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- ItemApplication of the transferred matrix method to a unified evaluation of the cathodic electron emission(New York, NY : American Inst. of Physics, 2018) Baeva, M.The work is concerned with the Transfer Matrix Method for solving the steady-state Schrödinger equation applied for a unified evaluation of the emission current density from non-refractory cathodes. The method is applicable to arbitrary shapes of the potential barrier and its transmission probability is obtained without any analytical approximations. The Fermi-Dirac distribution for the free electrons in the metal is considered as a supply function. The results, obtained for a work function of the cathode material of 4.5 eV over a wide range of values of the surface temperature and the electric field strength, clearly show a growing deviation from those obtained by the classical Jeffreys-Wentzel-Kramers-Brillouin approximation with the increase of the electric field strength. Preliminary results are obtained to demonstrate the applicability of the Transfer Matrix method to the evaluation of the ion-assisted electron emission. A significant local enhancement of the emission current density is obtained as a result of the presence of an ion at a fixed position near the metal surface. The effect becomes very strongly pronounced at an appropriate value of the electric field strength, for which a resonant ion contribution appears.
- ItemEffect of a Spatially Fluctuating Heating of Particles in a Plasma Spray Process(Dordrecht : Springer Science + Business Media B.V., 2022) Zhu, T.; Baeva, M.; Testrich, H.; Kewitz, T.; Foest, R.The work is concerned with the effect of a spatially fluctuating heating of Al2O3 particles with diameters of 5–120 μm during a plasma spray process. A plasma jet is generated in a mixture of Ar (40 NLPM) and H2 (14 NLPM) and in pure Ar at an electric current of 600 A. The tracing of the injected particles in the plume region of the plasma jets is considered in the framework of a three-dimensional model taking into account a turbulent fluid flow. It is shown that the heat source for the injected particles exhibits a well pronounced spatially fluctuating structure due to the enhancement of the thermal conductivity resulting from dissociation and ionization of the molecular gas in the temperature range of 2500–4000 K and 13,000–14,000 K, respectively. During their travel towards the substrate, the particles are therefore repeatedly heated in the gas mixture in contrast to the case of pure argon. Particles injected in the gas mixture reach the substrate with a higher average temperature and velocity.
- ItemComparative studies of low-intensity short-length arcs(Praha : Czech Technical University in Prague, Faculty of Electrical Engineering, Department of Physics, 2019) Baeva, M.; Siewert, E.; Uhrlandt, D.We present results obtained by two non-equilibrium modelling approaches and experiments on low-intensity short-length arcs in argon at atmospheric pressure. The first one considers a quasi-neutral arc column combined with boundary conditions on the electrodes based on the energy balance in the space-charge sheaths. The second approach applies a unified description over the entire gap and solves the Poisson equation for the self-consistent electric field. The experiments provide the arc voltage.
- ItemAdvanced Nonequilibrium Modelling of DC Tungsten-Inert Gas Arcs(Praha : Czech Technical University in Prague, Faculty of Electrical Engineering, Department of Physics, 2017) Baeva, M.; Uhrlandt, D.The paper is concerned with the state-of-the-art nonequilibrium modelling of a DC tungsten-inert gas arc plasma. The advanced description involves the two-way interaction between the plasma and the electrodes. Results in atmospheric pressure argon demonstrating important features of the arc plasma are presented and discussed. First results in the presence of metal vapour released from the molten anode are presented. Outlook for further developments in nonequilibrium arc modelling are discussed.
- ItemModelling and experimental investigations of DC electric arcs in argon and carbon dioxide(Praha : Czech Technical University in Prague, Faculty of Electrical Engineering, Department of Physics, 2019) Mohsni, C.; Baeva, M.; Franke, S.; Gortschakow, S.; Gonzalez, D.; Araoud, Z.; Charrada, K.In this work an arc model is employed along with electric and spectroscopic measurements to study DC electric arcs in Ar and CO2. The model is aimed at describing the arc and the electrodes. Simulation and experimental results are shown for currents between 150 A and 210 A.
