Diagnostics of discharge modes of a free-burning low-current argon arc

We investigated the cathode region of free-burning argon arcs at atmospheric pressure and at currents from 0.3 A up to 25 A. Apart from the well known diffuse mode of operation we also found a more constricted form which we call the blue-core mode. Different diagnostic methods are simultaneously applied to these modes of operation, that is: emission spectroscopy in conjunction with a partial LTE model for the density and temperature measurements; Thomson scattering for the electron temperature and electron density determination; and Rayleigh scattering for a determination of the gas temperature. The large differences in appearance, densities, temperatures or current formation between the discharge modes are well defined by the measurements.