Two-dimensional kinetic model of short high-current vacuum-arc discharge

A two-dimensional mathematical model of a short high-current vacuum-arc discharge is developed, according to which magnetized electrons move in a hydrodynamic regime and fast cathode ions propagate in a free flight regime in a two-dimensional electric field. The proposed model takes into account the distribution of ions with respect to their escape angles from the cathode plasma boundary. A method for calculation of the plasma density distribution in the interelectrode gap is proposed. Two-dimensional distributions of the plasma density, electric field, and discharge current density in an external magnetic field are calculated. It is shown that ion trajectories exhibit mutual intersections, partly return to the cathode, and partly rotate in the oppositely oriented electric field at the side boundary of plasma. A decrease in the applied magnetic field intensity leads to a decrease in the number of ion trajectories reaching the anode (ion starvation), which can result in the violation of a stationary current transfer.     

A theoretical study of the impact of magnetic field of Hall current on the parameters of a high-current vacuum-arc discharge

It is demonstrated that the magnetic field of Hall current in a short high-current vacuum-arc discharge significantly distorts the external axial magnetic field and affects the distribution of current density in the discharge gap. This effect decreases with increasing external magnetic field, with decreasing arc current, and with decreasing ratio of the length of discharge gap to its transverse dimension. A 2D magnetohydrodynamic mathematical model is used to calculate the discharge parameters for different values of induction of external magnetic field. The calculation results are compared with similar results obtained using a 1.5D model in which the impact made by magnetic field of Hall current is ignored. It is inferred that the simpler 1.5D model may be employed in calculations of parameters of vacuum arc in a wide range of variation of arc current and of induction of external magnetic field.     

An experimental study of the effect of nonuniform axisymmetric magnetic field on the breaking of the current of a vacuum-arc discharge

An experimental investigation is made of the effect of breaking of current in a vacuum-arc discharge as a result of superposition of a pulsed axisymmetric magnetic field with longitudinal and transverse components. A study is made of the dependences of the probability of breaking of current on the current magnitude, the pulse amplitude of the magnetic field, and the value of emf of the electric circuit in the current range of 100 to 600 A. The main factors affecting the probability of breaking of current are analyzed.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 850–855.Original Russian Text Copyright © 2004 by E. F. Prozorov, K. N. Ulyanov, and V. A. Fedorov.     

Minimum voltage principle in the theory of a high-current vacuum-arc discharge

The effect of an inhomogeneous magnetic field with axial and radial components on the structure of a high-current vacuum-arc discharge (VAD) has been theoretically studied. The characteristic features of current passage in a short VAD are considered using analytical expressions for the slope of the total current lines (TCLs) relative to the discharge axis, the axial components of the electric and magnetic fields, and the effective conductivity of discharge plasma. A two-dimensional mathematical model has been used to calculate the TCL shapes and the discharge voltage for various dimensions of the region of cathode spots. Calculations showed that the voltage drop on the discharge gap as a function of the arc discharge radius on the cathode has a minimum. The shape of TCLs and the arc radius on the cathode at this minimum agree with the available experimental data.     

Critical flow of high-speed ions in a high-current vacuum-arc discharge

A physical model of vacuum arc is developed, which is used to construct a quasi-one-dimensional model and a two-dimensional two-fluid mathematical model. The two-dimensional magnetohydrodynamic model is based on the method of trajectories, according to which a set of partial equations is reduced to a set of ordinary differential equations written for derivatives along the lines of current. The quasi-one-dimensional model is based on the principle of compensation of radial magnetic forces, which is valid for a short high-current vacuum arc in axial magnetic field. A criterion is obtained, which defines the range of validity of the quasi-one-dimensional model in external axial magnetic field B ₀. Calculations are performed of the dependence of critical current on B ₀ for discharge gaps of different geometries. The calculation results agree with experiment.     

Plasmachemical synthesis of aluminum-based nitride compounds in vacuum-arc discharge plasma

A vacuum-arc plasmachemical synthesis of aluminum-based nitride compounds (AlN and Ti-Al-N) is described. The compositions and structures of the obtained coatings have been studied. It is shown that the composition and structure of a coating can be effectively controlled by varying the discharge current, ion current density distribution, and gas pressure, provided that the plasma flow is free of droplets.     

