Mode Of Arc Attachment Research Articles

Arc dynamics in a vortex-stabilized non-transferred plasma torch with a tangential gas feed

The mode of anode arc attachments in a thermal plasma torch, such as constricted arc, multi-arc or diffused arc attachment, has operational as well as economic consequences on the use of plasma torches in industrial applications. The mode of arc attachment is strongly dependent on the gas flow dynamics, gas vortex formation and wall energy exchange. To gain more insight into arc attachment behaviour, this article presents a transient flow plasma model implemented using OpenFOAM computational fluid dynamics code to study the effect of gas swirl and arc rotation on modes of arc attachments. A tangentially fed plasma gas creates a gas swirl that helps to induce the arc rotation. The investigations were carried out at arc currents between 200–450 A and input gas flow rates in the range of 5–20 slpm. High gas swirl and low arc currents are found to favour constricted attachment on the anode surface. The local gas vortex driven by the plasma swirl creates hot spots inside the torch that dictate the arc attachment. The transition of arc attachment from the diffuse to constricted mode was simulated by imposing a step-change in the gas flow which shows the evolution of different arc attachment behaviour. During the transition from constricted to diffused attachment, the arc tends to undergo a transitionary multi-attachment mode at specific selected current and flow parameters.

Numerical simulation of constricted and diffusive arc–anode attachments in wall-stabilized transferred argon arcs

A numerical simulation is conducted to investigate arc–anode attachment behavior, especially the formation mechanism of the constricted arc attachment mode for the water-cooled anode of wall-stabilized transferred argon arcs. Argon molecular ions and the corresponding kinetic processes are included to the finite-rate chemistry model in order to capture the chemical nonequilibrium characteristics of the arc near the anode region. Modeling results show that constricted and diffusive arc–anode attachments can be self-consistently obtained at different arc currents while keeping other parameters unchanged. The dominant kinetic processes contributing to ionization and recombination in the arc center and fringes are presented. The results show that in arc fringes and the arc attachment region, molecular ion recombination plays an important role which leads to the rapid loss of electrons. The radial evolution of the production, loss and transport processes of electrons is further analyzed. It is found that for the constricted arc attachment mode, both the recombination and convection transport caused by the anode jet result in the loss of electrons at the arc fringes, which leads to the shrinkage of the arc column at the anode. The formation of the anode jet is due to the combined action of radial and axial Lorentz forces in the anode region.

Chemical Non-equilibrium Simulation of Anode Attachment of an Argon Transferred Arc

The diffuse and constricted arc attachment modes to the anode of an argon wall-stabilized transferred arc are investigated using a 2D chemical non-equilibrium model. Detailed argon chemical kinetic processes and self-consistent effective binary diffusion coefficient approximation treatment of diffusion are coupled with a single-fluid, two-temperature model to investigate the plasma characteristics of the near-anode region. The diffuse and constricted arc attachment modes to the anode are obtained self-consistently by changing the gas flow rates. The results show that the high current density and associated self-induced magnetic field of the constricted arc attachment give rise to a large Lorentz force near the anode, resulting in an anode jet. The convection process dominated by the cathode jet is conducive to the radial transport of electrons in the diffuse arc attachment mode, while in the constricted arc attachment mode the direction of electron transport due to the convection caused by the anode jet is opposite to that in a diffuse arc attachment. The effect of diffusion on electron transport is relatively small in the arc central region, while it becomes important in the arc fringe. The recombination processes, especially the dissociative recombination reactions, restrict the electron radial diffusion in the arc fringe. The diffusion current density is the main component of current density in front of the anode due to the steep gradient of electron pressure. It is found that when the diffusion current is not considered, the electric field and Joule heating in front of anode promote a more constricted arc attachment.

Change of the arc attachment mode and its effect on the lifetime in automotive high intensity discharge lamps

In modern cars, the new generation Hg-free high intensity discharge (HID) lamps, the so called xenon lamps, take an important role. The long lifetime of these lamps is achieved by doping the tungsten electrodes with thorium. Thorium forms a dipole layer on the electrode surface, thus reducing the work function of tungsten. However, thoriating the electrodes is also an issue of trade and transport regulation, so a substitute is looked into. This work shows the influence of the arc attachment mode on the lifetime of the lamps. The mode of the arc attachment changes during the run-up phase of automotive HID lamps after a characteristic time period depending, i.e., on the filling of the lamps, which is dominated by scandium. It will be shown that this characteristic time period for the change of the attachment mode determines the long term performance of Hg-free xenon lamps. Measurements attributing the mode change to the scandium density in the filling are presented. The emitter effect of scandium will be suggested to be the reason of the mode change.

