Numerical investigation of AC arc ignition on cold electrodes in atmospheric-pressure argon

Abstract

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.

Investigation of the interaction of dense noble gas plasmas with cold cathodes: I—Experimental setup and application to Al, Cu, Ti, and graphite cathodes

“…The experimental results confirm the statement obtained by modelling that secondary electron emission, induced by ions impinging on the electrode surface, of a positive space charge layer in front of the cathode and an adjacent ionization layer is starting the current transfer to cold cathodes. [38] A look at Table 2 discloses a quite different increase of the commutation time t c with decreasing applied voltage between the arc plasma and CEs consisting of different materials. The increase is lowest for graphite electrodes, which are uncovered with an oxide layer after polishing.…”

“…The experimental results confirm the statement obtained by modelling that secondary electron emission, induced by ions impinging on the electrode surface, of a positive space charge layer in front of the cathode and an adjacent ionization layer is starting the current transfer to cold cathodes. [ 38 ]…”

Investigation of the interaction of dense noble gas plasmas with cold cathodes:III—Arc spot ignition on pure and doped W cathodes

“…The interpretation confirms the role of secondary electron emission obtained by numerical modelling cold cathodes of high‐pressure arc plasmas. [ 36 ]…”

“…The interpretation confirms the role of secondary electron emission obtained by numerical modelling cold cathodes of high-pressure arc plasmas. [36] The electron emissivity varies in general as a function of the position on the end face of the cathode. It is increased first of all by a locally enhanced secondary electron emission coefficient 𝛾 i and less distinctly by a locally lowered work function or by locally increased electric field strength in front of tips on the electrode surface.…”

Modeling of Melt Flow and Heat Transfer in Stationary Gas Tungsten Arc Welding with Vertical and Tilted Torches

“…They reported that the region of the strongest electron overpopulation appeared at the intersection of the plasma fringes and the electrode surface. Some other relevant studies can be read elsewhere [42][43][44][45][46][47].…”