Leader effects on femtosecond-laser-filament-triggered discharges

Abstract

Dynamics of laser filaments in strong nonuniform electric fields is studied with high temporal and spatial resolution. Considerable reduction of the breakdown potential is found and is attributed to a filament-induced leader. Two breakdown modes, fast and slow, are found in 0.4MV positive dc-voltage discharges activated by filaments that are induced by 65fs, 170mJ laser pulses. In the fast mode with duration order of a few microseconds, the filament may acquire the electrode potential and temporarily maintain it, becoming a leader. This gives rise to an average electric field over the attachment instability threshold between a leader head and cathode. Ionization waves precede the breakdown with maximal voltage reduction up to 40% for this mode. The slow mode with its duration order of 1ms appears with a considerably smaller voltage reduction when the leader decays before the secondary streamer; the breakdown delay depends on negative and positive ion mobilities in this case.