Nonequilibrium phenomena and determination of plasma parameters in the hot core of the cathode region in free-burning arc discharges.

We present spectroscopic measurements of plasma parameters (electron density n(e), electron temperature T(e), gas temperature T(g), underpopulation factor b) in the hot-core region in front of the cathode of a low-current, free-burning arc discharge in argon under atmospheric pressure. The discharge is operated in the hot-core mode, creating a hot cathode region with plasma parameters similar to high-current arcs in spite of the fact that we use comparatively low currents (less than 20A). We use continuum emission and (optically thin) line emission to determine n(e) and T(e). We apply relaxation measurements based on a power-interruption technique to investigate deviations from local thermodynamic equilibrium (LTE). These measurements let us determine the gas temperature T(g). All measurements are performed side-on with charge-coupled-device cameras as detectors, so that all measured plasma parameters are spatially resolved after an Abel inversion. This yields the first ever spatially resolved observation of the non-LTE phenomena of the hot core in the near-cathode region of free-burning arcs. The results only partly coincide with previously published predictions and measurements in the literature.