Mechanisms of signal uncertainty generation in laser-induced breakdown spectroscopy within different flame regions

Abstract

The inhomogeneous matrix of flame has distinct effects on signal uncertainty of laser-induced breakdown spectroscopy (LIBS) which hindered quantitative equivalence ratio measurement across the entire combustion field. The spectral signal uncertainty in different flame regions, including low-temperature premixed reactants and high-temperature products, were analyzed respectively. Using time-resolved spectra and fast photography techniques, the generation mechanism of spectral signal uncertainty was elucidated. The generation and evolution processes of plasma were affected by incident laser energy and properties of target gas medium. Under atmospheric conditions, excessive laser energies (≥ 100 mJ) in low-temperature premixed gases led to a rise in signal RSD due to the great disturbance caused by intense plasma rebounding process and consequent chemical reactions. In high-temperature reacting regions, only a small portion of laser energy could be deposited in plasma when the incident laser energy was low (< 60 mJ). Due to the distinct process of breakdown and laser-plasma interaction in reacting regions, a two-lobe structure plasma was discovered. The interaction between these lobes disrupted the plasma evolution process and caused high fluctuation in plasma morphology. Clear understanding of the generation mechanism of signal uncertainty is favor to optimize experimental parameters and improve LIBS quantitative performance in combustion diagnostics.