Laser-induced gas breakdown and ignition

Laser-induced gas breakdown and ignition are studied in atmospheric pressure gas flow. The nanosecond-pulsed, 1064-nm Nd:YAG laser was used to create the cascade-type optical breakdown in air, oxygen, ammonia, and the combustible am-monia/oxygen mixture. We investigate the formation of the initial plasma and the chemical and gasdynamic development of the breakdown kernel. The spatial and temporal features of the energy deposition process are presented for laser breakdowns in still air. The generation of air-breakdown events is very stable between laser pulses when the incident laser power is two times larger than the threshold value. The effects associated with the ammonia flow-speed in the range of 1-7 cm/sec are shown to be significant for the plasma formation and stability of both laser-induced breakdown and ignition kernel, even though the flow field is laminar. The post-breakdown de-velopment of laser breakdown and ignition is studied using high-speed photographic and spectroscopic techniques including shadowgraphs, planar laser-induced fluores-cence (PLIF), spontaneous emission and Rayleigh scattering. These time-resolved two-dimensional images provide gasdynamic, radiative and NH radical concentration and temperature information to aid the understanding of the kernel dynamics. The asymmetric feature of the initial plasma and the gas dynamics that leads to the backstreaming effect in laser-induced breakdown is suggested and evaluated.