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.

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.

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.