Effect of high-power laser divergence on the plasma structural parameters during multiple filamentation in air

Multiple filamentation of an infrared high-power laser pulse in air is considered. Based on the numerical solution to the unidirectional pulse propagation equation, the effect of radiation external focusing on the spatial structure of the plasma area produced in the filamentation region is studied. We show that the number of generated plasma channels in the beam wake and the density of their spatial distribution over the filamentation region depend on the initial divergence of laser radiation. We found that in a specific range of beam focusing the number of produced plasma channels could be minimized due to the formation of a consolidated thick plasma bunch at the beam axis.

Figure 1

The number of plasma channels along an optical path during filamentation of a pulse with (a) $E_0=10$ and (b) 50 mJ and different focal distances $f$.

Figure 2

(а) Maximal number $N_m$ and (b) total cross-sectional area $s_m=S_m/\left(\pi R_0^2\right)$ of plasma channels as functions of the focal length $f$.

Figure 3

Plasma density ${\rho }_e$ transverse profiles for pulse with $E_0=50\mathrm{mJ}$ and (a–d) $f=15$, and (e–h) $f=100\mathrm{cm}$ at $z=8$ (a), 12 (b), 14 (c), 18 (d), 72 (e), 101 (f), 150 (g), and 200 cm (h).

Figure 4

Volume distributions of free-electron density (isosurfaces) and optical fluence (tonal figures) during laser pulse filamentation in air for $E_0=50\mathrm{mJ}$, (a) $f=15\mathrm{cm}$ and (b) 100 cm. The laser radiation is directed from left to right.