On Conductivity of Cold Gas Layer Separating Arc Column and Nozzle in Nontransferred Plasma Arc (Anode Reattachment Process in Plasma Spray Systems)

The nozzle serves as an anode in systems for plasma arc spraying. In these systems, the anode attachment moves downstream along the nozzle under the influence of the gas flow. The arc length increases and the arc voltage rises. At a specific arc voltage, the anode attachment jumps upstream and the arc voltage drops: reattachment occurs. The frequency of reattachment and the location of the new anode spot are determined by the conductivity of the cold gas layer that separates the extended arc column and the metallic anode. The conductivity of this layer in calculated in this paper. Two important effects are taken into account: 1) the difference between the electron and the heavy particle temperatures and 2) deviation from ionization equilibrium. Disregarding these effects leads to, practically, a completely insulating characterization of the cold gas layer. Calculations for an argon plasma showed that as opposed to the frequently accepted opinion, the electrical conductivity of the layer in the arc trail remains at a very substantial level. They also showed, in accordance with experiments, that admixing even small amounts of helium to argon decreases the layer conductivity that leads to reattachment delay and high voltage fluctuations. Analysis of the spatial distribution of the layer conductivity allows one to predict the new location of the anode spot after reattachment.