Investigation of the electro-thermal dynamics of a low pressure DC plasma spray torch

The dynamics a DC plasma torch is studied experimentally in the pressure range of 1 – 30 mbar, with a focus on low pressure spraying and coating applications. The low pressure plasma spray technique bridges the gap between physical vapor deposition (PVD), chemical vapor deposition (CVD) techniques and atmospheric plasma spray processes. Hence, it becomes imperative to study the plasma torch dynamics. In this study, the effect of various operating conditions such as (i) pressure (ii) gas flow rate (iii) type of gas (iv) arc current and (v) anode geometry on the electro-thermal output of the plasma torch is investigated. Current-voltage characteristics (CVC) of plasma torch with different operating parameters are explored, highlighting their differences. A semi-empirical relation between voltage and current is constructed using theory of dynamic similarity to predict the behaviour of the DC plasma torch under low pressure conditions (1 – 30 mbar). The influence of input controllable parameters on the output of the plasma torch is highlighted using dimensionless numbers. This study also unravels interesting correlations between the anode exit diameter and the threshold limit for subsonic to supersonic transition of the jet at 30 mbar chamber pressure. The spectral analysis reveals the different fluctuations associated with the arc voltage, classified into three different modes (i) restrike (ii) Helmholtz and (iii) acoustic. While the restrike mode is superimposed over the complete spectrum and the power density is constant, the power associated with the Helmholtz mode and acoustic mode oscillations have direct correlation with the kinetic energy component influencing the plasma jet velocity. The power density corresponding to the Helmholtz mode and acoustic mode acts as a signature of the transition from subsonic to supersonic regime.