Loading...
2 results
Search Results
Now showing 1 - 2 of 2
- Numerical investigation of AC arc ignition on cold electrodes in atmospheric-pressure argonPublication . Santos, D. F. N.; Lisnyak, M; Almeida, N.; Benilova, L. G.; Benilov, M. S.Since experiments cannot clarify the mechanism of current transfer to non-thermionic arc cathodes, this can only be done by means of numerical modelling based on first principles and not relying on a priori assumptions. In this work, the first quarter-period after the ignition of an AC arc on cold electrodes in atmospheric-pressure argon is investigated by means of unified one-dimensional modelling, where the conservation and transport equations for all plasma species, the electron and heavy-particle energy equations, and the Poisson equation are solved in the whole interelectrode gap up to the electrode surfaces. Results are compared with those for DC discharges and analysed with the aim to clarify the role of different mechanisms of current transfer to non-thermionic arc cathodes. It is found that the glow-to-arc transition in the AC case occurs in a way substantially different from the quasi-stationary glow-to-arc transition. The dominant mechanisms of current transfer to the cathode during the AC arc ignition on cold electrodes are, subsequently, the displacement current, the ion current, and thermionic emission current. No indications of explosive emission are found. Electron emission from the impact of excited atoms can hardly be a dominant mechanism either. The introduction of the so-called field enhancement factor, which is used for description of field electron emission from cold cathodes in a vacuum, leads to computed cathode surface temperature values that are appreciably lower than the melting temperature of tungsten even in the quasi-stationary case. This means that pure tungsten cathodes of atmospheric-pressure argon arcs can operate without melting, in contradiction with experiments.
- Account of diffusion in local thermodynamic equilibrium and two-temperature plasma modelsPublication . Santos, D. F. N.; Lisnyak, M; Benilov, M. S.A self-consistent account of the effect of diffusion on charge transport in local thermodynamic equilibrium (LTE) and two-temperature (2T) ionization-equilibrium plasmas amounts to introducing into Ohm’s law, in addition to the conventional term proportional to the electric field (conduction current) and thermal-diffusion terms, also terms describing the diffusion due to plasma composition variations, which are proportional to the temperature gradient (or, in the case of 2T plasmas, to ∇Te and ∇Th) and to the plasma pressure gradient. These terms are calculated, with the use of the Stefan–Maxwell equations, for the particular case of 2T ionization-equilibrium atomic plasmas with singly charged ions. Also proposed is a simple way of approximate evaluation of reactive thermal conductivity in such plasmas. An online tool performing evaluation of the relevant coefficients for 2T argon, xenon, and mercury plasmas has been deployed on the internet. Representative modelling results show that the new form of Ohm’s law, when introduced into standard LTE or 2T models, may describe the electric field reversal in front of arc anodes, an effect that has been simulated previously only by means of (more complex) models taking into account deviations from ionization equilibrium.