Transient development of backward stimulated Raman and Brillouin scattering on a picosecond time scale measured by subpicosecond thomson diagnostic

The excitation and the relaxation of the plasma waves and ion acoustic waves (IAW), respectively, driven by stimulated Raman (SRS) and Brillouin (SBS) backscatterings have been experimentally investigated with short-pulse lasers. The spectra have been obtained with a 0.3 ps time resolution. It is shown that SRS develops before SBS and suddenly decays around the peak of the pump, as the IAW reaches saturation. On this short time scale, electron kinetic effects play a major role for SRS saturation, contrary to ion dynamics. These results are supported by particle-in-cell simulations.     

Enhanced spatiotemporal laser-beam smoothing in gas-jet plasmas

Spatiotemporal smoothing of large-scale laser intensity fluctuations is observed for a laser beam focused into underdense helium plasmas. This smoothing is found to be severely enhanced when focusing the laser beam into a helium gas jet. In contrast to other experiments with preformed plasmas, the average and the peak laser intensities are well below the threshold for ponderomotive self-focusing. The coherence characteristics of the transmitted light are measured for various electron densities, and the smoothing effect is explained by multiple scattering of laser light on self-induced density perturbations.     

Ultraintense laser-produced fast-electron propagation in gas jets

We study the propagation of fast electrons in a gas at different densities. A large relativistic electron current is produced by focusing a short-pulse ultrahigh-intensity laser on a metallic target. It then propagates in a gas jet placed behind the foil. Shadowgraphy in the gas shows an electron cloud moving at sub-relativistic average velocities. The experiment shows (i) the essential role of the density of background material for allowing propagation of fast electrons, (ii) the importance of the ionization phase which produces free electrons available for the return current, and (iii) the effect of electrostatic fields on fast-electron propagation.     

Experimental evidence of the effect of heat flux on thomson scattering off ion acoustic waves

Thomson self-scattering measurements are performed in a preionized helium gas jet plasma at different locations along the laser propagation direction. A systematic and important variation of the intensity ratio between the blue and the red ion spectral components is observed, depending on whether the location of the probed region is in front of or behind the focal plane. A simple theoretical calculation of Thomson scattering shows that this behavior can be qualitatively understood in terms of a deformation of the electron distribution function due to the return current correlated with the classical thermal heat flux.     

A summary of the beatwave experiments at Ecole Polytechnique

We present a summary of the beatwave particle acceleration program developed at Ecole Polytechnique. In dedicated experiments, plasma formation, plasma wave generation and saturation, and particle acceleration were successively studied and understood in detail. A maximum energy gain of 1.3 MeV was obtained, which is compatible with an accelerating gradient of 0.7 GV/m     

Interaction of ultraintense laser pulses with an underdense,preformed plasma channel

We report on experimental results regarding the propagation of ultraintense laser pulses in a preformed plasma channel. In this experiment, the long (4-mm) fully ionized plasma channel created by the amplified spontaneous emission (ASE) was measured by interferometry before and after the propagation of the short laser pulse. Forward spectra show a cascade of Raman satellites, which merge with one another when the laser power was increased up to critical power for relativistic self-focusing P_c. The number of filaments measured by interferometry increases when the laser power increases. High conversion efficiency (≈10%) of second harmonic generation was observed in the interaction     

Search for stimulated photon-photon scattering in vacuum

We have searched for stimulated photon scattering in vacuum at a center of mass photon energy of 0.8 eV. The QED contribution to this process is equivalent to four wave mixing in vacuum. No evidence for scattering was observed. The corresponding upper limit of the cross-section is . Received 29 September 1999