Characteristics of laser-triggered electric discharges in air

An experiment of laser-triggered electric discharges has been conducted in a uniform background electric field. By focusing the beam of a pulsed Nd-YAG laser (wavelength 532 nm, pulse duration 5 ns) in a narrow (15 mm) plane-parallel air gap exposed to a DC voltage, streamer discharges and disruptive discharges were triggered. The laser energy and the background electric field strength were varied in the ranges 0-180 mJ and 0.8-1.4 MV/m, respectively, and the laser plasma was induced either close to the anode, close to the cathode or mid-gap. From images of the electrode gap and from current measurements, the occurrence of and time to the streamer discharge and the disruptive discharge were determined. No distinct level was found of either the laser energy or the plasma energy for the transition from one discharge case to another, even if the general trend was that the energy required for a certain discharge case was reduced when increasing the background electric field. These and other observations make the method of laser triggering a less suitable method for studies of streamer initiation and streamer propagation, but revealed several interesting features of laser-triggered electric discharges.