Experimental Characterization of Laser-Induced Plasmas and Application to Gas Composition Measurements

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Date
2005-08-08Author
Ferioli, Francesco
Advisor
Buckley, Steven G
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In this dissertation new applications of Laser Induced Breakdown
Spectroscopy (LIBS) are investigated. When a powerful laser beam
is focused to a high enough fluence breakdown occurs and a hot,
short-lived plasma is formed. In the first part of this
dissertation, an experimental study of laser plasmas generated in
air and argon is presented. The breakdown is investigated starting
from the formation of the plasma, through the subsequent phase of
adiabatic expansion, until the final decay. The results are
interpreted on the basis of an hydrodynamic model that describes
the plasma as a strong shock wave propagating through the fluid.
Simple dimensional and energy considerations allow derivation of
the general scaling laws that govern the regimes of motion. The
time and length scales appropriate to describe the motion of the
fluid at different stages are defined and, for every regime, the
comparison between experimental observations and theory is
presented. The proposed model appears to be consistent with the
observations, and the theory outlined in the paper can be used to
derive estimates of the fluid properties.
In the second part of the dissertation, atomic emission
from the plasma is used to perform direct measurements of atomic
species in mixtures of hydrocarbons and air. Atomic emission from
the laser-induced plasma is observed and ratios of elemental lines
are used to infer composition in reactants and in flames.
Equivalence ratio can be determined from the spectra obtained from
a single shot of the laser, avoiding time averaging of signals
with a spatial resolution on the order of a few mm. The strength
of the C, O, and N lines in the 700 - 800 nm spectral window is
investigated for binary mixtures of C3H8,
CH4, and CO2 in air. The dependence of the
atomic emission on the concentration of carbon and hydrogen is
investigated, as well as the influence of experimental parameters
such as the laser power and the temporal gating of the
detector.