The Petawatt laser at LLNL has opened a new regime of laser-matter interactions in which the quiver motion of plasma electrons is fully relativistic with energies extending well above the threshold for nuclear processes. In addition to -few MeV ponderomotive electrons produced in ultra-intense laser-solid interactions, we have found a high energy component of electrons extending to -100 MeV apparently from relativistic self-focusing and plasma acceleration in the underdense pre-formed plasma. The generation of hard bremsstrahlung, photo-nuclear reactions, and preliminary evidence for positron-electron pair production will be discussed.

We have measured the characteristics of saturated ion-acoustic waves in inertial confinement fusion plasmas. A 263-nm probe laser has been applied to simultaneous Thomson scatter on both ion-acoustic waves excited by thermal electrostatic fluctuations and by stimulated Brillouin scattering of a kilojoule laser beam of varying intensity. The Thomson scattering spectra show saturated ion-wave amplitudes for intensities above 5x10(14) W cm(-2) consistent with three dimensional nonlinear wave modeling.

Channeling and filamentation of a laser beam is investigated experimentally and via computer simulations.A two-dimensiotial, fully nonlinear, fluid code has been developed and used to simulate the pmpagation of a 1aser pulse through an underdense plasma when v /v, -1. Comparison to interferometrically measured density depressions shows good agreement. The limited axial extent of the density perturbation seen in experiment is linked by the simulation to filamentation and divergence of the laser beam. A characteristic angle of divergence of the laser beam is derived, and agreement is found with experiment and simulation.

Experimental results are presented which show the formation of density channels in a preformed plasma by a 100 ps laser beam focused with different f/number lenses. The density channels are diagnosed by an interferometer. The experiments are made with both line foci and circular foci up to intensities of 5×1016 W/cm2. The experimental channel size and density perturbation compare favorably with the predictions of two-dimensional theoretical models. The limited axial extent of the channels is shown to be due to the onset of the filamentation instability.

Spatial, temporal, and polarization smoothing schemes are combined for the first time to reduce to a few percent the total stimulated backscatter of a NIF-like probe laser beam (2x10(15) W/cm (2), 351 nm, f/8) in a long-scale-length laser plasma. Combining temporal and polarization smoothing reduces simulated Brillouin scattering and simulated Raman scattering (SRS) up to an order of magnitude although neither smoothing scheme by itself is uniformly effective. The results agree with trends observed in simulations performed with the laser-plasma interaction code F3D simulations [R. L. Berger et al., Phys. Plasma 6, 1043 (1999)].