Non-Local Thermodynamic Equilibrium in Laser-Sustained Plasmas
Zerkle, David Karl
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https://hdl.handle.net/2142/72234
Description
Title
Non-Local Thermodynamic Equilibrium in Laser-Sustained Plasmas
Author(s)
Zerkle, David Karl
Issue Date
1992
Doctoral Committee Chair(s)
Krier, Herman
Department of Study
Mechancial Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Physics, Fluid and Plasma
Abstract
An argon laser sustained plasma (LSP) at atmospheric pressure has been studied spectroscopically 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 in 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. Typical electron and heavy particle kinetic temperatures are 14000 K and 8000 K, respectively. Electron number density ranges from 6 $\times$ 10$\sp{22}$ m$\sp{-3}$ to 2.1 $\times$ 10$\sp{23}$ m$\sp{-3}$.
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. Botlzmann 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 particle number density.
The results of the non-LTE spectroscopic analysis are compared to the results of a single temperature (LTE) numerical LSP model. It is found that the numerical model poorly predicts the plasma location with respect to the laser focus. Peak electron number density is comparable in both analyses, as is the overall plasma length, thus the prediction of total laser power absorbed by the plasma is approximately the same for both analyses.
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