Heating of a current filament and formation of constrictions in a pulsed vacuum discharge

It is shown that a rapid current rise in a pulsed vacuum discharge is accompanied by enhanced compression of the current filament by its self-induced magnetic field. As a result, a constriction forms at a distance L≃1 mm from the cathode and the electron temperature increases to 10²–10³ eV at currents of order 1 kA. This behavior explains the observed increase in the degree of ion charge and the appearance of x-rays as the current pulse length decreases. The criterion for a rapid rise is the condition τ<L/V≃10⁻²⁷ s, where τ is the characteristic current amplification time and V≃10⁶ is the velocity of the cathode plasma. Pis’ma Zh. Tekh. Fiz. 24, 50–56 (September 26, 1998)     

The ion charge-current strength relationship in stationary and pulsed vacuum discharges

It is shown that the electron temperature in the region where the vacuum-discharge plasma is in equilibrium with the intrinsic magnetic field is proportional to the discharge current I and inversely proportional to the coefficient of erosion of the cathode material. For I≥300 A, this region features additional electron-impact ionization of ions coming from the near-cathode layer. As a result, the average ion charge Z increases with the current strength, the Z(I) dependence being more pronounced at a lower discharge pulse duration.     

脉冲真空电弧离子镀在不锈钢上沉积类金刚石薄膜的研究

利用脉冲真空电弧离子镀技术在3Cr13不锈钢上制备了类金刚石(DLC)薄膜,通过Raman光谱分析了膜的结构特征,采用摩擦磨损试验机测试了薄膜在不同载荷下的摩擦系数,运用划痕仪研究了膜基的结合强度.结果表明:所镀制的薄膜具有典型类金刚石结构特征,膜中ID/IG为1.33;摩擦系数随着载荷的增大而减小,载荷为5 N,转速120 r/min时的摩擦系数为0.12;Ti过渡层的引入显著地提高了膜基结合力.     

The current–voltage characteristics of high-voltage spark discharge over a dielectric surface in vacuum

Investigation of temporal dependences of the voltage and current of high-voltage spark discharge over a dielectric surface in vacuum has shown that, in the presence of a longitudinal magnetic field, delay of the leading edge of the pulse-breakdown voltage takes place and the current, in turn, is converted from a well-defined impulse into alternating oscillations. Deceleration of the breakdown process in the presence of a low-current discharge that precedes the spark has been observed.     

Production of high-velocity flows of finely divided particles using a pulsed jet diaphragm discharge in vacuum

A new method is described of producing a flow of finely divided particles using a pulsed jet diaphragm discharge (JDD) in vacuum. This method enables one to produce high-velocity (up to 30 km/s and higher) flows of stable microparticles of different chemical compositions sized 0.1 μm to 900 μm under vacuum conditions (p _vac = 10^?4–1.0 Pa). The possibility is demonstrated of employing this method for the simulation of combined impact made on the surface of optical materials by flows of micrometeoroids and finely divided particles of different sizes and velocities resulting from anthropogenic pollution of outer space in low circumter-restrial orbits.     

Pressure and discharge current dependence of production rate of single-walled carbon nanotubes by the bipolar pulsed arc-discharge method

Bipolar pulsed arc-discharge method has been studied for the efficient production of high quality single-walled carbon nanotubes (SWNTs). Gas pressure and discharge current dependence of this method has been carried out to obtain the optimum production condition. The experimental results show no cathode deposition, and almost all the sublimated carbon becomes soot containing SWNTs. The production rate increases with increasing gas pressure and discharge current. The high production rate with high quality SWNTs is obtained around the conditions of helium gas pressure, p (He) = 50 – 70 kPa and discharge current, I_d ? 55 A. The morphology of the SWNTs is measured by a TEM and the quality is analyzed by a Raman spectrometer.     

Angular dependence of the ion energy,charge, and mass distributions in surface discharge plasma

Previously, the ion energy, charge, and mass distributions in vacuum discharge plasma at the surface of polymeric insulators were studied using an analyzer oriented perpendicularly to the surface, and both multiply charged and high-energy ions were found in the plasma. This Letter presents the first experimental results on the dependence of the ion energy, charge, and mass distributions in the plasma on the particle take-off angle relative to the insulator surface.     

Field emission current and vacuum breakdown by a pointed cathode

Vacuum breakdown of very small gap in the range of 30 nm to 2 μm, between a pointed cathode of thin tungsten wire and a plane anode of stainless steel, was experimentally investigated. The experimental setup, which consists of both electrodes and precise positioning mechanisms actuated by piezoelectric devices, was installed in scanning electron microscope (SEM). Breakdown voltage decreases with decrease of gap length and/or radius of curvature at the tip of cathode. Theoretical consideration shows that the evaporation from the anode surface heated by field emission current is a dominant factor of triggering vacuum breakdown.     

