Abstract: Energy deposition in air-breathing laser propulsion usually occurs during the formation and evolution of laser-supported detonation (LSD) wave. Therefore, analyses of propagation laws and influence factors of LSD wave play a fundamental role in disclosing the mechanism of energy deposition. The schlieren photographs of CO₂ laser-induced air breakdown at different focusing angles are taken with a nanosecond framing highspeed camera. During the process, an optical lens is used and ray path of the schlieren system is extended. Further, the wave velocity is also analyzed through the one-dimensional theoretical formula. The results indicate that ① under the given experimental conditions, the process of the air breakdown in air-breathing laser propulsion includes two stages: the LSD wave stage and the laser-supported combustion (LSC) wave stage. The duration of the former stage lasts approximately 7.1 μs; ② at three different focusing angles, the overall trend of LSD wave velocity obtained in experiment is generally consistent with the estimated one predicted by the theoretical formula. The smaller the focusing angle is, the faster the initial LSD wave velocity is; it reaches about 14 km/s when the angle is 4.29°.

Key words: laser propulsion, focusing angle, air breakdown, laser-supported detonation wave, laser-supported combustion wave, schlieren systems