From Field Emission to Vacuum Arc Ignition: A New Tool for Simulating Copper Vacuum Arcs

Understanding plasma initiation in vacuum arc discharges can help to bridge the gap between nano‐scale triggering phenomena and the macroscopic surface damage caused by vacuum arcs. We present a new twodimensional particle‐in‐cell tool to simulate plasma initiation in direct‐current (DC) copper vacuum arc discharges starting from a single, strong field emitter at the cathode. Our simulations describe in detail how a sub‐micron field emission site can evolve to a macroscopic vacuum arc discharge, and provide a possible explanation for why and how cathode spots can spread on the cathode surface. Furthermore, the model provides us with a prediction for the current and voltage characteristics, as well as for properties of the plasma like densities, fluxes and electric potentials in a simple DC discharge case, which are in agreement with the known experimental values. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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     

Rapid breakdown mechanisms of open air nanosecond dielectric barrier discharges

The discharge initiation mechanism of nanosecond dielectric barrier discharges in open air has been clarified with time-dependent measurement of the discharge electric field by electric-field-induced coherent Raman scattering and optical emission. Our experimental observations have revealed that, in the prebreakdown phase of a nanosecond dielectric barrier discharge, the externally applied fast-rising electric field is strongly enhanced near the cathode due to large accumulation of space charge, which then strongly enhances ionization near the cathode. Once a sufficiently large number of ionizations take place, the location of peak ionization forms a front and propagates toward the cathode with strong optical emission, which establishes the discharge. This process is essentially different from the well-known Townsend mechanism for slower discharges.     

Triggered discharges with high arc voltages in a vacuum interrupter

Studies of nonsustained disruptive discharges (NSDDs), isolated cases of which can occur in vacuum interrupters, indicate lateral discharges between the cathode and shield, which can initiate a brief discharge between the contacts. To facilitate the study of such discharges, the sample discharges were triggered by a surface discharge induced by a spark gap, built into the side of the cathode, and observed with a high-speed film camera and image-converter camera. The tests showed a cathode spot after igniting. The emitted electrons first charge the shield negatively and then are directed toward the anode. The discharge burns at a high voltage, with current ranging from 10 to 100 A. After a period of up to 400 μs, the current demand increases abruptly; an arc discharge occurs between the contacts and discharges the capacitances near the switch. The contact gap undergoes a rapid dielectric recovery, and the restored voltage is maintained. These types of discharge were also observed with NSDDs; thus it can be assumed that the triggered discharges studied correspond to the NSDD type     

Application of Electron-Beam Ionized Discharges to Switches?a Comparison of Experiment with Theory

A theoretical investigation of high-pressure discharges ionized by an external electron beam (e-beam) was conducted. Only when secondary emission from the cathode and electron-impact ionization of metastable states were included in the analysis did calculated current-voltage (I-V) characteristics for argon and methane discharges compare well with experimental data. The I-V characteristics obtained reveal a sharp rise in the current at a certain threshold voltage. This threshold voltage and the entire I-V characteristic are shifted to lower voltages when metastable ionization is significant. Below the threshold voltage and at low external ionization source strengths, a region of negative differential conductivity is obtained. In the high-current region, the I-V slope is controlled by the secondary emission coefficient. The additional cathode sheath ionization from secondary emission and ionization from metastable states significantly reduces the discharge voltage. This important effect can be used to reduce e-beam switch losses and increase lifetime through judicious gas mixture selection and proper cathode conditioning.     

Main reaction process simulation of hydrogen gas discharge in a cold cathode electric vacuum device

Based on the related theory of plasma discharge process and the COMSOL multiphysics software, and considering the corresponding boundary conditions, the related reaction types in the hydrogen plasma discharge were simulated and analysed, and the main reactions of hydrogen discharge in small electric vacuum components at low pressure and weak ionization were confirmed. Among the 21 types of reactions in hydrogen discharge process, 11 of them play important roles under low pressure and weak ionization in cold cathode electric vacuum device. The simulated results are consistent with the test result.     

