When a laser beam of irradiance in the order of 1010 W/cm2 interacts with a gas, a spark plasma of high temperature and pressure is generated. Sparks created this way are strong enough to ignite a gaseous combustible mixture, liquid fuel sprays, or even to extinguish a diffusion flame. Comparing with other conventional ignition methods such as electric spark plus, plasma jet ignitor, or rail plugs, etc., laser ignition is more advantageous because, with laser ignition, the control over ignition location, ignition timing, ignition energy can be easily carried out. Also, laser ignition is non-intrusive, and it is capable of providing center ignition and/or multiple ignition sites. Thus, for better understanding of laser-induced spark ignition of gas mixtures, it is important to study the evolution of the laser spark during the ignition process. The result showed that after breakdown, the spark expands asymmetrically. The initial velocity can be greater than 107 cm/s and it moves towards the lens faster than it does away from the lens. Numerical analysis was carried out and many loss mechanisms were quantified. The calculations indicated that the shock radius is proportional to t0.4 as functionally described by the blast wave theory.
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