Instabilities in the anode region of atmospheric pressure arc plasmas

To understand the origin of the multiple arc–anode attachments in the anode region of high intensity arcs, a linear stability analysis of a non-uniform argon plasma consisting of electrons, argon neutrals and singly ionized argon ions and an arc–anode interface instability analysis are performed and presented. Plasma conditions typical of the anode region of high intensity argon arcs are emphasized. The calculations have shown that small-amplitude fluctuations presented in the plasma excite two different wave modes, space-charge relaxation mode and electron thermal mode, on a time scale shorter than the one in which the multiple-attachment mode forms. The space-charge relaxation mode has been found to be stable and not affected by the non-uniformity of the plasma. The electron thermal mode can be unstable, depending on the distributions of the current flow, the electric field, the electron temperature and the electron density. The analysis, together with a consideration of the evaporation–ionization instabilities at the arc–anode interface, has been applied to previous experimental data. The observed multiple-attachment mode, consisting of several constricted attachment spots in the arc fringes, can be explained by this analysis.