Nanosecond Laser Plasma Shock Wave Of The Initial Phase Of The Optical Diagnostic Experimental Study

The basic theory of laser-induced air plasma and shock wave transmission is summarized in this paper as well as the overall introduction of the general experimental methods in diagnosing laser-induced plasma. Plasma spark photography and optical deflection methods are used to study the initial evolution process of plasma and shock wave.Through the experiment of plasma spark shooting by industrial high-speed CCD under different laser energy, the shape, size and color of plasma are studied. The results show that plasma plume looks like a liquid-droplet in space and expands in the opposite direction of the laser beam. With increased laser energy, initially the size of plasma increases greatly both in the horizontal and vertical directions. As time goes, the expand trend will slow down. The photos of plasma are handled by Matlab software through denoising and sharpening which will strengthen the brightness and clearness. Three layers of Plasma plume can be divided by color. The most internal layer shows white with the largest area. The middle layer shows mixed color inclined to blue with middle temperature while the outermost layer is green with the lowest temperature. Comparisons on plasma caused by different focal lengths show that the use of long focal length lens in air is easy to produce multiple breakdown.Optical deflection method is used to study the changes in velocity attenuation of the plasma and shock wave. The results show that the plasma in a relatively short range from the probe-light moves the same speed as shock wave. With the increase of detection range, the shock wave detaches from the plasma. The plasma gradually slows down and its wave-form is broadened. When the distance is up to 3mm away from the detecting light, it extinguishes. The experimental date is fitted by least square method and by derivation the velocity attenuation curve of the shock wave is got. The curve tells that the velocity of the shock wave attenuates more severe in pre-5mm from the initial velocity 10 times of the sound speed to 500 m/s. Later the velocity attenuation of shock wave tends to smooth till reaching the speed of sound at the position of 10mm.