激光引致水诱导空气击穿所获无约束流场演化过程的实验研究

Experimental Study on Evolution Process of Unconfined Flow Produced by Laser-induced Air Breakdown with Water as an Induction

流场演化过程是揭示激光推进机理的重要研究内容之一。利用纹影系统和PCO-HSFC高速相机,首次拍摄了自来水诱导CO2激光击穿空气产生的激波向空气和水中的演化过程。阐述了产生两次气蚀空穴的原因:初始时刻产生的空穴为激波作用于水面所致,第二次空穴出现在相机被触发后约8μs时,是激光烧蚀水蒸气作用于水面所形成的低压区。实验结果表明:击穿后流场向激光入射方向(空气中)演化较快,激波初速度较大,约为6km/s;流场向水中演化相对较缓,激波初速度约为3.33km/s。两者都服从指数衰减,分别在约73.368μs和41.649μs时衰减到声速。研究结果对于把水作为工质应用于激光推进有一定意义。

It is important to study on evolution of flux-field to discover the mechanism of thrust formation in laser propulsion. The process of shock wave spread in air and water is obtained by a set of schlieren systems and a high speed framing camera, during which the shock waves are generated by CO2 laser-induced air breakdown with water as induction. It is found that cavitation appears two times. The first is caused by action of shock wave on water surface, which appears at initial time;the second is a low-pressure area because of action of evaporation induced by laser ablation on water surface, which appears at about 8 μs after the camera is activated. Experimental results show that after the breakdown, the evolution of flux-field which transmits in incident direction of the laser in air is faster than that in opposite direction in water,with initial velocities of shock wave being about 6 km/s and 3.33 km/s respectively. Both velocities of shock wave follow exponential damping and attenuate to voice velocities at about 73. 368μs in air and 41. 649 μs in water. This work is of promoting significance for laser propulsion with water as propellant.