Fast deposition of metallurgical coatings and production of surface alloys using a pulsed high current vacuum arc

A preliminary investigation was conducted on the metallurgical treatments produced on a planar workpiece connected as an anode in a pulsed 1 kA vacuum arc. Generally a combination of deposition and heating effects could be produced. Very high deposition rates (approximately 25 μm s^-1) are calculated on the basis of the ion flow, and even higher rates (exceeding 100 μm s^-1) are observed if part of the macroparticle flow is incorporated into the coating. The concurrent heating of the workpiece can, depending on the arcing conditions, promote interdiffusion of the coating and substrate materials forming a metallurgical bond, can melt the substrate surface in the presence of the plasma flow forming surface alloys and can induce martensite formation near the surface of steel workpieces.     

Radiometric characterization of a Penning discharge in the vacuum ultraviolet

We present a Penning discharge as a possible radiometric transfer standard source in the vacuum UV, primarily in the spectral region below 20 nm. Following the concept of Finley et al., we have designed a Penning source using NdFeB permanent magnets. Emphasis was put on simple operation, quick electrode exchangeability, and easy source readjustment. The radiant intensities of the emission lines from different ionization stages of both buffer gas atoms and atoms sputtered from the cathodes have been studied in various discharge conditions. For selected Al and buffer gas emission lines we determined the absolute radiant intensities by a comparison with the calculable spectral radiant power of the Berlin Electron Storage Ring. A comparison with data from our hollow-cathode transfer standard source is given.     

结构缺陷对电子束诱发纯铝表面熔坑的影响

用Nadezhda-2强流脉冲电子束轰击不同预变形量的纯铝表面,对铝表面的结构缺陷和轰击后产生的表面熔坑进行了系统的表征,研究了表面熔坑与结构缺陷之间的关系.结果表明,结构缺陷对表面熔坑的形成有重要的影响,熔坑的数量随着结构缺陷数量的增加而增加,并且有沿晶界和高位错密度区择优形成的趋向,结构缺陷是表面熔坑形成的萌芽.另外,强流脉冲电子束还能在晶界和滑移带上诱发出大量由于空位聚集而产生的孔洞.     

Kinetics of decolorization of azo dye by bipolar pulsed barrier discharge in a three-phase discharge plasma reactor

Removal of amaranth, a commercial synthetic azo dye widely used in the dye and food industry, was examined as a possible remediation technology for treating dye-contaminated water. Effects of various parameters such as gas flow rate, solution conductivity, pulse repetition frequency, etc., on decolorization kinetics were investigated. Experimental results show that an aqueous solution of 24 mg/l dye is 81.24% decolorized following 30 min plasma treatment for a 50 kV voltage and 0.75 m(3)/h gas flow rate. Decolorization reaction of amaranth in the plasma reactor is a pseudo first order reaction. Rate constant (k) of decolorization increases quickly with increasing the applied voltage, pulse repetition frequency and the gas flow rate. However, when the applied voltage is beyond 50 kV and increases further, increase rate of k decreases. In addition, k decreases quickly when the solution conductivity increases from 200 to 1481 microS/cm. The decolorization reaction has a high rate constant (k=0.0269 min(-1)) when the solution pH is beyond 10. Rate constant k decreases with the decrease of pH and reaches minimum at a pH of about 5 (k(min)=0.01603 min(-1)), then increases to 0.02105 min(-1) when pH decreases to 3.07. About 15% of the initial TOC can be degraded only in about 120 min non-thermal plasma treatment.     

On the mechanism of the external magnetic field action on the electron temperature and ion charge state distribution in a vacuum arc plasma

When an external axial magnetic field is applied to a vacuum arc, the radial expansion of plasma from cathode spots transforms into a plasma flow along the magnetic field, provided that the electron-ion collision frequency is smaller than the Larmor frequency. As the magnetic field strength increases, the diameter of the resulting cylindrical channel decreases. This leads to an increase in the electron temperature and the ion charge due to enhanced Joule heating of the plasma. Unlike the intrinsic azimuthal magnetic field, the external axial field only restricts expansion of the plasma, rather than compressing the plasma jet.     

The formation and decay of atoms in a pulsed low-frequency glow discharge in a chlorine-hydrogen mixture

The mechanisms of formation and decay of atoms in the pulsed low-frequency plasma of a chlorine-hydrogen mixture have been studied. The contributions of processes involving all particles of plasma that form the composition of a plasma gap are analyzed in detail. The atoms have been shown to undergo the decay on the reactor walls according to a first-order rate law. Using the pulse relaxation technique, the absolute values of the rate constants and the probabilities of chlorine (hydrogen) atom recombination in the plasma Cl₂/H₂ of variable composition have been obtained.     

