Influence of the tip effect of a carbon nanostructure on low current electrical arc initiation

During their setting off, circuit breakers and vacuum switch devices are accompanied by an electric arc whose physical and chemical properties are governed by emissive current sites at the cathode surface called cathode spots. Assuming the continuity of the current on the cathode surfaces, the cathode spots contribute to supply the electrical arc by electron emission and material ejection. Thus they cause erosion on the contact electrodes inducing a dysfunction of contactors. The apparition of these cathode spots is due essentially to dust particles and surface irregularities at different scales. This experimental work represents a contribution to the understanding of the electrical arc behaviour by an investigation at submicronic scale on the influences of nanostructured carbon tips on the electric arc initiation and the cathode spots birth.     

Anodic current density of an electric arc with carbon electrodes

We present the results obtained in measuring the current density of anode spots in a carbon arc in air and we show its relationship to the magnitude of the arc current, analogous to the cathode spot.     

Anode Mode of Vacuum Arc in a Multirod Electrode System

The effect of the plasma of a high-current vacuum arc on the anode rods in a rod electrode system, which leads to the melting of the surface of copper anode rods and to the formation of droplets of the electrode material, is investigated. High-speed photography is used to register the formation of anode spots, and the sizes of the spots are determined at discharge currents from 30 to 100 kA. A one-dimensional nonstationary model of the melting of the anode and of the formation of droplets in the anode spot of a vacuum arc is proposed. The model is in satisfactory agreement with the results of observation of traces of melting on the anodes of used dischargers in a rod electrode system and with high-speed photographs of glow of the discharge plasma and spots on electrodes in a model of the rod electrode system.     

The effect of axisymmetric two-dimensional magnetic field on the configuration of vacuum-arc plasma

An experimental investigation is made of the effect of axisymmetric two-dimensional magnetic field on the forming of plasma and on the configuration of cathode spots in a vacuum-arc discharge. It is demonstrated that a magnetic field with a transverse (relative to the discharge axis) component has a significant effect on the shape of plasma column and on the rate of expansion of the cathode spot region. In a magnetic field, arc plasma has the form of truncated cone expanding toward the anode. The cathode spots take up a part of the cathode area which decreases with increasing magnetic field. Arguments are given in support of the assumption that the arrangement of cathode spots and the form of arc plasma are defined by the minimum principle similar to the Steinbeck principle. In so doing, the displacement of spots is caused by their emergence in a new region corresponding to a lower arc voltage. Also discussed is the mechanism associated with retrograde motion of cathode spot in view of the effect of azimuthal magnetic field on the axial component of current and of the effect of axial magnetic field on the azimuthal component of current.     

Products on electrodes in an argon-methane magnetically rotating arc at atmospheric pressure

The arc discharge method in a hydrocarbon atmosphere has a promising application in the nanocarbon synthesis field due to its high yield. In this paper, products on electrodes in an argon-methane magnetically rotating arc at atmospheric pressure are investigated. Different nanocarbons are obtained on the cathode and anode surfaces, respectively. Material analysis indicates that the products on the cathode are mainly carbon tubes. Some carbon tubes are well-aligned in one direction, and the others have a coil-like structure. The products on the anode are a mixture of graphene nano-flakes and aggregate hollow carbon particles. The hollow carbon particles exhibit a chain-like structure with internal space to be connected. To the authors’ knowledge, this is possibly the first time that aggregate hollow carbon particles with a chain-like structure have been observed in the absence of any catalyst. Based on the configuration of arc plasma, the formation mechanism of electrode products is briefly proposed.     

Dynamics studies of cathode spots in a vacuum-arc discharge with ring electrodes

The effect of an external pulse magnetic field with axial and radial components on electric characteristics of the discharge and the configuration of cathode spots of a vacuum arc discharge with ring electrodes is studied experimentally. For arc currents within the range of 0.05–2 kA, shots of the cathode spots at different instants of time are obtained. The dependences of the number of the spots on the discharge current and the mean current per spot are determined. It was found that the expansion rate of the cathode spots area depends significantly on the instant value of the discharge current. It is shown that, when the pulse magnetic field is applied, the discharge voltage increases and the discharge current and number of the cathode spots decreases. It was found that the current interruption is a probability process. The probabilities of the current interruption depending on the maximal value of the external pulse magnetic field induction are determined.     

The Effect of External Magnetic Field on a Vacuum Arc

The effect of an external longitudinal magnetic field on the time required for anode spot formation in a high-current (5 to 12 kA) vacuum arc is experimentally studied, and the minimal value of a magnetic field inhibiting the anode spot formation is determined. The experimental results are compared with theory. The phenomenon of break of current in a low-current (100 to 300 A) vacuum discharge upon superposition of a magnetic field with transverse component on the discharge is investigated. The probability of break of current increases with the magnetic field. The possible mechanism of break of current is discussed.     

