激光诱导空气击穿的二维高速双帧阴影成像测量
We present two-dimensional measurements of thelaser-induced plasma development and shock wave evolutionin air. The breakdown is induced by a Q-switchedNd:YAG laser (λ = 1064 nm) with a pulse duration of 4 ns.To study these fast laser-induced phenomena, we have developeda high-speed, two frame shadowgraph method. Itenables 2D visualization of a laser-induced event in two timeinstances, which are delayed by an arbitrary time intervalin the range from 300 ps to 30 ns. The established methodis based on 30 ps, green (λ = 532 nm), and linearly polarizedlaser pulse, which is split into two orthogonally polarizedillumination pulses for direct and delayed illuminationof the breakdown area. Exploiting polarization of the probepulses, we capture two temporally and spatially separatedframes with two CCD cameras. Special attention is given tothe subsequent data processing, especially to the minimizationof the systematic error due to alignment of both images,and to the determination of 2D velocity distribution from thecaptured image pairs.