[en] 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 < P_c, the critical power for self-focusing. By contrast, self-guiding of subpicosecond pulses is observed for P ∼ P_c. 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 P_c range. It is also demonstrated that ion motion may significantly reduce the effective P_c

[en] Experiments carried out with a laser beam focused into a vacuum chamber onto a 3-mm long, pulsed hydrogen jet, at powers close to the critical power required for relativistic self focusing, have shown that an underdense plasma is able to significantly reduce the divergence of an intense laser pulse. The propagation mode is in good agreement with theoretical predictions of relativistic self focusing. 2 figs., 8 refs