Arc voltage behavior as indication of different operating modes in a plasma torch

Summary form only given. An uncontrolled plasma instability is a phenomenon that most processes try to avoid. In plasma spraying, such an instability can lead to a deterioration of the coating quality. In our investigation, the voltage fluctuations of a subsonic DC plasma torch have been analyzed and correlated with appearance of plasma jet instabilities. The torch has a straight bore anode nozzle and a button shape cathode. The arc gas can be injected forming either a straight flow or a swirling flow. Three are operating modes have been found: the "restrike" mode which is characterized by a saw-tooth shape voltage waveform and a large fluctuating amplitude, the "take-over" mode which is represented with a more or less sinusoidal or triangle shape waveform with a varying amplitude, and the "steady" mode which shows a nearly flat voltage profile with a low mean voltage. In addition to the analysis of the voltage waveform, analysis of high speed video images obtained from end-on observation of the arc in the anode nozzle has been used as diagnostics.