Triggering and guiding large scale leader-discharges with ultrashort laser pulses

Summary form only given, as follows. Ultrashort pulse lasers, which can produce long continuous plasma channel with a relatively small amount of energy, are interesting candidates to achieve laser triggering and guiding of lightning. In order to investigate this possibility, we have used picosecond pulses of 400 mJ delivered by a Ti:sapphire laser to trigger and guide leader discharges in gaps of 3 to 7 m. We used two types of discharge configuration: (i) gap between a positive rod and a grounded plane and (ii) gap between a negative plane and a rod on a grounded plane. The discharges were studied by means of optical and electrical diagnostics, such as a streak camera and a current probe. First, we have observed that the laser pulse can initiate a corona at the tip of the positive rod and instantaneously trigger leader propagation, at a voltage that can be 30% lower than the normal minimum inception voltage of the first corona. Also, we have seen that the leader propagation can be guided by the laser-created plasma channel on a distance of up to 4 m, with a tenfold increase of its velocity. The leader current records revealed that this latter effect is accompanied by a significant reduction (by 4 to 5 times), of the amount of charge, fed to the leader tip by its corona, needed to generate by Joule heating a given length of leader channel. Our results show that the lower leader inception voltage combined to the higher leader speed can lead to a 40% smaller gap breakdown voltage. The length and the quality of the leader guiding has been studied as a function of the focal length of the lens used to produce the plasma channel and of the laser pulse characteristics (energy, duration).