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

首先，研究了激光光束特性对空气击穿的影响。击穿所需的能量由击穿区域的透射激光能量精确估算。当改变激光束特性时，意外地发现当使用最窄脉冲宽度（8 ns）时击穿阈值增加，并且在f=100 mm的聚焦透镜下阈值约为2.5 mJ。发现透镜的像差是影响激光器击穿阈值的重要因素。诱导击穿是在具有短焦距（例如f=50 mm）的透镜下进行的。其次，我们使用卡塞格伦光学器件和条纹照相机观察了来自由大气压力下的空气的激光诱导击穿形成的等离子体的化学发光光谱。当激光脉冲能量为23 mJ，用f= 100 mm的透镜聚焦时，条纹图像显示了等离子体中的三个特征区域。中央 ...更多信息 等离子体区域的特征在于其最弱的发射，但是发现激光击穿的开始在该区域中开始。此外，在该区域观察到两个化学发光发射峰。这一现象可能是由激波在等离子体中传播引起的，其传播速度约为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一个完美会聚的球面波阵面，但实际光学系统的球差会导致焦点附近的光强分布复杂，这将严重影响可燃混合物的激光点火。为了阐明这一点，在空气中的实际透镜的激光诱导击穿进行了实验和数值研究。在不同的聚焦条件下的击穿阈值和激光束的聚焦轮廓的变化进行了研究。我们发现在常规的激光诱导击穿实验中，球差对击穿的影响很大。因此，在进行精密激光点火实验时，应充分考虑球差的影响。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