Electron temperature and current density measurements at the anode of free burning, high intensity arcs

摘要

The plasma near the anode of free-burning, high-intensity argon arcs characterized by strong deviations from thermal and chemical equilibrium has been investigated by means of electric probes. This diagnostic technique provides data that can be interpreted without assumptions about the thermodynamic state of the plasma. Two-temperature nonequilibrium has been shown to prevail, with electron temperatures remaining around or above 10000 K in spite of the presence of a water-cooled anode. The electron current density data for atmospheric pressure arcs showed a nonmonotonic dependence on the overall arc current, displaying a minimum that, for the shorter electrode gaps, is very pronounced. The probe characteristics indicated that the anode may also collect ions from the plasma. The experimental results suggest the existence of a transition from a boundary layer dominated by the externally applied field to a boundary layer dominated by diffusion. This can be explained in terms of the direct influence exerted on the anode boundary layer by the cathode induced flows, which control the arc attachment and the establishment of more or less steep gradients in front of the anode.