Simulation of the hot core mode of arc attachment at a thoriated tungsten cathode by an emitter spot model

Recently, a constricted attachment of an atmospheric pressure low-current argon arc in the centre of the flat end face of a thoriated tungsten cathode was observed and spectroscopically analysed. Its diameter of 0.6 mm and its length of the free standing part of 10 mm are the typical dimensions of electrodes for high-intensity discharge lamps. This paper gives a physical interpretation of the axially symmetric arc spot by a simulation of its properties with a cathodic sheath model which takes into account a reduction in the work function above a critical temperature of the cathode surface by a thorium ion current.At first the optical observation and spectroscopic investigations are recapitulated. Then, an overview is given on the essential elements which are needed to simulate the cathodic arc attachment on a hot electrode. A simulation of a central cathode spot with these elements gives results which are far away from the experimental findings if a constant work function φ is used. Therefore, a temperature-dependent work function φ(T) is introduced. This φ(T) transitions from 4.55 to 3 eV above temperatures of the order of 3000 K. With this emitter spot model a constricted arc attachment is obtained by simulation in the centre of the flat end face of the cathode in accordance with experiment. For currents below iarc,max ≈ 15.5 A, two spot solutions with different cathode falls are found. They form a current–voltage–characteristic consisting of two branches which extend from a turning point at iarc,max to lower currents. For iarc > iarc,max, only a diffuse mode of cathodic arc attachment is obtained. It is shown by a comparison with measured data for iarc = 7.5, 10, 12.5 and 15 A that the solution with the lower cathode fall is observed experimentally.

Temperature measurements at thoriated tungsten electrodes in a model lamp and their interpretation by numerical simulation**The paper is dedicated to the memory of the late Dr G M J F Luijks in appreciation of his outstanding pioneering work on high pressure discharge lamps, particularly his contribution to an understanding of the gas phase emitter effect in these lamps in cooperation

An atmospheric pressure argon arc is operated with dc currents of different amplitudes in a model lamp between electrodes made of pure and thoriated tungsten. Temperature measurements are performed at these electrodes with a CCD camera being calibrated at λ = 890 nm in absolute units of surface radiance and an interference filter for this wavelength. Temperature distributions are deduced from the CCD camera records of the electrodes assuming that they are grey body radiators. The records show a diffuse mode of attachment at the cathode. Doping the electrode with ThO2 causes a reduction in the cathode temperature by an amount of the order of 1000 K. On the other hand the anode temperature is weakly increased by a doping with ThO2. A reduction in the work function ϕ of the cathode from 4.55 to 3 eV is found by a comparison with cathode temperatures obtained by a numerical simulation of the diffuse mode of arc attachment with a well established cathode boundary layer model. Moreover, it is noted that the reduction is independent of the amount of ThO2 by which the electrode material is doped indicating that the work function of thoriated cathodes is the result of a self adjustment to the work function minimum at a thorium coverage of Θ ≈ 0.5. The weak influence of ThO2 on the anode temperature shows that the average work function of the anode does not depend on the thorium content of the electrode. The results are explained by a thorium ion current, by which evaporated thorium is repatriated to the cathode surface.

Development of the arc attachment at HID lamp electrodes in the range from low to RF-frequencies

At cathodes of high intensity discharges two different modes of arc attachment are observed: a diffuse mode characterized by a low current density and a uniformly distributed high global temperature, and a spot mode with a high current density and a high local but low global surface temperature. For dc and low frequency operation an investigation of these different modes has already been presented. Different modes of arc attachment are shown in the case of RF operation (up to 1 MHz operation frequency) of the Bochum model lamp. Images of the arc attachment demonstrate that with increasing frequencies the occurrence of the spot mode is reduced. But it cannot be excluded that it arises accidentally at high frequencies. It is found that with increasing frequency the differences between the electrode tip temperature in the cathodic and the anodic half period are levelled and that the course of the electrode sheath voltage (ESV(t)) adjusts to the sinusoidal current waveform resulting in an increase in the average power input into the electrodes with proceeding adjustment. Moreover, it indicates a reshaping of the electrode boundary layers.

Experimental investigations of the anode boundary layer in high intensity arcs with cross flow

The anode boundary layer in high intensity arcs determines the anode lifetime and processing efficiency in many industrial applications and can be controlled by cold lateral gas flows (LGFs) parallel to the anode. In this paper, properties of the anode boundary layer in a high intensity argon arc with argon and nitrogen LGFs have been studied. Anode heat and current transfer and restrike frequency and restrike length have been measured using a segmented anode and a high speed camera. Electron temperature, electron density and current density have been measured by a Langmuir probe setup. The correlation between these properties and the previously defined arc attachment mode has been described and explained.

Different modes of arc attachment at HID cathodes: simulation and comparison with measurements

Based on a model for the plasma boundary layer of high intensity discharge cathodes, simulations are performed and compared with experimental results. To solve the power balance of the cathode body different methods are used, namely a 1D integral solution as well as 1D, 2D and 3D finite-element calculations. The simulations are done for cylindrical tungsten cathodes operated in different pure noble gas discharges (0.1–1.0 MPa) and with currents between 0.5 and 10 A. Under these conditions different modes of arc attachment are found, both in simulations and experiments.For the diffuse mode of arc attachment an excellent quantitative agreement between measurements and the simulations is obtained, reflecting an improved accuracy of measurements and simulation. In addition, different spot modes are found. At least one of these modes is also observed in the experiment. Also for this spot mode the agreement between measurements and simulation for the integral quantities is good. There are still some open questions concerning the spot mode of cathodic arc attachment.Varying the geometric dimensions of the cathode, the proper simulation of the heat conduction problem of the cathode body is shown. Variations of the plasma properties, like gas type and pressure, prove the conceptional capability of the boundary layer model for the simulation of different modes of arc attachment. Evaluating the cathode fall characteristics, regions of existence for the different modes are found, which are similar to the experiments.

