The Effect of Laser Imprint on Perturbation Growth In Direct Drive Inertial Confinement Fusion

Imprint of nonuniformities in the laser intensity into mass perturbations in the target is one of the major sources for geometrical distortions during direct drive Inertial Confinement Fusion (ICF) implosions. Small perturbations introduced by the target surface roughness and by the laser imprint grow due to the Rayleigh-Taylor instability, and may reduce significantly the efficiency of the nuclear fusion. In the present work we present a numerical simulation based theoretical analysis of static and dynamic laser imprint. Using two-dimensional numerical simulations, the equivalence between laser nonuniformities and mass perturbations in the target is analyzed as a function of perturbation wavelength, laser intensity and pulse shape.The reduction of laser imprint by temporal changes in the laser beam is analyzed by calculations of the integrated on-target nonuniformity, by comparisons to other models and to experiments and by full numerical simulations of dynamic nonuniformity imprint. Applications of the resulting static and dynamic imprint efficiencies are presented. The spectrum of mass modulations which is equivalent to laser imprint is calculated, and its nonlinear growth due to the Rayleigh-Taylor instability is calculated using a Haan modal model for several experimental configurations