SIMULATIONS OF LASER IMPRINT FOR NOVA EXPERIMENTS AND FOR IGNITION CAPSULES

In diner drive inertial confinement fusion (ICF), nonuniformities in l aser illumination seed ripples at the ablation front in a process call ed ''imprint.'' These nonuniformities grow during the capsule implosio n and, if initially large enough, can penetrate the capsule shell, imp ede ignition, or degrade burn. Imprint has been simulated for recent e xperiments performed on the Nova laser [Campbell rt nl., Rev. Sci. Ins trum. 57, 2101 (1986)] examining a variety of beam smoothing condition s. Most used laser intensities similar to the early part of an ignitio n capsule pulse shape, I similar or equal to 10(13) W/cm(2). The simul ations matched most of the measurements of imprint modulation. The eff ect of imprint upon National ignition Facility (NIF) direct drive igni tion capsules has also been simulated. Imprint is predicted to give mo dulation comparable to an intrinsic surface finish of similar to 10 nm rms. Modulation growth was examined using the Haan [Phys, Rev. A 39, 5512 (1989)] model, with linear growth factors as a function of spheri cal harmonic mode number obtained from an analytic dispersion relation . Ablation front amplitudes are predicted to become substantially nonl inear, so that saturation corrections are large. Direct numerical simu lations of two-dimensional multimode growth were also pet formed. The capsule shell is predicted to remain intact, which fives a basis for b elieving that ignition can be achieved. (C) 1997 American Institute of Physics.