Performance of 1-THz-Bandwidth, 2-D Smoothing by Spectral Dispersion and Polarization Smoothing of High-Power, Solid-State Laser Beams

LLE Review, Volume 98 49 Introduction A direct-drive inertial confinement fusion (ICF) implosion of a spherical capsule containing thermonuclear fuel is initiated by the ablation of material from the outer shell surface with overlapped, intense laser beams.1 The ablated shell mass forms a coronal plasma that surrounds the target and accelerates the shell inward via the rocket effect.1,2 Perturbations at the ablation surface resulting from target imperfections and laser irradiation nonuniformities (known as laser imprint) are amplified by the ablative Rayleigh–Taylor (RT) instability as the shell accelerates inward and are further amplified during the deceleration phase.3–9 The RT instability can reduce the thermonuclear yield of the implosion.1,2 The direct-drive ICF program strives to reduce laser imprint levels by uniform laser irradiation of the target. High-compression direct-drive experiments require a 1% rms level of the on-target laser irradiation nonuniformity averaged over a few hundred picoseconds.10 This is accomplished on the 60-beam, 30-kJ, 351-nm OMEGA laser system11 using two-dimensional smoothing by spectral dispersion (2-D SSD),10,12–14 distributed phase plates (DPP’s),15,16 polarization smoothing (PS) utilizing birefringent wedges,17–19 and multiple-beam overlap.20 These techniques are directly applicable to direct-drive ignition target designs21 planned for the 1.8-MJ, 351-nm, 192-beam National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory.22