Protecting the anode foil of a high-current electron accelerator against arc-discharge damage

The mechanism of anode foil damage during the extraction of a high-power pulsed electron beam from a high-current diode has been experimentally studied on a TEU-500 electron accelerator [1]. It is established that the breakage of the anode foil is caused by the appearance of cathode spots on its surface, the intense electron emission from these spots during positive voltage pulses (postpulses following the main negative pulse of accelerating voltage), and the formation of arc discharge in the interelectrode gap. The improvement of diode matching to the pulse-forming line of the accelerator and the use of an auxiliary electrode (anode) forming additional vacuum discharge gap (crowbar) with the cathode practically excludes the anode foil breakage by arc discharge and significantly increases the working life of the foil (up to ∼10⁵ electron beam pulses).     

Electric field-oriented growth of carbon nanotubes and Y-branches in a needle-pulsed arc discharge unit: mechanism of the production

Carbon nano-onions, multiwall carbon nanotubes and Y-branched nanotubes are synthesised in a simple production apparatus. A pulsed plasma is generated by discharging a high voltage needle pulse between two graphite electrodes. A strong electric field is presented along anode and cathode electrodes. The pulse width is 0.3 μs. Acetone vapour, as a precursor, is introduced to the plasma through a graphite nozzle in the cathode assembly. A magnetic field, perpendicular to the plasma path, is provided. The possibility of carbon nanotube production through a short-pulsed arc discharge technique is investigated in this article. The results show that adding an electric field between electrodes prevents carbon ions’ dispersion, facilitates charge transferring between ions and electrodes, orients the growth of carbon nanotubes along the applied electric field and finally makes it possible to produce functionalised carbon nanoparticles such as Y-branch nanotubes and nanoknees. In this work, the growth mechanism of carbon nanotubes in a needle-pulsed arc-discharge reactor is discussed. And a possible explanation is provided for the synthesis of Y-branch carbon nanotubes. The products are examined by using scanning probe microscopy technique.     

Multiple Arc Contraction in the Anode Region

Magnetogasdynamic equations are used to calculate the characteristics of an open high-current arc burning between a tapered cathode and a flat anode in argon at atmospheric pressure for different values of current strength and interelectrode spacing. The possibility is established of splitting the arc in the anode region into current-conducting channels with contracted connections to the electrode surface. The calculation results are qualitatively consistent with the experimental data.     

阴极弧斑放电的机理分析

介绍了阴极弧斑放电的基本特征。参考前人的研究模型和最新的实验结果,系统地分析了阴极弧斑放电的机理和过程：电子的爆裂发射模型;热场致发射理论;阴极电位降区满足的Ｍａｃｋｅｏｗｎ 公式;阴极弧斑在磁场中的运动机理。最后讨论了气压对弧斑运动的影响,解释了阳极弧斑放电特征。     

Cathode spot movement on a continuous carbon fiber reinforced Cu matrix composite in vacuum

The cathode spot movements on a carbon fiber reinforced copper matrix composite (C_f/Cu) were investigated by a scanning electron microscope and a digital high-speed video camera. The composite was prepared by infiltrating a Cu alloy doped with Ti and Cr into a porous carbon/carbon composite in vacuum. It is found that the carbon fibers have a higher ability to withstand the vacuum arc erosion than the Cu matrix. The cathode spot walks randomly on the matrix, rather than on the carbon fibers. The cathode spots are restricted on the Cu matrix in C_f/Cu. The directional movement in the high-speed images is caused by the restricted random walk of the cathode spots. A gradient distribution in the size and density of the cathode spot is present on the Cu matrix. The average arc spreading velocity is estimated to be about 0.36 m/s and the transient arc spreading velocity is in the range of 0.12–0.7 m/s.     

Analysis of metrological potentialities of a high-current arc in plasma generators with a diverging channel

The paper is concerned with substantiating a high metrological “quality” of the cathode section of a stationary electric arc in plasma generators which have a current density of over 2 kA/cm², a diverging anode channel, and vortex stabilization. An automated spectroscopic system and a system for fast visualization of plasma filament are used to demonstrate the high reproducibility and stability of the electrical parameters of the arc and of the space and time radiating characteristics of highly ionized plasma of the cathode region of the discharge. The results of analysis of the thermodynamic state of plasma in the cathode region of the discharge point to the validity of the conditions of partial local thermodynamic equilibrium in this region. The use of the developed system of automatic acquisition and processing of spectroscopic information enables one to obtain unique (as regards the scope and accuracy) data on the radiating characteristics of lines of atoms and ions of different multiplicity.     