On the Polishing Effect of Nanosecond Discharges in Vacuum

When a metal surface in ultra high vacuum is subjected to electric discharges with a sufficiently short duration the surface becomes perfectly smooth. This polishing is caused by the inertia of the molten metal within a cathode spot that prevents the formation of a crater by acceleration of the liquid. The present paper reports an investigation of this phenomenon in dependence on the discharge parameters. The effect was found to be determined by the pulse duration, it occurs with pulses of less than 10 nanoseconds. But also long discharges of 500 ns show a polishing if the current rise rate is below a critical value (0.5 — 1 A/ns), depending on the cathode material. Thus long discharges consist of elementary steps of about 10 ns duration, in accordance with the formation time of cathode craters. The polishing effect causes an abrupt decrease of the breakdown probability of rectangular high voltage pulses from one to zero at a critical pulse duration.     

Contribution to the theory of non-self-maintaining volume discharges in molecular and noble gases

Results are presented of investigations of non-self-maintaining volume (NV) gas discharges. The structure is investigated and the current-voltage characteristics are plotted of NV discharges in molecular gas. The characteristic times of the transient processes in the cathode layer of a nonstationary electron-beam-controlled (EBC) discharge are obtained. It is proposed to use an NV discharge as the nonlinear element of a relaxation-oscillation laser. Results are presented of computer experiments with such a laser. The influence of the hydrodynamic motion of the mixture on the characteristics of the cathode layer of an open-cycle cw EBC laser is investigated. It is shown that the ultraviolet emission of excimer molecules alters radically the structure of the cathode layer of an EBC discharge in a noble gas, viz., the intense photoeffect on the cathode leads to the appearance of a negative cathode potential drop. Self-maintaining photoionization and EBC discharges in a noble gas are considered; in these discharges the plasma density is maintained by impurity ionization or by electron-excitation of the atoms by emission from excimer molecules.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 142, pp. 46–94, 1983.     

Über den Gleichspannungsdurchschlag im Ultrahochvakuum

DC-breakdowns with arc-conditioned electrodes in vapour-free vacuum have been investigated with high temporal resolution. The breakdowns started always with a cathode flare, the ignition voltage of which was strongly correlated with prebreakdown field emission. The temporal development of gap plasma, discharge current and gap voltage during the breakdowns could be satisfactorily explained by the models of explosive electron emission and space-charge limited diode conduction. No change of mechanism was observed with gap distances up to 6 mm and voltages up to 120 kV. Generally the resistance of the discharge channel became low enough for a transition to the arc stage. However, in a few cases a self-limitation of the discharge occurred due to starvation of the material transfer from the cathode. At higher gap distances the surrounding walls had a distinct influence on the ignition of the discharges.     

氘化物阴极真空弧放电光斑分布

氘化物真空弧放电在许多领域均有应用,如无损检测、石油探井、中子活化分析等。和金属阴极不同,氘化物阴极放电时会释放大量的气体分子,表现出许多不同性质。采用放大镜头和ICCD相机观察了氘化物阴极真空弧放电光斑分布。测量系统的空间分辨率约为5 μm,时间分辨率最小2 ns。放电脉冲半高全宽（FWHM）0.9 μs,弧流波形为半周期正弦波。实验结果表明,氘化物真空弧放电时,所有阴极斑聚集为一个群落,表现为一个大光斑;在液滴作用下,阴极斑群落偶尔也会分裂为两个或多个群落;光斑形状不受弧流影响,但面积和亮度会随弧流增加而增大。氘化物阴极放电斑点聚集有利于产生高密度等离子体,提高放电效率。     

Investigation of the Directional Velocities of Ions in a Vacuum Arc Discharge by Emission Methods

This paper is devoted to an investigation of the directional velocities of the ions generated in cathode spots of vacuum arc discharges. By using emission methods of studying the processes in a vacuum arc discharge, which involve the determination of the parameters and characteristics of the discharge plasma by analyzing the ion current extracted from the plasma and the ion charge states, the velocities of ions have been determined for the majority of cathode materials available in the periodic table. Is has been shown that at a low pressure of the residual gas in the discharge gap the directional velocities of the ions do not depend on the ion charge state. Comparison of the data obtained with calculated values allows the conclusion that the acceleration of ions in a vacuum arc occurs by the magnetohydrodynamic mechanism.     

