Arc Column Oscillation: A New Mode of Arc Fluctuation in Laminar Plasma Torch

Laminar plasma torch has been widely employed to generate laminar plasma jet with notable advantages, e.g. high specific enthalpy, long jet length, low axial temperature gradient, etc. However, it is challenge to maintain the stability of laminar plasma jet due to the existence of different modes of arc fluctuation, e.g. steady mode, take-over mode and restrike mode. Previous studies on arc fluctuation mainly regard the arc current as a constant value, ignoring the influence of the dynamic control performance of the DC power supply on the variation of the actual arc current. The authors found that when the variation of the arc current exceeds the control capability of the power supply, the arc column expands and contracts periodically, exhibiting a new mode of arc fluctuation named arc column oscillation (ACO) in this paper. To reveal the characteristic phenomena, conditions of the laminar plasma torch and reason for the occurrence of ACO, experiments were conducted on a home-made DC laminar arc plasma torch. Results showed that ACO occurs when the operating conditions were low current, high gas flow rate, and high nitrogen proportion of the plasma-forming gas. It was found that ACO shows characteristics of high fluctuation of the arc current and the pressure in the cathode cavity, and periodic expansion and contraction of the arc column as observed by the high speed camera. Additionally, the arc current and pressure in the cathode cavity oscillated at the same frequency along with the occurrence of ACO, showing the coupling between the arc current and the pressure in the cathode cavity. Furthermore, with the decrease of the arc current, increase in the gas flow rate, and increase in the nitrogen proportion of the plasma-forming gas, the fluctuation of the arc current tends to increase when the ACO occurs.