The equations for time-averaged diffraction beams describing self-focusing of femtosecond laser radiation in air are derived on a basis of nonlinear Schrodinger equation. It is shown numerically that these beams pass three stages along propagation distance: 1) nonlinear focus formation; 2) light filament; and 3) post-filamentation evolution. The diffraction beams at the second and third stages are characterized by two structures; one of them, localized in a paraxial area, has an angular divergence, less than the diffraction one after output from a filamentation zone. Another structure is a peripheral area with characteristic divergence, corresponding to divergence of the laser beam on the whole after passing through the nonlinear focus zone. The principal role of diffraction interaction between central and peripheral parts of a laser beam in formation of stable light channel near the beam axis is established.
femtosecond laser radiation, diffraction beams, filamentation