On the possibility of negative ion concentration growth between pulses of discharge current in oxygen

We present a critical analysis of data available in literature on the degradation of low-pressure oxygen plasma. Special attention is paid to two effects repeatedly observed in the initial stage of plasma degradation: a sharp growth in the flux of negative ions incident on the wall and an increase in the probing current during the laser-induced photodetachment. Both these phenomena are attributed by various authors to an absolute growth in the negative ion concentration. Our analysis shows that the whole body of experimental data cannot be explained based only on the influence of bulk plasmachemical reactions without taking into account special features of the transport processes in electronegative gases. An alternative mechanism is proposed which involves a two-stage degradation of the electronegative gas plasma accompanied by a change in the diffusion regime.     

The thermodynamic properties of erosion plasma produced by diaphragm discharge in jets in vacuum

An experimental study is made of the thermodynamic properties of plasma flowing from the orifice of a plasma-forming diaphragm as a function of the size of the orifice of the diaphragm made of textolite (C₃₇H₄₇O₁₆) under conditions of a diaphragm discharge in jets in vacuum (P _init = 13.3-1.3 Pa). The data are obtained for a fixed energy supply in a wide range of variation of the diaphragm opening size, namely, the diaphragm thicknessl ₀ = 2.0–20.0 mm and the diameter of the orifice 2r ₀ = 2.0–12.0 mm. The increase of pressure and temperature in the jet is shown to slow down with an increase of the thicknessl ₀ with the constant diameter of the diaphragm orifice 2r ₀.     

The intermediate products in the degradation of 4-chlorophenol by pulsed high voltage discharge in water

Degradation of 4-chlorophenol by pulsed high voltage discharge is an intricate process involving a series of complex chemical reactions. Hydroxylation of 4-chlorophenol to form hydroquinone, 4-chlororesorcinol and 4-chlorocatechol is the first step, though a very small amount of direct cleavage products of the C₁-C₂ or C₅-C₆ bond are observed. The yield of 4-chlorocatechol is about twice as much as that of hydroquinone. Less 4-chloresorcinol is produced. The free chloride ions dropped from the 4-chlorophenol degradation can obtain reactivity again from the discharge, and react with undegraded 4-chlorophenol to form 2,4-dichlorophenol. Some ring-opened products have also been identified and their possible reaction routes are proposed. Several compounds are verified by use of authentic samples. The more stable ring-opened products are low molecular weight (LMW) acids such as formic, acetic, oxalic, malonate, maleic and malic acid. By discharging 4-chlorophenol aqueous solution for 36 min, the amount of carbons obtained from organic acids is more than 50% while that of carbons from aromatic products less than 20% in the carbons of degraded 4-chlorophenol, which is about 94% of initial carbons. After 60 min of discharge, all the 4-chlorophenol and its aromatic intermediates have been removed completely and the organic carbons are mainly presented as organic acid such as acetic and oxalate acid. At the end of the 120 min discharge, the amount of the remaining organic carbons is not more than 14% of the initial carbons.     

The formation and structure of current cells in a vacuum arc cathode spot

A physical model describing the structurization and localization of electric current in the surface layer of a cathode in the initial stage of a vacuum arc discharge is proposed. According to this model, each current cell on the cathode surface represents a dislocation cell containing a ring electron vortex with the axis perpendicular to the electrode surface. It is shown that localization of the electric current at the vortex center leads to the formation of a spatial structure of hot spots acting as precursors of the explosive emission centers, the dynamics of which determines operation of the vacuum discharge.     

The hydrodynamic theory of a positive discharge column in an axial magnetic field

The influence of an axial magnetic field on the performance of a low-pressure cylindrical positive discharge column is studied from the hydrodynamic point of view. It is shown that the magnetic field affects the distribution of the plasma density, its speed, and the energy of electrons. The energy of electrons, the concentration and the speed of plasma, and the azimuth speed of electrons and ions as functions of the radius have been found for a helium atom in a magnetic field of varying intensity. It has been noticed that the electron and ion azimuth movement equations should account for inertia. The obtained hydrodynamic results significantly deviate from the ones obtained in the wide-spread diffusion model of a positive column. It is shown that the distribution of plasma concentration and the radial speed in the positive column are generally close to the results using the diffusion approach, if the axial inductance of the magnetic field and the gas density are increased. However, major differences are found near the walls.