Spatial characterization of laser-induced sparks in air

Images and emission spectra of sparks produced by laser-induced breakdown in air were investigated as functions of the laser energy and optical configuration. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using an intensified CCD camera and a Cassegrain optics system coupled to an ICCD spectrometer. The results provided information about the first stages of laser-induced spark breakdown. Good reproducibility of the plasma location and shape was observed; these parameters depended largely on the optical configuration and plasma energy absorption rate. The high spatial resolution of the Cassegrain optics system was used to observe different ionization levels in the plasma kernel, which confirmed the electron cascade mechanism for plasma formation. The different ionization levels partially explained the asymmetry of the ignition induced by the plasma generation in gaseous mixture. Backward propagation of the plasma along the laser path was observed using the high spatial and temporal resolution of the experimental apparatus. The propagation was largely due to the thickness of the plasma relative to the laser wavelength, which created different ionization levels and energy absorption rates throughout the plasma. This observation was correlated with images obtained using the ICCD camera.