Modeling Laser Imprint for Inertial Confinement Fusion Targets

LLE Review, Volume 80 185 In inertial confinement fusion (ICF), a spherical shell filled with a DT-gas mixture is compressed to high densities and temperatures to achieve ignition condition.1 Degradation from spherical symmetry during the implosion, however, limits the achievable compression ratios and could quench the ignition. The main source of such asymmetry is hydrodynamic instabilities (such as the Rayleigh–Taylor and Bell–Plesset instabilities) seeded by both irradiation nonuniformities and impurities in the target materials. In this article we describe a process that generates mass perturbations on an initially uniform target driven by a modulated laser illumination. Such a process is referred to as a “laser imprint.” The control of laser imprint is of crucial importance for the successful implosion of direct-drive ICF targets. To evaluate the imprint growth, the following two physical problems must be considered: (1) generation of nonuniformities in ablation pressure due to spatial modulations in a laser intensity, and (2) mass perturbation growth on a target driven by nonuniform ablation pressure. A detailed analysis of the first problem can be found in Refs. 2. The second problem, however, has not been adequately treated in the past. In Ref. 3, for example, perturbation growth was derived by using the Chapman–Jouguet deflagration model. As discussed in Refs. 4, such a model neglects thermal smoothing of perturbations in the conduction zone (a region between the critical surface and ablation front), and in addition, it does not reproduce the main restoring force, which is due to a difference in the dynamic pressure at the peaks and valleys of the front distortion.5,6 An improved model has been proposed in Ref. 4, where thermal smoothing of the pressure perturbations has been included. At the ablation front, however, the authors used the “Landau–Darrieus” boundary condition that, similar to the result of Ref. 3, neglects the main stabilizing force due to the dynamic overpressure.