Peculiarities of self-focusing and filamentation of spatially focused ultrashort laser radiation in air are theoretically investigated. The influence of beam initial focusing on dimensional and energetic parameters of formed filaments is studied. It is established, that focusing of femtosecond laser radiation can lead to formation of several spatial zones characterized by weak change of radius of a beam. These zones have various extent and, depending on focusing strength, are characterized by various values of peak intensity, minimal beam size and density of free plasma electrons.
ultra-short laser radiation, self-focusing, filamentation, photoionization, spatial beam focusing