Laser spectroscopy for in situ diagnostics


B. Attal-Trétout, F. Grisch, D. Packan, I. Ribet-Mohamed and M. Lefebvre

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Laser spectroscopy based on nonlinear processes such as four wave mixing and laser-induced gratings or based on spontaneous Raman and Thomson scattering is widely used, from near IR to near UV, to probe reactive media. The bulk of this monograph is devoted to the application of these techniques to the specific area of plasma, combustion and hypersonic flow field. For each technique a brief description of the signal amplitude is given from which spectral and/or temporal evolution can be predicted as a function of temperature and pressure. Experimentally, the dependence of the signal on parameters such as temperature, pressure, gas composition, or the energy of pump and probe laser pulses have been investigated. Comparison of the experimental results with theoretical predictions allows us to accurately measure these parameters in situ. Therefore, temperature, velocity or number density of important species (major or minor) can be derived from the spectral or temporal content. The synchronization of pulsed laser with particular physical phenomena provides time evolution of the sample over a μs or ms duration, depending on the probed events, thanks to the time resolution (ns) of the lasers. Several examples are given. CARS is applied to a buoyant H2/air flame, a monodisperse droplet stream in combustion and a pulsed plasma in a methane/air flow. In each case, time and space evolution of temperature is recorded. DFWM has proved particularly useful for probing NO, especially when spectral interference is overwhelming the LIF signal; LIF is used otherwise. Laser induced gratings are also valuable tools in some typical cases. Examples are given to illustrate their potential in measuring NO2 molecules in a flame and in measuring the temperature and velocity of a high speed flow. Finally, electron density can be inferred using Thomson scattering and an application in the pulsed plasma is presented.