Lineshape Analysis of Laser Light Scattered by AN Argon Thermal Plasma Jet

Radial gas temperature and velocity profiles in the exit plane of an atmospheric pressure argon thermal plasma jet were determined from high resolution lineshape analysis of elastically scattered laser light. Unlike emission spectroscopy, lineshape analysis of scattered laser light does not rely on assumptions of local thermodynamic equilibrium (LTE) or specific non-LTE models to measure gas or kinetic temperature, and has a high degree of spatial resolution. At atmospheric pressure, if electron densities are > 10^{22} m^{-3}, coherent Thomson scattering dominates, while at lower electron densities, Rayleigh scattering dominates. Using a frequency doubled injection-seeded Nd:YAG laser and a scanning tandem Fabry-Perot interferometer, high quality lineshapes were measured in the exit plane of a commercial subsonic plasma torch at several radial positions with torch operating currents ranging from 300 to 900 A. The experimental lineshapes were fitted by the lineshape theory and the gas temperatures determined. The scattered wavevector had a component along the flow axis of the jet resulting in a Doppler shift of the frequency of scattered light relative to the frequency of the incident laser. From this Doppler shift the gas velocity was calculated. Maximum centerline temperature and velocity values observed were 13350 K +/- 7% and 100 m s ^{-1} +/- 3%. The fitting errors for the centerline lineshape data were 2-3% for all operating currents. The minimum velocity measured in the fringe of the jet was 45 m s^ {-1} +/- 45%. Comparison of temperature profiles calculated from the lineshape data with temperatures determined from LTE emission spectroscopy shows a severe departure from LTE in the outer regions of the jet. Centerline temperature and velocity values determined from lineshape analysis of scattered light increase with torch operating current to about 600 A after which they remain essentially constant. However, spectroscopic measurements show that the electron temperature and density continue to increase with current. This indicates that increasing the electrical power dissipated by the torch drives the plasma away from LTE. Measured radial velocity profiles are very nearly parabolic and differ significantly from velocity profiles assumed by computational modelers as boundary conditions for models of the plasma jet.