Effect of the Angular Distribution of Cathode Ions on the Current Flow in a High-Current Vacuum Arc

The effect of the angular distribution of cathode ions on the parameters of a vacuum arc is investigated. It is shown that, at current densities characteristic of a high-current arc, collisions of flows of fast ions from different cathode spots may result in a high value of the ion temperature at the cathode plasma boundary. The boundary in current density of the region of a stable current flow as a function of the width of angular distribution of cathode ions is determined. It is found that, in the region of supersonic flow of ions, the permissible value of the current density drops rapidly as the angular distribution width increases, and in the subsonic region, it rises rapidly. An expression is obtained for the maximum possible ion sound velocity and for the equivalent critical ion velocity. Two- and three-fluid hydrodynamic models are used to perform calculations that illustrate characteristic distributions of parameters in the discharge gap. Analysis of the calculation results makes it possible to discuss the characteristic features of current flow in a vacuum discharge.     

A multichannel discharge between a stream electrolytic cathode and a metal anode at atmospheric pressure

An experimental investigation is performed of the structure and current-voltage characteristics (CVC) of a multichannel discharge (MD) between a stream electrolytic cathode (SEC) and a metal anode, as well as of the density of SEC and anode current in a wide range of values of current I = 0.02–10 A, interelectrode spacing l = 5–50 mm, and anode diameter d _a = 5–40 mm. The development of cathode spots on the SEC surface is studied. It is found that a cathode spot of SEC may have various shapes and structures (regular circle, horseshoes, filamentary spots, and so on). The CVC of a multichannel discharge between SEC and metal anode depends significantly on I, l, and d _a , as well as on the material of the anode and on the composition and concentration of electrolyte. The dependence of the density of SEC current on discharge current is nonmonotonic. It is for the first time that a bundle-like MD between SEC and metal anode and multichannel and diffuse discharges between the plasma region and stream electrolytic cathode are observed, and their characteristics are investigated.     

Numerical simulation of argon twin torch plasma arc for high heating efficiency

Plasma arc heating technology has been applied for volume reduction through melting of bottom and fly ash, and for producing slag. Recently, a twin torch plasma arc, which has two torches at the cathode and anode, has been anticipated for application to disposal of medical waste because it can treat a wide area and can treat non-conductive materials. For this study, a numerical simulation model of a twin torch plasma arc at opposite electrodes was developed to elucidate high-efficiency heating using a twin torch plasma arc. It is defined as Local Thermal Equilibrium (LTE) and calculated in Magneto-Hydro-Dynamic (MHD) equations. Furthermore, the temperature distribution and conditions of high heating efficiency with the radiation loss were addressed and compared to those of a single torch. The heating efficiency decreases with increasing radiation efficiency because of the temperature increment caused by the current and input power. The radiation efficiency of a twin torch is about 5% higher than that of single torch.     

The Effect of the Flow of Atoms Sputtered from the Anode on the Flow of Current in a Vacuum Arc

The paper deals with the problem on the effect of atoms sputtered from the anode on the flow of fast cathode ions in a vacuum discharge at relatively low (100 A/cm²) values of current density. The problem is solved using a model of instantaneous ionization and a model allowing for the finite ionization length. The calculations result in determining the boundary of the region of stable passage of current depending on the density of arc current, the density of the flow of sputtered atoms, and on their velocity. It is demonstrated that, in the studied range of currents, it is impractical to include the fast ion pressure in the equations of motion and balance for energy, because these ions hardly collide with one another during the time of their transit through the discharge gap. The calculation results enable one to determine the range of validity of the model of instantaneous ionization beyond which one must allow for the dynamics of the process of ionization of atoms in the anode region.     

Carbon Nanotubes Grown on the Surface of Cathode Deposit by Arc Discharge

Carbon nanotubes prepared by de arc discharge of graphite electrodes in He and CH₄ gas took markedly different morphology. Thick nanotubes embellished with many carbon nanoparticles were obtained by evaporation under high CH₄ gas pressure and high arc current. Thin and long carbon nanotubes were obtained under a CH₄ gas pressure of 50Torr and an arc current of 20A for the anode with a diameter of 6mm.     

Metallurgical characterization of arc-conditioned carbon coating on copper electrodes

During arc discharge between two parallel electrodes, the electrode surface is eroded by the arc. Carbon coating of the electrode surface often results in a decreased erosion. The surface structure of the arc-conditioned carbon-coated copper electrode was metallurgically characterized by the use of (1) scanning electron microscopy and (2) Auger in-depth profiling. Two different types of a.c. conditioning with varying pulse characteristics were studied.It was found that the peak current density, the pulse duration and the total number of pulses were the most important parameters in determining the metallurgical structure of the conditioned electrode surface. The vapor arcs which were produced by pulses with a high current density and short pulse duration were most effective in compacting the carbon coating and promoting good adhesion between the carbon and the copper electrode. The compaction and the adhesion are effected by a high ionic beam pressure on the cathode reaching up to 1000 atm.A lower degree of carbon penetration was effected when surface melting was produced by thermionic arcs which have a lower current density and longer pulse duration. In this case, carbon adhesion is due to the mechanical entrapment of carbon powder in the thin molten copper layer produced on the electrode surface by the arcs.