Guiding large-scale spark discharges with ultrashort pulse laser filaments

Using the nonlinear propagation properties of ultrashort laser pulses in ai r, we were able to produce long ionized filaments that served to guide spar k discharges. With a laser pulse energy of 20 mJ, one or two ionized filame nts were created and could guide streamer discharges over 2 m air gaps, whe re the electric field was fairly uniform and had an average value of 0.6 MV /m. Such a guiding effect was observed for times of 1-3 mu s after the lase r pulse created the ionized filaments. Longer delays (10-15 mu s) were reco rded at higher laser pulse energy, with a larger number of filaments. Image s of the early stages of the discharge of a uniform air gap show that the l aser-produced ionized filaments do not initiate the discharge process but a ct rather as preferred channels where the leader growth is accelerated. In the end, these straight conductive channels carry the arc current as the vo ltage in the gap breaks down. (C) 2000 American Institute of Physics. [S002 1-8979(00)06214-9].