Determination of gas-temperature and velocity profiles in an argon thermal-plasma jet by laser-light scattering.

Abstract

Gas-temperature and velocity profiles at the exit plane of a thermal argon plasma torch have been determined directly from a high-resolution Doppler-shifted line-shape analysis of laser light scattered by the plasma. Peak temperature and velocity values observed were 13 350 K\ifmmode\pm\else\textpm\fi{}7% and 1100 m ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$\ifmmode\pm\else\textpm\fi{}3%. Velocities as low as 45 m ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$\ifmmode\pm\else\textpm\fi{}45% were measured in the fringe of the jet. An injection-seeded pulsed neodymium-doped yttrium aluminum garnet laser was used as the laser source and the scattered laser light was analyzed with a scanning tandem Fabry-P\'erot interferometer. Temperature data obtained from laser scattering are compared with values obtained from emission spectroscopy and show a severe departure from local thermodynamic equilibrium (LTE) in the outer regions of the jet. Gas temperatures were observed to increase as the torch operating current increased from 300 to 500 A, but remained constant as the operating current increased to 900 A. However, electron temperatures and densities continued to increase with operating current. This suggests that increasing the electrical power drives the plasma away from LTE. Measured temperature and velocity profiles were found to be nearly parabolic.