Nonlocal thermodynamic equilibrium in laser-sustained plasmas

An argon laser-sustained plasma at atmospheric pressure has been studied spectroscopically and the existence of a nonlocal thermodynamic equilibrium state has been determined. The spectroscopic data consist of argonneutral and −ion line emissions used to spatially resolve electronic energy level population densities in each plasma species. A hydrogen seed is added to the argon flow for the purpose of determining electron number density by Stark broadening analysis of the Balmer series alpha line. Electron and heavy particle kinetic temperatures are calculated through the use of an appropriate nonequilibrium model. The dominant nonequilibrium effect in this plasma is kinetic nonequilibrium where the electron kinetic temperature can be more than twice the heavy particle kinetic temperature in high laser power flux regions. Typical electron and heavy particle kinetic temperatures are 14,000 K and 8000 K, respectively. Electron number density ranges from 6 × 10¹⁶ cm⁻³ to 2.1 × 10¹⁷ cm⁻³.