Optical and Plasma Smoothing of Laser Imprinting in Targets Driven by Lasers with SSD Bandwidths up to 1 THz

LLE Review, Volume 84 173 Introduction A key issue for inertial confinement fusion (ICF)1–3 is the Rayleigh–Taylor (RT) hydrodynamic instability.4,5 In directdrive ICF, nonuniformities in the drive laser produce pressure variations that “imprint” perturbations into the target. Unstable RT growth can amplify these perturbations, resulting in sufficient distortion of the target shell to degrade implosion performance. As coronal plasma is formed around the target, a region of thermal conduction is produced where nonuniform energy deposition can be thermally smoothed6–8 before perturbations reach the ablation surface. A mode of laser nonuniformity couples to (or seeds) a mode of hydrodynamic instability up to the time that sufficient plasma smoothing occurs, and then imprinting ceases.8 The time to produce this plasma and decouple the laser nonuniformities from the unstable ablation region depends on the perturbation wavelength and the energy deposition rate, i.e., the laser pulse shape. Fast-rising pulses produce plasma more rapidly than slow-rising pulses and therefore cause imprinting over shorter times.9 During the time that this plasma evolves, nonuniformities in the drive must be minimized. To do so, many ICF target designs employ laser-beam-smoothing techniques,10 most of which are time dependent. These techniques are quite effective and have produced marked improvements in the performance of directdrive targets.11