Analysis of Modulations Observed in X-Ray Self-Emission Images of OMEGA Direct-Drive Inertial Confinement Fusion Implosions *

We discuss the observations of a pattern of the x-ray self-emission corresponding to the overlapping of laser beams on OMEGA spherical direct-drive implosions using a small-spot distributed phase plate (SG5-650) in all 60 laser beams. The targets were spherical plastic shells filled with deuterium gas, and the diagnostic used to record the gated x-ray self-emission images was an x-ray framing camera. The state-of-the-art 3-D ASTER simulations performed for these implosions with and without including the laser-imprinting effect also show qualitatively similar patterns in the self-emission images. Inclusion of the laser-imprinting effect in the simulations increases the contrast in the images by about 5% compared to the simulations performed without imprinting. Analyses of the total intensity distributions in the self-emission images suggest that these emission patterns in the images arise from the x-ray self-emission from shell (toward the line of sight). The measurements show greater contrast than in simulated self-emission images, thereby suggesting that beam-mode–induced hydrodynamic perturbations may be underestimated. The difference in electron temperature and density gradients in the experimental and simulated implosions has been inferred using the emission discrepancies in simulated and experimental images.