A widely-used approach to simulation of high-pressure arc discharges is based on the system of magneto-hydrodynamic equations written in the approximation of local thermodynamic equilibrium (LTE). In this work, boundary conditions on the surface of the electrodes are formulated with the use of equations of balance of energy in the non-equilibrium near-electrode layers that separate the LTE bulk plasma and the electrodes. As an example, numerical simulations of a free-burning arc in atmospheric-pressure argon plasma in the current range from 20 to 200 A are reported. Simulation results are in a reasonably good agreement with those given by more sophisticated models and with the experiment. Simulations performed for cathodes of slightly different geometries have predicted a strong effect produced by details of the cathode geometry over the distribution of the current density along the cathode surface and therefore over the plasma temperature; an interesting and potentially important result worth of further numerical investigation and experimental verification.

Experiments on high-current arcs carried out in an ultrahigh vacuum chamber are presented in this paper. For contact separation the lower electrode is moved by a mechanicalpneumatic device simulating the conditions of a vacuum circuit breaker. The arc behavior of the Cu-Cr test electrodes after the electrode separation without application of external magnetic field is observed by a high-speed video camera. Besides the usual electrical measurements, the emission in the gap along the electrode axis is investigated by spatially resolved optical emission spectroscopy. Differences are found in the intensity distribution of atomic and ionic lines along the gap.

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.

Spectroscopic investigations of free-burning vacuum arcs in diffuse mode with CuCr electrodes are presented. The experimental conditions of the investigated arc correspond to the typical system for vacuum circuit breakers. Spectra of six species Cu I, Cu II, Cu III, Cr I, Cr II, and Cr III have been analyzed in the wavelength range 350-810 nm. The axial intensity distributions were found to be strongly dependent on the ionization stage of radiating species. Emission distributions of Cr II and Cu II can be distinguished as well as the distributions of Cr III and Cu III. Information on the axial distribution was used to identify the spectra and for identification of overlapping spectral lines. The overview spectra and some spectral windows recorded with high resolution are presented. Analysis of axial distributions of emitted light, which originates from different ionization states, is presented and discussed.

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 ∇T e and ∇T h ) 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.

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In our previous work is shown that in the visual spectral region of radiation of the high-current vacuum arc with copper electrodes there is a spectral "window" that allows to register continuum radiation (including at high-current densities). This fact was used for elaboration of the method of determination of the electron density in the gap of high-current vacuum arc. The method was verified by comparison of the obtained results with known results received by different methods.

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