method, which indicates possible areas of arc operation in the T_cj plane, where T_c is the spot temperature and j is the current density. A numerical evaluation for Cu gives the results which are qualitatively similar to Ecker's theory. The existence areas are quantitatively enlarged and shifted to lower current densities