- ItemOn the interaction of a microwave excited oxygen plasma with a jet of precursor material for deposition applications(Praha : Czech Technical University in Prague, Faculty of Electrical Engineering, Department of Physics, 2019) Methling, R.; Hempel, F.; Baeva, M.; Trautvetter, T.; Baierl, H.; Foest, R.A plasma source based on a microwave discharge at atmospheric pressure is used to produce an oxygen plasma torch. A liquid precursor material is evaporated and injected into the torch through a nozzle, causing oxidization and deposition of silica at a nearby quartz substrate. The temperature generated inside the plasma source and in the plume, in the region of treatment, and at the substrate surface are key parameters, which are needed for process description and optimization of plasma-chemical reactions. Optical emission spectroscopy and thermography were applied to observe and characterize the jet behavior and composition. The experimental results are compared with self-consistent modeling.
- ItemA collisional-radiative model of iron vapour in a thermal arc plasma(Bristol : IOP Publ., 2017-05-15) Baeva, M.; Uhrlandt, D.; Murphy, A.B.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.
- ItemModelling and experimental evidence of the cathode erosion in a plasma spray torch(Bristol : IOP Publ., 2022) Baeva, M.; Benilov, M.S.; Zhu, T.; Testrich, H.; Kewitz, T.; Foest, R.The lifetime of tungsten cathodes used in plasma spray torches is limited by processes leading to a loss of cathode material. It was reported in the literature that the mechanism of their erosion is the evaporation. A model of the ionization layer of a cathode is developed to study the diffusive transport of evaporated tungsten atoms and tungsten ions produced due to ionization by electron impact in a background argon plasma. It is shown that the Stefan-Maxwell equations do not reduce to Fick law as one could expect for the transport of diluted species, which is due to significant diffusion velocities of argon ions. The ionization of tungsten atoms occurs in a distance of a few micrometers from the cathode surface and leads to a strong sink, which increases the net flux of tungsten atoms far beyond that obtained in absence of tungsten ions. This shows that the tungsten ions are driven by the electric field towards the cathode resulting in no net diffusive flux and no removal of tungsten species from the ionization layer even if convection is accounted for. A possible mechanism of removal is found by extending the model to comprise an anode. The extended model resolves the inter-electrode region and provides the plasma parameters for a current density corresponding to the value at the center of the cathode under typical arc currents of 600 A and 800 A. The presence of the anode causes a reversal of the electric field on the anode side, which pulls the ions away from the ionization layer of the cathode. The net flux of tungsten ions can be further fortified by convection. This model allows one to evaluate the loss of cathode material under realistic operating conditions in a quantitative agreement with measured values.
- ItemCathode fall voltage of TIG arcs from a non-equilibrium arc model(Heidelberg : Springer, 2014) Uhrlandt, D.; Baeva, M.; Pipa, A.V.; Kozakov, R.; Gött, G.This work presents modelling results concerning a tungsten inert gas (TIG) welding arc. The model provides a consistent description of the free burning arc, the arc attachment and the electrodes. Thermal and chemical non-equilibrium is considered in the whole arc area, and a detailed model of the cathode space-charge sheath is included. The mechanisms in the cathode pre-sheath are treated in the framework of a non-equilibrium approach which is based on a two-fluid description of electrons and heavy particles and a simplified plasma chemistry of argon. A consistent determination of the electrode fall voltages and temperature distributions is achieved. The model is applied to arcs in pure argon at currents up to 250 A, whereby welding of a workpiece made of mild steel with a fixed burner is considered. Arc voltages in the range from 12 to 17 V are obtained at 50 at 250 A, respectively. The space-charge sheath voltage is found to be about 7 V and almost independent of the current. The corresponding temperatures of the cathode tip are in the range from 3,000 K to about 3,800 K. The results obtained are in a good agreement with measurements.