An experimental study of the dynamics of formation of plasma in a vacuum-arc discharge

An experimental study is made of the dynamics of formation of plasma in a vacuum-arc discharge (VAD) at different currents and different transverse dimensions of CuCr50 electrodes. High-speed multipleframe photography is used to obtain the time dependence of the current channel diameter and determine the rate of plasma expansion. It is demonstrated that the rate of expansion under the experimental conditions is independent of the discharge current, as well as of the presence or absence of external axial magnetic field. Two modes are identified, which differ from one another by the rate of expansion. Experiments are performed with anodes and cathodes of different diameters in the range of currents from 2 to 15 kA. It is demonstrated that, with electrodes of different sizes, a discharge initiated at the cathode center expands with time to the size of the smaller electrode while retaining the cylindrical shape. The anode temperature is calculated for two modes in view of the variation of the source power and of the transverse dimension of the zone of heating. It is demonstrated that the anode spot in the mode of rapid expansion is formed under insignificant heating of the anode. In the case of slow expansion, a strong heating of the anode is possible, under which its intense evaporation occurs. The anode temperature is calculated in view of the evaporation losses for modes with different numbers of jumpers arising as a result of the breaking of vacuum switch contacts.     

The effect of pulsed vacuum-arc discharge on the surface of elements of a discharger

Regularities of erosion and transport of the material of discharger elements have been studied by the methods of surface structure visualization and X-ray fluorescence elemental microanalysis. Microrelief and oxide film on the surface of cathode material are found to affect the development of arc discharge in a commutator.     

An investigation of nitrided layer prepared by direct current nitrogen arc discharge

A simple direct current (DC) nitrogen arc discharge method is presented, which allows for in situ nitriding of titanium at atmospheric pressure. The microstructure and microhardness of the nitrided layer and effects of the arc discharge current were investigated. The nitrided layer was mainly composed of TiN dendrites and small amounts of TiN_0.3. The density and size of the TiN dendrites gradually decreased from the surface towards the titanium substrate. The layer had a good adhesion with titanium. With an increase of the arc discharge current from 40 to 80 A, the TiN dendrites coarsened, the layer thickness and amount of TiN increased and the layer hardness enhanced. The nitrided layer with the highest hardness value of 1600 HV and thickness of 1800 μm was obtained for an arc discharge current of 80 A.     

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.     

Amorphous carbon films in direct current magnetron sputtering from regenerative sooting discharge

We present results of carbon coatings on metal substrates in cylindrical hollow cathode (CHC) direct current magnetron sputtering. This is a new technique for making amorphous carbon films by CHC magnetron sputtering from a regenerative sooting discharge. The carbon films are deposited on Cu and Al substrates in a Ne atmosphere and compared with the films of carbon soot on the same materials produced from a conventional 80A arc discharge between graphite electrodes in a He background gas. Raman spectroscopy reveals the existence of graphite and diamond-like structures from the arc discharge while in CHC magnetron sputtering, graphite-like structures are dominant. X-ray diffraction data of samples from the arc discharge show nano-size precipitates of Al₄C₃ of rhombohedral and hexagonal form for the aluminium sample and probable formation of diamond and hexagonal carbon in copper whilst in magnetron sputtering we obtain amorphous carbon films. Scanning electron microscope images of the surface show a collection of loose agglomerates of carbon particles in the arc discharge whereas, for magnetron sputtering, structures are regular with smooth edges and fine grains.     

Ionization mechanism in a direct current discharge plasma in an argon-oxygen mixture

We present the results of calculating the charged particle formation and decay rates in a dc discharge positive column in argon-oxygen mixture. The calculations are based on a combined solution to the Boltzmann equation; vibration kinetics equations for the ground state of the O₂ molecule; and chemical kinetics equations for Ar and O atoms, O₂ molecule, and their excited states under the experimentally determined reduced electric field strength, E/N, for various gas phase compositions, currents, and pressures. The role is clarified of particular components in the ionization processes. Electron and positive ion decay is shown to take place predominantly at the reactor walls on which they fall due to joint diffusion. The negative oxygen ions affect the diffusion rate.     

Rare Earth oxide equilibria in pulsed direct current glow discharge mass spectrometry

Glow discharge mass spectrometry is used to examine the equilibria existing between La(+) and LaO(+). A pulsed discharge permitted temporal comparison of spectra taken at varying intervals after discharge initiation. Postdischarge peaks are observed for both atom and oxide ions. By varying the pulse period while sustaining a fixed "on" time, the degree of deposition of gaseous constituents on the cathode surface can be controlled. Injection of normal water and isotopically labeled water for compacted and noncompacted samples allows insight into the source of water signals.     

