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The dependence of laser-induced breakdown spectroscopy (LIBS) on ambient conditions has not been clearly clarified as yet for element qualitative and quantitative analysis. To fill this gap, a laser-induced air plasma was produced by focusing a 1064-nm laser beam of Q-switched Nd:YAG pulse laser and the influence of relative humidity on the air plasma was systematically investigated by varying humidity at different laser irradiances. In contrast to the early findings, i.e., weak dependence of plasma properties on the humidity, some distinctive characteristics have been observed and explained. Examining the time-resolved emission spectra shows that the temporal evolution of spectral lines almost remains unchanged with the humidity and the influence of humidity on emission spectra can be neglected under a short gate width less than several hundred nanoseconds. As for the time-integrated emission spectra obtained at a higher laser irradiance, the increase in the emission intensity of H and O with the humidity is attributed to the increasing mole fraction of H and O and more laser energy that can be absorbed by the increasing water vapor. A faster growth of emission intensity of H and a slower increase of emission intensity of O respectively result from the relatively large increase of mole fraction of H and the relatively small increase of mole fraction of O with the humidity. Observations indicate that a shorter gate width or a lower laser irradiance is preferable in reducing the humidity influence and a certain magnitude of emission intensity of H and O spectral lines should be subtracted from the measured spectra for a more accurate and more reliable LIBS examination. [ABSTRACT FROM AUTHOR]

A one-dimensional self-consistent fluid model was employed to investigate the influence of longitudinal argon flow on the DC glow discharge at atmospheric pressure. It is found that the charges exhibit distinct dynamic behaviors at different argon flow velocities, accompanied by a considerable change in the discharge structure. The positive argon flow allows for the reduction of charge densities in the positive column and negative glow regions, and even leads to the disappearance of negative glow. The negative argon flow gives rise to the enhancement of charge densities in the positive column and negative glow regions. These observations are attributed to the fact that the gas flow convection influences the transport of charges through different manners by comparing the argon flow velocity with the ion drift velocity. The findings are important for improving the chemical activity and work efficiency of the plasma source by controlling the gas flow in practical applications. [ABSTRACT FROM AUTHOR]

We explore an effective way to promote propane combustion by applying a plasma discharge for efficiency enhancement. A coaxial-cylinder, dielectric barrier discharge is used to activate propane and air before they are mixed with each other and ignited for combustion. The characteristics of the combustion flame are well studied and evaluated by varying various operational parameters. It is found that the combustion process can be enhanced by applying a plasma on either the propane or air stream, and the combustion stability is found to be somewhat sensitive to the lean burning conditions and confined to a relatively narrow operating window. The temperature and spectrum of the flame in the main combustion zone are investigated with a 4 W plasma in the on or off state. The main components are identified, and the possible physical and chemical reaction mechanisms are discussed. A comparative analysis of these spectra and temperatures obtained in the main flame suggests that the energy generated from the 4 W plasma is partially used to heat the reaction gases in the flame, and another part of the energy is used to increase the luminosity, especially for activation of air. We also observe that combustion of high flow rate propane and/or air requires more discharge energy density under certain conditions. A comparison of combustion enhancement through different activation methods in flame blowout tests shows that reactive species derived from activation of air play a more critical role in the blowout limit of propane combustion flame than those generated by activation of propane at low equivalence ratio and propane flow. [ABSTRACT FROM AUTHOR]