Spectroscopic study of a supersonic plasma jet generated by an ICP torch with a convergent-divergent nozzle

A supersonic argon and argon-hydrogen (3-5%) plasma jet generated by an induction plasma torch is studied by means of the methods of optical emission spectroscopy. The torch was operated at the input power of 20 kW and near atmospheric pressure. The supersonic jet with a periodic structure of expansion and compression zones is created by expanding the plasma through the Laval nozzle into a chamber maintained at the pressure around 1.8 kPa. Atomic argon lines with the upper level energies ranging from 13.3 to 15.5 eV, continuum emission and H line profile are used to evaluate plasma parameters such as temperature and electron number density. Analysis based on the Boltzmann diagram, line-to-continuum ratio, population of continuum extrapolated level and Stark broadening reveals various stages of departure from thermodynamic equilibrium in the plasma flow. It is shown, among others, that the temperature derived from Boltzmann diagram does not follow the jet structure and reliable determination of electron temperature is questionable. An addition of several percent of hydrogen results in a significant quenching of populations of atomic states and nonequilibrium behaviour of continuum radiation.