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

The authors report on the plasma near the anode of free-burning high-intensity argon arcs, characterized by strong deviations from thermal and chemical equilibrium, investigated by means of electric probes. This diagnostic technique is appropriate since it provides data which can be interpreted without assumptions about the thermodynamic state of the plasma. Measurements of electron and ion current density and electron temperature on the arc centerline were obtained using a probe flush with the anode surface. The results clarify the effects of arc current, electrode gap, and operating pressure on the properties of the boundary layer. In addition, they may affect the scale-up, optimization and electrode life expectancy criteria for such arcs in terms of the correct estimation of anode heat fluxes. Two-temperature non-equilibrium is shown to prevail with electron temperatures remaining around or above 10000 K in spite of the presence of the water cooled anode. The electron current density data for atmospheric pressure arcs show a non-monotonic dependence on the overall arc current, displaying a minimum which, for the shorter electrode gaps, is very pronounced. The probe characteristics indicate that the anode indeed may also collect ions from the plasma