Rayleigh Scattering Measurements of Heating and Gas Perturbations Accompanying Femtosecond Laser Tagging
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Copyright 2016 by Joseph I. Milluzzo III and J. Gordon Leishman. Planar Rayleigh scattering and Rayleigh scattering polarimetry are applied toward measurements of molecular dissociation, thermometry, and the visualization of fluid dynamics in the afterglow of a femtosecond laser plasma under conditions relevant to the femtosecond laser tagging diagnostic. The generation of shock waves and a hightemperature region are observed, with a corresponding temperature increase of 280 40 K (320/iJ, N2) and 860 160 K (780/iJ, N2) produced with a 175 mm focusing lens. The spatial profile of the temperature rise along the beam is found to scale linearly with the optical emission intensity. The transverse width of the emission region is also observed to correlate with increased temperature. Rayleigh polarimetry measurements show a high dissociation fraction of up to 0.46 0.04 (780 J, N2). Modeling of three-body recombination into the N2 (B3ng,u ') state using the measured dissociation fraction is able to reproduce the emission intensity time history between 1 and 50 s. Local heating is found to play a key role in the observed trends in the spatial profile, width, intensity, and temporal evolution of femtosecond laser tagging at laser intensities near or above 3.9 1014 W/cm2.