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Title:Microwave Interferometric Measurements of Electron Density in Laser-Generated Plasma Channels
Author(s):Keister, Kathryn E.
Director of Research:Eden, James G.
Doctoral Committee Chair(s):Cooper, S. Lance
Doctoral Committee Member(s):Eden, James G.; Hertzog, David W.; Carney, Paul S.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
dissociative recombination
multiphoton ionization
electron density
plasma filaments
Abstract:Measurements of the temporal decay of the absolute electron density in a laser-produced plasma channel have been made with a 9.2~Ghz microwave interferometer. The plasma channels were generated by sub-picosecond laser pulses ($\lambda=$248~nm, 40~mJ per pulse) produced by a hybrid Ti:sapphire/KrF excimer amplifier system. The ultrashort pulse duration resulted in a $\delta$-function excitation source, allowing the subsequent plasma decay to be explored without further excitation of the gas and without deconvolving the excitation source profile from the electron density decay data. The temporal resolution of the interferometer has been demonstrated to be a few nanoseconds (bandwidth of $\approx$1~GHz), and electron density decay profiles have been measured in argon and nitrogen at pressures in the range of 1-650~Torr. Gas kinetic models in argon and nitrogen have been developed, and have shown good agreement with the measured electron density profiles. Model predictions of the dissociative recombination rate constants, $\alpha_D$, in argon and nitrogen are reported, \mbox{$\alpha_D\,(\mathrm{Ar})=1-6 \times 10^{-6}$~cm$^3$~s$^{-1}$} and \mbox{$\alpha_D\,(\mathrm{N}_2)=1-3.25 \times 10^{-6}$~cm$^3$~s$^{-1}$}. At pressures $>$200~Torr, the reported values show good agreement with previous measurements, and in the pressure range 20-200~Torr, the values presented here are the first to be reported. The multiphoton ionization (MPI) cross section for argon, $\sigma_{(4)}$, is estimated to be in the range $10^{-117}-10^{-118}$~cm$^8$~s$^3$, which is within an order of magnitude of the one previous measurement, and the experimental technique is shown to have the potential to improve the precision of this estimate.
Issue Date:2011-05-25
Rights Information:Copyright 2011 Kathryn E. Keister
Date Available in IDEALS:2011-05-25
Date Deposited:2011-05

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