Contents I. Introduction
The interaction between an electric arc and its electrodes has become an indispensable feature of up-to-date arc plasma models due to the need for understanding and description of the arc attachment to the electrodes. The arc attachment to refractory cathodes and non-refractory anodes has been considered in review articles during the last decade (see e.g. [1]–[4]). Refractory cathodes withstand temperatures above the melting point of non-refractory materials and the emission current from them is chiefly due to thermionic emission. Non-refractory cathodes, as used e.g. in gas-metal welding arcs, circuit breakers, and plasma torches, have been considered to produce electron emission due to the high electric field near the cathode surface [5]. Although it has been proposed that instead of field emission, electrons may be produced as a result of photoionization of neutral atoms by radiation from the arc [6] or impact of excited atoms at the cathode [7], the dominant emission mechanism has not been clearly established. It has been shown by Ecker et al. [8] that the thermo-field emission current can be significantly enhanced by the field of ions coming from the plasma without having to enhance the temperature or the electric field at the cathode surface.
