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 less than or equal to P less than or equal to 15 TW) in an underdense plasma. We present results obtained with picosecond (tau = 1 ps) and subpicosecond (tau = 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 approximate to 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.