Experimental study of relativistic self-focusing and self-channeling of an intense laser pulse in an underdense plasma

This paper reports on experimental investigations on relativistic self-focusing and self-channeling of a terawatt laser pulse (0.7 TW ≤ P ≤ 15 TW) in an underdense plasma. The authors present results obtained with picosecond (τ = 1 ps) and subpicosecond (τ = 0.4 ps) pulses. In the ''long pulse'' regime, modifications in the laser propagation are observed for P < Pc, the critical power for self-focusing. By contrast, self-guiding of subpicosecond pulses is observed for P ∼ Pc. Using a paraxial envelope model describing the laser propagation and taking into account the plasma response to the ponderomotive force, it is shown that a maximum laser intensity of 5--15 times that reached in vacuum may be achieved when P is in the (1.25--4) x Pc range. It is also demonstrated that ion motion may significantly reduce the effective Pc