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Home > Seminars and Workshops > Past seminars, PhD & HDR defense > 2018 > Séminaires 2018 > Le jeudi 4 octobre 2018 à 10h30

Le jeudi 4 octobre 2018 à 10h30

Salle de réunion Jean Lascoux, aile 0, RdC

10h30: Accueil café

11h00: Séminaire
Igor V. Adamovich (Department of Mechanical and Aerospace Engineering, Ohio State University) - Electric Field Measurements in Atmospheric Pressure Plasmas and Flames by Ps Four-Wave Mixing and Ps Second Harmonic Generation"

Résumé :
Non-intrusive laser diagnostic measurements of temporal and spatial distributions of electric field in atmospheric pressure plasmas are essential for development of engineering applications such as plasma flow control, plasma-assisted combustion, plasma materials processing, and plasma medicine. This work presents an overview of electric field measurements in atmospheric pressure plasmas by ps four-wave mixing and, more recently, by ps second harmonic generation. In both cases, absolute calibration is obtained from measurements of a known Laplacian field.
Picosecond four-wave mixing measurements have been done in ns pulse discharges in ambient air for several electrode geometries. For short voltage rise times of several ns, peak electric field considerably exceed DC breakdown threshold. Sub-nanosecond time resolution is obtained by monitoring the timing of individual laser shots relative to the voltage pulse, and post-processing four-wave mixing signals saved for each laser shot, placing them in the appropriate “time bins”. The main advantages of second harmonic generation over four-wave mixing are that it is considerably more sensitive and species independent, such that is can be used in any high-pressure plasma. Ps second harmonic generation are used to measure electric field in dielectric barrier discharge plasma flow actuators, in atmospheric pressure flames enhanced by transient plasmas, and in atmospheric pressure plasma jets. Similar to four-wave mixing, individual electric field vector components are isolated by measuring second harmonic signals with different polarizations. In ns pulse plasma actuators, the data show that surface charge accumulation strongly affects the electric field in the plasma, and that a significant energy fraction is coupled to the plasma at a relatively low electric field, limiting the effect of localized thermal perturbations (“rapid heating”) on the flow. In flames, combining ns pulse and AC waveforms results in a strong effect of the plasma on the flame, due to the ionization produced by the ns pulses and the ion wind generated by the AC field. In atmospheric pressure plasma jets, inference of the electric field is complicated by mixing of the jet with ambient air species.
The results demonstrate considerable potential of these techniques for electric field measurements in high-pressure gas discharges, providing quantitative insight into kinetics of ionization, charge transport, molecular energy transfer, energy thermalization rate, plasma chemistry, and coupling with the gas flow.


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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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