Generation and characterization of laser sustainted plasmas of argon and AR-C02 mixtures under various flow conditions

An experimental study into generation and sustainment of near-atmospheric pressure laser sustained plasmas (LSPs) of argon and Ar-CO₂ mixtures, under varying flow conditions, has been conducted. The plasmas were generated and sustained using a 2 kW CO₂ laser in a coaxial Ar/Ar-CO₂ flow. Optical emission spectroscopy was used to map both the excitation and electron temperatures of the plasma, utilizing relative line and line to continuum intensity techniques. The electron density was measured from the Stark broadening of the H_alpha line. A high speed CCD camera was used to image the plasma. The plasma size and shape were found to vary with the gas flow velocity. Threshold flow rates for generation and sustainment of the LSP were established. The addition of CO₂ to the gas mixture quenched the plasma resulting in lower electron densities. The laser-sustained plasma comprised a plasma core surrounded by a hot gas envelope. Ar-CO₂ LSP had a similar shape to that of the Ar plasma with a larger hot gas envelope. An inverted funnel type of flow developed at the bottom of the plasma for Ar-CO₂ plasma at lower flow rates. This study of the composition and flow structure of LSPs will assist in understanding gas phase chemistry and spatial distributions of species concentrations. This will aid design and development of near-atmospheric pressure deposition process for hard coatings such as carbides and diamonds.