Atmospheric pressure air direct current glow discharge ionization source for ion mobility spectrometry

A new atmospheric pressure air direct current glow discharge (DCGD) ionization source has been developed for ion mobility spectrometry (IMS) to overcome the regularity problems associated with the conventional (63)Ni source and the instability of the negative corona discharge. Its general electrical characteristics were experimentally investigated. By equipping it to IMS, a higher sensitivity was obtained compared to that of a (63)Ni source and corona discharge, and a linear dynamic range from 20 ppb to 20 ppm was obtained for m-xylene. Primary investigations showed that alkanes, such as pentane, which are nondetectable or insensitively detectable with (63)Ni-IMS, can be efficiently detected by DCGD-IMS and the detection limit of 10 ppb can be reached. The preliminary results have shown that the new DCGD ionization source has great potential applications in IMS, such as online monitoring of environment pollutants and halogenated compounds.     

Liquid sample injection using an atmospheric pressure direct current glow discharge ionization source.

An atmospheric pressure DC glow discharge in helium has been used as an ionization source for organic samples introduced by liquid injection into atmospheric pressure ionization mass spectrometry (API/MS). The glow source operates typically in the range up to 1 mA of current at less than 1 kV, although the source can be operated up to a discharge current of 10 mA. Even at the high current used in this work, the protonated molecule, MH+, is observed with little or no fragmentation for many of the samples studied. The detection limits achieved for API glow discharge detection are typically in the low femtomole region for small organic molecules including small biological neurotransmitters, drugs, pesticides, phenylthiohydantoin-substituted amino acids, and explosives. A detection limit of approximately 2 pg has been achieved for tyramine with linear quantitation over at least 3 orders of magnitude. The sensitivity in these experiments has been further improved by optimization of the skimmer-interface system and the liquid injection/nebulization design.     

Grid-controlled current switching in cathode-spot discharge

The first results of an investigation of the complete current switching (initiation and quenching) using a fine-mesh grid in discharge with a cathode spot on liquid cesium are reported. Experimental data show the possibility of using this method of control at current densities within 5–25 A/cm² in the grid plane.     

Stoichiometry, phase composition, and properties of superhard nanostructured Ti-Hf-Si-N coatings obtained by deposition from high-frequency vacuum-arc discharge

Superhard nanostructured Ti-Hf-Si-N coatings (films) possessing high mechanical properties were obtained by deposition from high-frequency vacuum-arc discharge. The elemental and phase composition and morphology of the films were studied by a combination of methods including RBS, SIMS, GDMS, SEM-EDXS, XRD and nanoindentation techniques at various pressures and bias voltages applied to the coated samples. It is established that, as the average grain size in nc-(Ti,Hf)N (nanocrystalline) coatings decreases from 6.7 to 5 nm and a-Si₃N₄ (amophous or quasi-amorphous phase) interlayers are formed between the nanograins, the nanohardness of coatings increases from 42.7 to 48.4–1.6 GPa. However, the further grain refinement of nc-(Ti,Hf)N to 4.0 nm leads to a slight decrease in the nanohardness. The stoichiometry of the coatings changes from (Ti₂₅-Hf_12.5-Si_12.5)N₅₀ to (Ti₂₈-Hf₁₈-Si₉)N₄₅, which is accompanied by variation of the lattice parameter of (Ti,Hf)N solid solution grains.     

The effect of axial magnetic filed on the process of anode spot formation in a vacuum-arc discharge with CuCr50 electrodes

An experimental study is made of the effect of an external axial magnetic field on the process of anode spot formation in a pulsed vacuum-arc discharge in the range of currents from 5 to 12 kA in a discharge gap with CuCr50 electrodes. The times and currents, at which an anode spot is formed, are determined for each amplitude value of current depending on the magnitude of magnetic field. The minimal value of magnetic field preventing the anode spot formation is determined for each current. The measured values of diameters of the current channel are used to calculate the anode temperature. It is demonstrated that, under experimental conditions, the heating of anode is insufficient for marked evaporation, and the anode spot formation is associated with the critical flow of fast cathode ions.     

Effect of N2/Ar gas flow ratios on the nitrided layers by direct current arc discharge

Nitrided layers were prepared on titanium substrates by direct current (DC) arc discharge technique. The effect of nitrogen to argon flow ratios on the microstructures and mechanical properties of the samples was investigated. The X-ray diffraction results show that the surfaces of the layers consist mainly of TiN and a very little TiN_0.3 (101) under the condition of nitrogen/argon gas mixture. A strong TiN (200) preferential orientation in nitrided layers can be seen. With increasing the nitrogen concentration ratio in nitrogen/argon gas mixture, the density and size of TiN dendrites in the nitrided layers, and the hardness and wear resistance of the layers enhanced significantly.