A model of laser imprinting

Irradiation nonuniformities in direct-drive (DD) inertial confinement fusio n experiments generate, or "imprint," surface modulations that degrade the symmetry of the implosion and reduce the target performance. To gain physic al insight, an analytical model of imprint is developed. The model takes in to account the hydrodynamic flow, the dynamics of the conduction zone, and the mass ablation. The important parameters are found to be the time scale for plasma atmosphere formation and the ablation velocity. The model is val idated by comparisons to detailed two-dimensional (2D) hydrocode simulation s. The results of the model and simulations are in good agreement with a se ries of planar-foil imprint experiments performed on the OMEGA laser system [T.R. Boehly, D.L. Brown, R.S. Craxton et al., Opt. Commun. 133, 495 (1997 )]. Direct-drive National Ignition Facility's [J.A. Paisner, J.D. Boyes, S. A. Kumpan, W.H. Lowdermilk, and M.S. Sorem, Laser Focus World 30, 75 (1994) ] cryogenic targets are shown to have gains larger than 10 when the rms las er-irradiation nonuniformity is reduced by 2D smoothing by spectral dispers ion (SSD) used in the current DD target designs. [S1070-664X(00)93605-X].