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

首先，研究了激光束特性对激光诱导空气击穿的影响。击穿所需的能量由击穿区域发射的激光能量精确计算。随着激光束特性的变化，当使用最窄的脉冲宽度（8 ns）时，击穿阈值意外增加，在f=100 mm聚焦透镜下，击穿阈值约为2.5 mJ。此外，在焦距较短的透镜（如f=50 mm）下进行激光诱导击穿时，发现透镜的像差是影响击穿阈值的重要因素。其次，我们利用卡塞格伦光学系统和条纹相机观察了大气压力下激光诱导击穿空气形成的等离子体的化学发光光谱。当激光脉冲能量为23mJ时，用f=100mm透镜聚焦等离子体，条纹图像显示出三个特征区域。等离子体的中心…更多区域的特征是其最弱的发射，但发现激光击穿的开始是在这个区域。此外，在该区域还观察到两个化学发光峰。这一现象似乎源于等离子体中冲击波的传播，其速度约为12.7km/s。最后，研究了激光聚焦引起的球差对点火性能的影响。虽然理想的光学元件可以产生完美会聚的球面波前，但实际光学系统的球差导致焦点周围的强度分布复杂，这将显著影响可燃混合物的激光诱导点火。为了澄清这一点，用实际透镜对空气中激光诱导击穿进行了实验和数值研究。研究了不同聚焦条件下激光束的击穿阈值和聚焦曲线的变化。我们发现，在常规的激光诱导击穿实验中，球差对实验结果有相当大的影响。因此，在进行精密激光点火实验时，球差问题是必须认真考虑的。少

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