Cathodic arc attachment in a HID model lamp during a current step

Changes in the cathodic arc attachment mode during a current step are studied both experimentally under well-defined and reproducible conditions, and by finite element calculations. A transition from diffuse to spot and back to diffuse attachment is observed as the response to the step. Spot formation is identified experimentally by a sharp peak in the light intensity in front of the cathode and a steep decrease in the lamp voltage. Numerical three-dimensional solutions of the thermal-conduction equation in the cathode body exposed to nonlinear surface heating give good quantitative agreement of the time course of the observed changes in attachment modes. The model that describes the heating by the near cathode layer uses a simpler equation for the ion current density than other models in the recent literature. The model describes how an excess energy flux is created in the layer in front of a spot, which cannot be transported back to the cathode by the ion current and must, therefore, be supplied to the arc. The calculated integral excess power shows a time course similar to the observed light intensity.

Observation of an extremely constricted cathodic arc attachment to electrodes of high intensity discharge lamps

Usually two modes of arc attachment to cathodes for high intensitydischarge (HID) lamps are observed, the spot mode for highpressure, low currents and large electrode diameters and thediffuse mode for the opposite conditions. Recently, a veryconstricted attachment to cathodes was observed both in real HIDlamps and in Bochum's model lamp. In the model lamp, the extremelyconstricted arc attachment was observed in argon and xenonatmosphere at a pressure p = 0.26 MPa and for currentsI = 1-6 A. Whereas the global electrode temperature andthe cathode fall of the diffuse and the spot mode differ onlyslightly, the extremely constricted arc attachment (so-called`super spot mode') is associated with a significantly decreasedglobal electrode temperature at similar parameters. The reductionin electrode temperature implies low cathode falls and lowelectrode losses, respectively. Scanning electron micrographs show that the surfacestructure of the electrodes has substantial influence on the modeof cathodic arc attachment.

Numerical modelling of a dynamic mode change of arc attachmentto cathodes of high-intensity discharge lamps

A commercial finite-element analysis code is used to calculateaxisymmetric transient temperature fields, current density distributionsand energy fluxes in the cathode of a high-intensity discharge lamppowered by a unipolar current I(t). The calculation is based on a sheathmodel of the near-cathode layer and the assumption that the resultingcathode fall voltage U(t) is spatially constant. So a calculation of thefields in the arc region is not necessary. Depending on the electrodedimensions and the frequency of I(t) hysteresis-like periodictransitions between the diffuse and spot modes of arc attachment areobserved in the numerical solutions for 0.62 MPa mercury discharges withpure, as well as thoriated, tungsten cathodes. These findings are comparedwith measurements from the literature. The simulation shows that thelatent heat has negligible influence on spot formation and that theaverage cathode power loss can be smaller for a time-varying current thanfor the direct current with equal mean value. The presented method can beused as a guide to better electrode geometries and current forms.

The nonequilibrium region of an electrode in contact with a flowing thermal plasma

A model for a high-pressure thermal plasma flowing onto an electrode in stagnation flow geometry based on a continuum description of the flow is used to establish anode current density limitations. These limitations can be overcome at high anode current densities when ionization in the nonequilibrium boundary layer supplies the required ion current to the distant plasma. Two modes of anode operation (low and high current density modes) are identified and interpreted to be a difference between a diffuse and constricted mode of arc attachment. Although the flow field is highly idealized, the results provide insight as to the coupling of the ionizational nonequilibrium boundary layer to the flow field and the concomitant effect that this coupling has on anode current limitation. The quasi-neutral approximation used is expected to accurately describe the dynamics within the ionization nonequilibrium boundary layer. The anode saturation current (current density above which the anode must emit ions) strongly increases with increasing free stream velocity and anode temperature. These results have important implications for the understanding of high-power arc jet operation, and the use of high-power arc jets in low thrust space propulsion. >

Plasma arc process systems, reactors, and applications

Thermal plasma arc systems are being applied to a wide variety of high-temperature applications. The use of electricity is proving to be a cost-effective alternative for the treatment of industrial wastes, recycling of scrap, upgrading of existing metallurgical processes, and new methods for the production of materials. Thermal plasma arc reactors are classified according to; (1) the mode of arc attachment, as nontransferred and transferred: and (2) the site for energy and/or mass transfer, as dispersed phase or condensed phase (bulk, film and packed bed). Unique features in the design of plasma reactor systems and applications in waste treatment and metals production are discussed.