The influence of electron density on the formation of streamers in electrical discharges triggered with ultrashort laser pulses

In an ongoing program using ultrashort laser pulses to provoke discharges i n air over considerable distances at electric fields below breakdown thresh old, me have studied the conditions for the onset of streamers in such lase r-produced plasmas, both experimentally and through numerical simulations. The results demonstrate the importance of the electron density and of its g radient on the generation of streamers. Also, a significant reduction of th e breakdown voltage for a 30-cm plane-plane gap in air was observed with a laser pulse energy of 15 mJ, Finally, a direct comparison of laser-induced breakdown in air and in nitrogen shows the influence of electron attachment to oxygen on the discharge process.