ИСТИНА

We investigate filament spatial interactions within femtosecond multifilaments in air under various focusing conditions in stochastic and regularized regimes. A bunch of filaments was formed by radiation of a Ti:Sa laser system delivering pulses with 55 fs duration and 5–25 mJ energy at 10 Hz repetition rate with 7mm FWHM beam diameter. Focusing conditions have been changing by introducing lenses with 20 cm -- 3 m focal lengths into optical path. Regularized multifilament was optained by introducing amplitude mask before the lens. We showed that superfilament is formed only in the case of weak additional focusing (3 m and 1 m lenses). In this regime a small number of filaments (several times smaller than P/P_cr ratio) is formed. Each of them contains higher plasma concentration than a single filament under the same focusing condition. The energy deposition from each filament of this bundle is more than 1 order higher than from the single filament. Otherwise, under more tight focusing filament bunch doesn't form similar unified stuctures. Energy deposition grows linearly with the pulse energy and plasma concentration doesn't considerably exceed this value for a single filament under the same focusing conditions. We also revealed that at the energies that correspond powers above the 5P_cr superfilament tends to split into several filaments (each of them with higher plasma concentration than single filament has), so one cannot fuse filaments in a bunch into one filament with arbitrarily high plasma concentration.