Evidence for self-channeling of a relativistically intense laser pulse in an underdense plasma is presented through Schlieren and $90°$ Thomson sidescatter images. Using collective Thomson scattering of a probe beam, we observe that relativistically propagating plasma waves are excited over the entire length of the channel, up to 12 Rayleigh lengths $\left(\approx 4\mathrm{mm}\right)$. From the wave amplitude, the intensity inside the channel is estimated to be $\sim {10}^{18}\mathrm{W}/\mathrm{cm}{}^2$.

• Received 1 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.81.100