Three-dimensional, unsteady behaviour of high-pressure electric arcs in argon is studied by means of numerical simulations. Attention is paid to argon arcs whose steady structure is fairly well understood. Results are reported for the case of a one centimetre long arc, burning in argon and driven by a total current of 200 A and 300 A. The influence of the boundary conditions for the electrical potential on the macroscopic structure of the arc is studied and it is found that the distribution of the current density near the cathode is one of the critical parameters which can significantly modify both the distribution of mean quantities and the stability of the arc. All mean quantities found from three-dimensional calculations are compared with two-dimensional axi-symmetric fields obtained by a previously used code for arc simulations. The results indicate good agreement between the results from three-dimensional calculations, two-dimensional axi-symmetric simulations and results from physical experiments documented in the literature.

The net flux of vapour from a solid surface in an ambient gas is analysed with the aim to estimate the effect of vaporization cooling on the energy balance of an arc cathode under conditions typical for a high-power current breaker. If the ratio of the equilibrium vapour pressure p v to the ambient pressure p ∞ is smaller than unity, the removal of vapour from the surface is due to diffusion into the bulk of the gas. As a consequence, the net flux of the vapour from the surface is much smaller than the emitted flux. An estimate of the diffusion rate under conditions typical for a high-power current breaker indicates that vaporization cooling plays a minor role in the energy balance of the cathode in this case. If ratio p v /p ∞ is above unity, the flow of the vapour from the surface appears and the net flux is comparable to the emitted flux. A simple analytical solution has been obtained for this case, which is in a good agreement with results of the Monte Carlo modelling of preceding authors. If p v /p ∞ exceeds approximately 4.5, vaporization occurs as into vacuum and the net flux is about 0.82 of the emitted flux.

A complete set of equations is proposed to model droplets of molten metal ejected from the contacts in a high voltage circuit breaker of the SF6 filled gas-blast type. The initial speed and diameter of the droplets are varied to characterize their behaviour and study the distribution of the metal vapour. A relation is derived that can be used in simplified arc models such as integral or two-zone models to take the effect of the droplets into account. It is found that, although the influence on usually measured quantities such as pressure and voltage is small, droplets affect velocity and temperature fields in the electric arc significantly and should not be neglected.

A one-dimensional model for the near-cathode region of electric arcs is applied to define the current, heat and mass transfer mechanisms in the electron temperature range of 10-40 kK and the pressure range of 10 4-10 7 Pa. The model considers details of the space charge zone, the ionization zone and evaporation of the cathode material. A copper-tungsten cathode is investigated, and it is found that evaporation is an important cooling mechanism of the cathode and should not be neglected. The plasma near the cathode is constituted of evaporated contact material in the main part of the investigated pressure-temperature domain. The results show that Schottky-enhanced thermoionic emission, together with the present model for the near-cathode layer, explains the current transfer mechanism for a wide range of pressures and temperatures and for refractory and non-refractory cathode materials.