Investigation of Subsonic to Supersonic Transition of a Low-Pressure Plasma Torch Jet

The plasma jet emanating out of a thermal plasma torch operating at low pressure (
$\sim 30$
mbar) has been investigated experimentally. Different anode exit diameters are chosen, and the torch is operated at various gas flows and discharge current and their thermodynamic outputs have been calculated. Current–voltage characteristics (CVC) of plasma torch with different anode geometries are explored highlighting their differences. Interesting correlations are found between the anode exit diameter and the threshold limit for subsonic to supersonic transition of the jet. The spectral analysis of the voltage fluctuations reveals the presence of different frequencies and also yields the threshold. The kinetic energy (KE) component of the plasma jet has a direct correlation with the oscillations in the arc voltage. In addition to restrike mode and Helmholtz mode, an acoustic mode of oscillations is also investigated. A new resonator model explaining the different oscillations occurring in the low-pressure operation is also presented. The Helmholtz mode oscillations are validated by comparing the experimental values with the theoretical calculations. The power associated with the Helmholtz mode and acoustic mode has direct correlation with the plasma velocity and acts as a signature of the transition from subsonic to supersonic regime.