Broadband Beam Smoothing on OMEGA with Two-Dimensional Smoothing by Spectral Dispersion

LLE Review, Volume 80 197 High laser-irradiation uniformity is an important requirement for successful implosions of inertial confinement fusion (ICF) targets,1 particularly for the direct-drive OMEGA laser system that directly illuminates ICF targets. Direct-drive laser irradiation that is not perfectly uniform imprints on the target surface and perturbs the spherical symmetry. This seeds the Rayleigh– Taylor hydrodynamic instability during the target acceleration phase and can severely degrade target performance. Target imprinting is determined when the critical surface decouples from the ablation surface. The critical surface corresponds to the highest density reached by the plasma, where the laser frequency equals the plasma frequency. The ablation surface corresponds roughly to the surface separating the inwardflowing (imploding) plasma and the outward-flowing “exhaust.” Direct-drive smoothing techniques must minimize the level of irradiation nonuniformity on a time scale comparable to or shorter than this imprinting phase. Initial target experiments indicate that this imprinting phase lasts for several hundred picoseconds on OMEGA and depends on the spatial wavelength of the most important perturbations.