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

Abstract

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.