Study of the electric arc dynamics in a cascaded-anode plasma torch

Abstract

In plasma spraying process, the plasma jet stability depends on the electric arc movement inside the plasma torch. For conventional plasma torches with rod cathode and a large anodic surface, arc roots are free to move along the anode wall, leading to high plasma jet fluctuations. To overcome these instabilities, cascaded-anode plasma torches have been developed, reducing considerably the electric arc movement and thus the plasma jet fluctuations. Unfortunately, a small number of studies reports on the behaviour of theses torches including the arc instabilities and on electrode erosion process as well. In this work, several diagnostics have been set up to further understand the cascaded-anode plasma torch behaviour under different sets of operating conditions. Influence of plasma forming gas composition on the electric arc dynamic has been studied through anodic imaging. The issue of the intense plasma radiation masking is discussed since the light emitted by the arc column interferes with anodic arc roots observations. Multiple constricted arc roots have been observed and a preferential site for anodic arc root has been identified for all experimental conditions. The addition of a secondary plasma forming gas promotes a higher number of reattachments. The influence of the electric arc movement on plasma jet stability is also studied by following the plasma jet brightness fluctuations. The low voltage fluctuation frequency is also observed on brightness spectra, which means that these fluctuations could have an impact on in-flight particle treatment.