Non-Local Thermodynamic Equilibrium in Laser Sustained Plasmas

摘要

An argon laser sustained plasma (LSP) at atmospheric pressure has been studiedspectroscopically and the existence of a non-local thermodynamic state has been determined. The spectroscopic data consist of several argon neutral 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. Neutral and ionic argon electronic excitation temperatures are calculated from the spectroscopic data. Electron and heavy particle kinetic temperatures are calculated through the use of an appropriate nonequilibrium model which includes multitemperature gas state, and ionization equations. 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. It is found that a local thermodynamic equilibrium (LTE) analysis of an ion upper energy level population density leads to an excellent prediction of ion number density. This is determined by comparison of the ion number density to the electron number density calculated through the hydrogen Stark broadening analysis, and assuming low temperature quasi-neutrality. Boltzmann equilibrium in the ionic argon system is indicated. LTE analysis of a neutral argon upper energy level population density leads to a very poor prediction of electron number density, but a fairly accurate prediction of neutral number density.