The filamentation of femtosecond light pulses in air is numerically and experimentally investigated for beam powers reaching several TW. Beam propagation is shown to be driven by the interplay between intense, robust spikes created by the defects of the input beam and random nucleation of light cells. Evolution of the filament patterns can be qualitatively reproduced by an averaged-in-time $(2\mathrm{D}+1)$-dimensional model derived from the propagation equations for ultrashort pulses.

• Received 11 July 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.225002