Flame control by high-energy light source and elucidation of interaction mechanism between light and flame by spectral measurement

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Firstly, Influence of the laser beam characteristics on the laser-induced breakdown of air was studied. Energy needed for the breakdown was precisely evaluated from the transmitted laser energy from the breakdown area. As the laser beam characteristics was varied, the breakdown threshold was unexpectedly found to be increased when the narrowest pulse width (8 ns) was used, and the threshold value was approximately 2.5 mJ under a focusing lens of f=100 mm. In addition, the aberration of the lens is found to be an important factor for the breakdown threshold when the laser-induced breakdown is performed under lens having a short focal length, such as f=50 mm.Secondary, we observed chemiluminescence spectra from the plasma formed by laser-induced breakdown of air at atmospheric pressure using Cassegrain optics and a streak camera. The streak images showed three characteristic regions in the plasma when the energy of the laser pulse was 23mJ and was focused by an f=100mm lens. The central … More region of the plasma was characterized by its weakest emission, but the onset of the laser breakdown was found to initiate in this area. In addition, two emission peaks of the chemiluminescence were observed in this area. The phenomenon was seem to be originated from the propagation of the shock wave in plasma, and which speed was calculated at approximately 12.7km/s.Finally, We investigated the influence of the spherical aberration on ignition performance owing to laser beam focusing. Although ideal optical elements can produce a perfectly converging spherical wave front, the spherical aberration of actual optical system results in a complicated intensity distribution around the focal point, which will significantly affect the laser-induced ignition of a combustible mixture. To clarify this, laser-induced breakdown in air with actual lenses was studied experimentally and numerically. Variations in the breakdown threshold and focusing profiles of the laser beam were investigated under various focusing conditions. We found that considerable influence due to spherical aberration existed in conventional laser-induced breakdown experiments. As a result, we should pay a careful consideration on the spherical aberration in performing precise laser ignition experiments. Less