Triggering and guiding leader discharges using a plasma channel created byan ultrashort laser pulse

In a 2.8 m positive rod-plane air gap, we have studied how a plasma channel produced by focusing a 200 mJ ultrashort laser beam is able to trigger and guide a leader discharge. We have observed that the plasma channel allowed the lowering of the leader inception voltage by 50% and the guiding of the leader propagation on a distance of up to 2.3 m, with a tenfold increase o f its speed. This led to an effective 40% reduction of the breakdown voltag e. For the conditions studied here, the laser energy per unit length requir ed to guide a leader is between 60 and 100 mJ/m. (C) 2000 American Institut e of Physics. [S0003-6951(00)03607-X].