Margarita Baeva

Novel non-equilibrium modelling of a DC electric arc in argon

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 Ohms 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.

Account of near-cathode sheath in numerical models of high-pressure arc discharges

Three approaches to describing the separation of charges in near-cathode regions of high-pressure arc discharges are compared. The first approach employs a single set of equations, including the Poisson equation, in the whole interelectrode gap. The second approach employs a fully non-equilibrium description of the quasi-neutral bulk plasma, complemented with a newly developed description of the space-charge sheaths. The third, and the simplest, approach exploits the fact that significant power is deposited by the arc power supply into the near-cathode plasma layer, which allows one to simulate the plasma–cathode interaction to the first approximation independently of processes in the bulk plasma. It is found that results given by the different models are generally in good agreement, and in some cases the agreement is even surprisingly good. It follows that the predicted integral characteristics of the plasma–cathode interaction are not strongly affected by details of the model provided that the basic physics is right.

Comparing two non-equilibrium approaches to modelling of a free-burning arc

Two models of high-pressure arc discharges are compared with each other and with experimental data for an atmospheric-pressure free-burning arc in argon for arc currents of 20–200 A. The models account for space-charge effects and thermal and ionization non-equilibrium in somewhat different ways. One model considers space-charge effects, thermal and ionization non-equilibrium in the near-cathode region and thermal non-equilibrium in the bulk plasma. The other model considers thermal and ionization non-equilibrium in the entire arc plasma and space-charge effects in the near-cathode region. Both models are capable of predicting the arc voltage in fair agreement with experimental data. Differences are observed in the arc attachment to the cathode, which do not strongly affect the near-cathode voltage drop and the total arc voltage for arc currents exceeding 75 A. For lower arc currents the difference is significant but the arc column structure is quite similar and the predicted bulk plasma characteristics are relatively close to each other. (Some figures may appear in colour only in the online journal)

Non-equilibrium Modeling of Tungsten-Inert Gas Arcs

The paper gives an overview of the most common non-equilibrium approaches for modeling of tungsten-inert gas arc plasma, which have been developed up to date, in particular two-temperature and fully non-equilibrium approaches. The first group implies thermal non-equilibrium but chemical equilibrium whereas the second group describes the arc plasma avoiding assumptions of both thermal and chemical equilibrium. The common and specific features of the physical description are discussed. Results of the most recent fully non-equilibrium model, which is applied for the first time to tungsten-inert gas arc arrangement with a truncated conical tip of a doped tungsten cathode, are compared with those of previously published non-equilibrium models and experimental data. The general diffusion representation and more accurate boundary conditions incorporating the properties of the space-charge sheaths adjacent to the electrodes enable a novel description of the arc core, the near-electrode regions and the arc fringes in a self-consistent manner and provides a deeper insight into the arc properties.

Microwave-based characterization of an atmospheric pressure microwave-driven plasma source for surface treatment

A plasma source operating at atmospheric pressure by continuous or pulsed microwave at 2.45 GHz with a maximum power of 1.7 kW is developed for surface treatment applications. The microwave power is coupled into a cylindrical cavity used as a process chamber. The device characteristics are studied in detail using a simple network analysis and finite integration technique simulations. Experimental results are compared with the outcome of the model. The TM01 mode in the process chamber is found to be appropriate for surface treatment. The results obtained are used to optimize and simplify the device performance and operation. It has been found that the electric field strength, responsible for plasma ignition, and the microwave power coupling into the plasma demonstrate a contradictory course—a maximum field corresponds to a minimum power in-coupled. A set of parameters representing a compromise between stable plasma ignitions and proper plasma treatment has been found.

Excited atoms in the free-burning Ar arc: treatment of the resonance radiation

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.

Effect of trapping of resonance radiation in a free-burning Ar arc

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.

Study of the Spatiotemporal Evolution of Microwave Plasma in Argon

The knowledge of the distribution of electromagnetic field and plasma parameters is of crucial importance for the development of microwave plasma sources. To that purpose, a time-dependent 2-D fluid model accompanying experiments has been applied to investigate the coupling between the plasma flow and kinetics and the propagating microwaves.

Boundary conditions at the plasma-cathode interface in high-pressure arcs

Summary form only given. Transport of electron energy from the near-cathode space-charge sheath into the bulk plasma is an important effect dominating heat exchange in cathodic part of high-pressure arc discharges. Therefore, a physically justified numerical model of bulk plasma in high-pressure arc discharges should take into account deviations between the electron and heavy-particle temperatures. As far as deviations from ionization equilibrium are concerned, two approaches are possible: to take them into account only in a near-cathode layer or in both the near-cathode layer and the bulk plasma. In the framework of the first approach, the model of near-cathode layer takes into account, in addition to the space charge, also non-equilibrium ionization. In the framework of the second approach, the near-cathode layer represents a space-charge sheath in which volume ionization and recombination are negligible. The two approaches have been compared1 between themselves and with the experiment on atmospheric-pressure argon arc, for which both approaches are justified. It was found that they predict values of the arc voltage which are close to each other and the experiment for arc currents between 100 and 200A, however for lower currents the second approach predicts a less constricted and colder arc attachment and, consequently, significantly higher arc voltages than those found in the experiment and predicted by the first approach. Since the second approach is at least no less accurate than the first one, its predictive capabilities with regard to the arc-cathode interaction may be improved. Physically justified boundary conditions at the interface between the bulk plasma and the cathode are needed to this end. These conditions should account for the existence of the near-cathode space-charge sheath and have not been derived up to now. A derivation and validation of these boundary conditions is the subject of this work.

PPPS-2013: Collisional-radiative modeling of free-burning arc plasma in argon

Summary form only given. Recent ambitions were aimed at improving a free-burning arc modeling by means of a self-consistent description of the arc plasma and the electrodes1. As next step, a non-equilibrium model of the arc in argon describing the heat transfer, electric and magnetic field, and excitation kinetics of a large number of levels of argon atoms is presented. The extended scheme of levels allows obtaining the populations of individual levels giving rise to important radiative transitions and enables better comparison with spectroscopic measurements. The model is based on the magnetohydrodynamic approach and starts from macroscopic parameters only. The Navier-Stokes equations provide a solution for the total mass density and the mass-averaged velocity. Separate energy equations are solved for heavy particles and electrons. The current continuity, Ohms law and Maxwells equations are considered to obtain the electric potential and the self-induced magnetic field. The heat transport in the electrodes accounts for thermal conduction, Joule heating and energy fluxes on their boundaries with the plasma due to ion and electron heating, electron emission and black body radiation. The unified description of the plasma and the electrodes is realized by implementing a sheath model2, in which the space charge sheath is treated as a zero-dimensional interface and the plasma characteristics of the presheath are obtained from the near-surface control volumes. The species transport is described by diffusion equations for excited atoms and ions. A set of reactions accounts for elastic scattering, excitation and de-excitation in collisions with electrons and atoms, step-wise ionization, recombination and spontaneous emission.