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 of its speed. This led to an effective 40% reduction of the breakdown voltage. For the conditions studied here, the laser energy per unit length required to guide a leader is between 60 and 100 mJ/m.

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