Multi-focus structure of light filament.
Dynamics of a light filament formation is theoretically considered upon non-stationary self-focusing of a femtosecond laser pulse with supercritical power in air. The qualitative interpretation of this process within the framework of filamentation model in terms of beam effective radius as a chain of nonlinear foci formed by slice-by-slice focusing of a light pulse is carried out. Thus, the smaller focal waist form of the temporary slices, located in the centre and on back front of a pulse, and the bigger sizes are characteristic to the pulse forward edge. The focal waist series of particular temporary slices of a pulse form an elongated waist of variable diameter, which represents a light filament.