Behavior of arc-anode attachment and its effect on anode state. Steam-argon plasma jet in air

Summary form only given. Interaction of arc and electrode was studied at conditions characteristic for non-transferred direct current arc plasma torches. Such torches are widely used in plasma spraying, sphereodization of powders, decomposition of chemicals and many other technologies. Plasma here is generated by Joule heating of a gas in the electric arc discharging in a cylindrically symmetric channel between a cathode and an anode. The anode is often a part of the channel or is situated at the end of the channel and its surface is parallel to the arc. As the anode surface is parallel to the main arc column the current path curves in an anode region to attach the electrode. The anode attachment behavior determines performance of the torch controlling plasma power, fluctuation and electrode erosion. Anode attachment position and movement determine arc length and thus control arc power and its fluctuation. Moreover the anodic jet which appears at the anode attachment in common regimes significantly disturbs the plasma flow. In this paper a structure and behavior of the arc-anode attachment is reported and corresponding type of anode erosion is documented. Plasma torch WSP®H2000 was used for generation of plasma from a mixture of steam with argon. Region of the arc near the anode was observed at different conditions. Two regimes of the arc-anode attachment were observed: constricted and rapidly moving attachment and almost stationary and wide arc channel. Distribution of copper evaporated from the electrode as measured by optical spectroscopy differed and erosion pattern was also substantially different. Low erosion mode was observed. Distribution of cracks was found on the anode surface in a mode with low surface melting. Largest of the cracks followed grain borders of the material and were as deep as 0.5 mm. Sharp edges of the cracks became melting centers. The results give insight to arc behavior in anode region and thermal processes in the anode material.