The irradiation of a KrF laser (248 nm) into the interelectrode space affects the electric flashover characteristics there. The results obtained show that negative oxygen ions dominate formation of the interelectrode arc in air. A laser beam is introduced parallel to the interelectrode axis and the equipment is arranged to prevent the occurrence of photoelectric effects on the electrode surfaces. The time separation between laser firing and application of the interelectrode voltage is varied from 10 ns to 10 ms. The interelectrode spacing is set in the range 2–12 cm. The arc formation time and the flashover probability are measured. The initial ion number density is estimated experimentally to be on the order of 1011 cm−3 throughout the channel.

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Electrical breakdown streamers have been triggered in atmospheric air by laser-induced ionization. Streamers were successfully triggered even when the voltage applied to the electrodes was too low for breakdown to occur by the influence of the electrical field alone. The lifetime of the observed triggering effect has been found to be no less than 1 ms. Laser-induced streamers were photographically studied and conveniently classified by their observed shapes. In addition, the statistics of the observed phenomena have been compiled according to the classified shape. The statistics indicate that the shape of streamer is likely to be determined by the degree of laser-induced ionization. It has also been found that the speed of streamer formation is strongly related to the degree of ionization.