  A theoretical study of power loss from periphery of an ultrashort pulse laser
beam and temporally resolved defocussing produced by laser induced plasma are
performed using paraxial approximation. Our analysis incorporate consideration
of spatial distribution of the laser beam irradiance and the results show that
substantial power losses (10%-80%) occur from the beam periphery limiting
length of a filament. It was also shown that generally accepted concept of
self-focusing critical power is inconsistent with consideration of self-induced
refraction of spatially distributed laser beam. A new criterion for
self-focusing and hypothesis for multiple filamentation are proposed.
