Suprathermal Electron Generation and Channel Formation by an Ultrarelativistic Laser Pulse in an Underdense Preformed Plasma

Relativistic electrons are produced, with energies up to 20 MeV, by the interaction of a high-intensity subpicosecond laser pulse ( $1\mu \mathrm{m}$, $300\mathrm{fs}$, ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$) with an underdense plasma. Two suprathermal electron populations appear with temperatures of 1 and 3 MeV. In the same conditions, the laser beam transmission is increased up to 20%–30%. We observe both features along with the evidence of laser pulse channeling. A fluid model predicts a strong self-focusing of the pulse. Acceleration in the enhanced laser field seems the most likely mechanism leading to the second electron population.