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     

真空弧放电TiH合金阴极表面形貌分析

TiH合金电极是一种含氢量非常高的金属材料,用它作真空弧离子源的电极,可在真空环境下产生强度非常高的氢离子流。相比纯金属材料电极,TiH电极除了出现真空弧放电特有的融蚀现象外,还存在气体释放过程,所以它的表面形貌具有一定的独特性。利用扫描电子显微镜观察了单次放电和多次放电后阴极表面形貌,发现阴极斑在阴极表面微裂纹附近连续分布,气体释放生成很多小孔,使阴极斑区域呈絮状结构;弧流越大,阴极斑数量越多;多次放电后,阴极斑朝含氢量多的地方移动。该结果有助于了解含氢电极的真空弧放电过程,对该类放电的应用具有一定参考意义。     

Formation of the microscopic structure of high-voltage nanosecond diffuse discharges in sharply nonuniform geometry

The possibility of the development of ionization instability in the avalanche and plasma phases (of the long-wave and short-wave type, respectively) is demonstrated for high-voltage nanosecond discharges in sharply nonuniform geometry. Specific implementation is determined by the electric field distribution in the cathode region and by the emissivity of the cathode. The spatial structure of the discharge formed in both cases is self-similar.     

Spatial and temporal characteristics of the X-ray emitted by a 1-J50-ns vacuum discharge

The X-ray emission from a source based on a small vacuum discharge is studied. Two different X-ray pulses whose intensities vary with the anode-cathode distance are identified. Time integrated pinhole images show that the whole anode and the teflon insulator emit X-rays. Some of the X-ray emission is found to originate also in the vicinity of the tungsten anode. The temporal behavior of the X-ray varies with respect to the distance between the anode and the cathode. Three different cathode geometries are tested: hollow cathode, hollow conical cathode, and massive or needle cathode. The spatial distribution of the X-ray sources is obtained by means of a sensitive imaging device. Some X-ray spots that appear in the discharge gap near the anode tip are similar to hot spots found in more powerful discharges     

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     

Experimental observation of high-voltage, low-current vacuum arcs

A poorly explored type of discharge has been investigated in high vacuum (10^-7 to 10^-6 torr), with a DC high voltage across 0.2- to 0.8-mm gaps. The discharge has been found to be quite different from other widely known types of vacuum and gas discharges by the combination of its voltage-current characteristics (hyperbola-type), source and carriers of current (mostly electrons), and spatial potential distribution (a considerable electric field across the gap and a steep potential fall near the cathode)     

Acceleration of a multicomponent plasma in the cathode region of a vacuum arc

A general solution to the problem of the steady-state spherical expansion of a current-carrying multicomponent plasma into a vacuum is derived. It is shown that, in vacuum arc discharges, the main force accelerating the cathode material, which becomes a plasma at distances of 1 to 300 μm from the cathode surface, is the electron pressure gradient force maintained by Joule heating. It is established that ions of different charges move with the same hydrodynamic velocity, which is uniquely determined by the mass and mean charge of the ions and the maximum electron temperature in the cathode region.     

Droplet spot as a new object in physics of vacuum discharge

It is established experimentally that the burning of a low-current (several and tens of amperes) pulsed (microseconds) vacuum discharge is accompanied by the formation of plasma microbunches around some of the droplets leaving the cathode spot. The parameters of these bunches (electron concentration n _e～10²⁶ m^?3 and equilibrium temperature T _e～1 eV) are close to the parameters of cathode-spot plasma. The data obtained suggest that the initial temperature of droplets and the thermionic emission from them play a key role in the formation of such plasma microbunches. By analogy with the well-known cathode and anode spots in vacuum discharges, these droplet plasma formations are classified as “droplet spots.” This work reports the first results on studying the formation dynamics and the characteristics of the droplet spots. It is noted that the concept of droplet spots will require a certain refinement of the plasma formation mechanism in vacuum discharges.