The results of the laboratory experiments on measurements of length and position of filamentation zone of femtosecond laser GW radiation at wavelengths of 800 and 400 nm with millimeter-sized aperture propagating in air under conditions of sharp external focusing are presented. Dependencies of length of the radiation filamentation zone on its initial radius and power are studied. Qualitatively new regularity is obtained, which consists in invariance of the observed length of the beam filamentation zone with respect to variation of its diameter with the assumption of equality of the initial intensities. The regularity is not a characteristic of collimated radiation and connected with geometrical focusing dominance over Kerr self-focusing of the beam during filament formation.
ultrashort laser radiation, sharp focusing, filamentation, plasma channel, Kerr effect