Strong absorption, intense forward-Raman scattering and relativistic electrons driven by a short, high intensity laser pulse through moderately underdense plasmas

The propagation of a short and intense laser pulse (1.057 μm, 350 fs, 1017 W/cm2–2×1019 W/cm2) through preformed undercritical plasmas (≈5%–40% of nc) has been experimentally investigated on the 100-TW laser facility at the Laboratoire pour l’Utilisation des Lasers Intenses. The transmission and reflection of the 1 μm laser pulse, the forward- and backward-Raman (respectively, F-SRS and B-SRS) scattered light and the emission of fast electrons are reported. Significant absorption occurs in these plasmas, which is found to increase with the laser intensity. B-SRS is strongly driven at 1017 W/cm2 and gradually decreases at higher intensities. It is shown that the transmission is low and only weakly dependent on the laser intensity. In contrast, the forward Raman scattering continuously increases with the laser intensity, up to 7% of the incident energy at 2×1019 W/cm2 in the lowest density case. The relativistic electrons accelerated in the forward direction appear to be correlated with the F-SRS. The experimental data are discussed in the light of recent theoretical and numerical investigations, indicating that intense electron heating is likely to play a major role in the temporal growth or inhibition of the instabilities. The theoretical predictions are in agreement with the experiments.