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Electrical measurements and numerical modelling of dc tungsten-inert gas arcs are carried out in order to determine the electric field and the voltage, and to study their importance to the plasma-electrode interaction. The configuration under consideration is very typical for tungsten-inert gas welding processes and includes a conically shaped cathode made of doped tungsten and atmospheric pressure argon as a shielding gas. The studies analyze the effects of the inter-electrode distance and the electric current over wide ranges of variation—currents from below 10 A up to 200 A, and distances from 20 mm down to 2 mm in the modelling and to the short-circuit in the experiment. The comparison of the experimentally obtained and the computed current–voltage and arc length-voltage characteristics clearly prove the predictive capability of the advanced non-equilibrium model of the arc and the electrodes. Moreover, the self-consistent model yields the structure of the electric potential and the electric field and is capable of describing the peculiarities of the plasma-electrode interaction. [ABSTRACT FROM AUTHOR]

The present work is concerned with the simulation analysis of a microwave plasma torch suitable for deposition applications prior to the admixture of any precursor. The self-consistent numerical model describes the electromagnetic field of the microwaves entering an R26 waveguide, the plasma generation, the gas flow through a tube crossing the waveguide, and the heat transfer in the gas. The plasma description avoids the assumption of quasi-neutrality and provides, therefore, a solution including the near-wall regions. The multiphysics model is applied to the source operated in argon at atmospheric pressure. Quasi-stationary solutions are obtained in a 2D Cartesian geometry and in a 3D geometry employing increasing degrees of complexity with respect to the physics, the reaction kinetics and the boundary conditions. The distribution of the electromagnetic field and the plasma parameters resulting from 2D fully coupled one- and two-ion models is analyzed for an incoming microwave power of 1 kW and a gas flow rate of 18 slm. The 2D model is capable of predicting the plasma parameters at a reasonable computational cost. The application of a 3D plasma-microwave model shows that the spatial distribution of the electromagnetic field and the plasma parameters is not, in general, axially symmetric. In the plane corresponding to the 2D work plane, the results of 3D one-ion plasma model show agreement with the 2D results, however, at significant computational costs. The 2D simulation analysis carried out allows us to draw up a decision-making with regard to the setup performance. [ABSTRACT FROM AUTHOR]

A collisional-radiative model for the ground state and fifty effective excited levels of atomic iron, and one level for singly-ionized iron, is set up for technological plasmas. Attention is focused on the population of excited states of atomic iron as a result of excitation, de-excitation, ionization, recombination and spontaneous emission. Effective rate coefficients for ionization and recombination, required in non-equilibrium plasma transport models, are also obtained. The collisional-radiative model is applied to a thermal arc plasma. Input parameters for the collisional-radiative model are provided by a magnetohydrodynamic simulation of a gas-metal welding arc, in which local thermodynamic equilibrium is assumed and the treatment of the transport of metal vapour is based on combined diffusion coefficients. The results clearly identify the conditions in the arc, under which the atomic state distribution satisfies the Boltzmann distribution, with an excitation temperature equal to the plasma temperature. These conditions are met in the central part of the arc, even though a local temperature minimum occurs here. This provides assurance that diagnostic methods based on local thermodynamic equilibrium, in particular those of optical emission spectroscopy, are reliable here. In contrast, deviations from the equilibrium atomic-state distribution are obtained in the near-electrode and arc fringe regions. As a consequence, the temperatures determined from the ratio of line intensities and number densities obtained from the emission coefficient in these regions are questionable. In this situation, the collisional-radiative model can be used as a diagnostic tool to assist in the interpretation of spectroscopic measurements. [ABSTRACT FROM AUTHOR]

The collisional–radiative model with an emphasis on the accurate treatment of the resonance radiation transport is developed and applied to the free-burning Ar arc plasma. This model allows for analysis of the influence of resonance radiation on the spatial density profiles of the atoms in different excited states. The comparison of the radial density profiles obtained using an effective transition probability approximation with the results of the accurate solution demonstrates the distinct impact of transport on the profiles and absolute densities of the excited atoms, especially in the arc fringes. The departures from the Saha–Boltzmann equilibrium distributions, caused by different radiative transitions, are analyzed. For the case of the DC arc, the local thermodynamic equilibrium (LTE) state holds close to the arc axis, while strong deviations from the equilibrium state on the periphery occur. In the intermediate radial positions the conditions of partial LTE are fulfilled. [ABSTRACT FROM AUTHOR]

A novel non-equilibrium model has been developed to describe the interplay of heat and mass transfer and electric and magnetic fields in a DC electric arc. A complete diffusion treatment of particle fluxes, a generalized form of Ohm’s law, and numerical matching of the arc plasma with the space-charge sheaths adjacent to the electrodes are applied to analyze in detail the plasma parameters and the phenomena occurring in the plasma column and the near-electrode regions of a DC arc generated in atmospheric pressure argon for current levels from 20 A up to 200 A. Results comprising electric field and potential, current density, heating of the electrodes, and effects of thermal and chemical non-equilibrium are presented and discussed. The current–voltage characteristic obtained is in fair agreement with known experimental data. It indicates a minimum for arc current of about 80 A. For all current levels, a field reversal in front of the anode accompanied by a voltage drop of (0.7–2.6) V is observed. Another field reversal is observed near the cathode for arc currents below 80 A. [ABSTRACT FROM AUTHOR]

A modified method for the solution of the Holstein-Biberman transport equation is proposed for cylindrical geometry, arbitrary absorption coefficient accounting for plasma inhomogeneity and a Lorentz spectral lineshape. The method is applied to the plasma generated by a free-burning arc in argon at atmospheric pressure in order to study the effect of trapping of resonance radiation on the population of the resonance state Ar(1s4). The results obtained are compared with those of a self-consistent nonequilibrium model of the arc including excited atomic levels. They show an enhanced (compared to the results of the nonequilibrium model) population of the resonance state outside of the arc core as an effect of the trapping of resonance radiation and almost negligible collisional de-excitation. In the arc core, the effect is negligible so that the calculated population of the Ar(1s4) state confirms the results obtained with the nonequilibrium model. [ABSTRACT FROM AUTHOR]