Initial Shell Convergence Measurements Using OMEGA’s 60 Unsmoothed Beams

LLE Review, Volume 66 57 One metric commonly used to evaluate the performance of inertial confinement fusion (ICF) targets is the ratio of the measured neutron yield to that predicted by a one-dimensional hydrodynamic simulation. When plotted versus the calculated convergence ratio of the implosion, this ratio [referred to as the YOC (yield-over-clean)] provides an indication of the nonuniformity present in the implosion [when targets that experience Rayleigh-Taylor (RT) similar growth factors are compared]. The convergence ratio is defined as the ratio of the initial and the final radii of the fuel/pusher interface. A concern, however, is that early experimental results never produced discernible, limb-brightened, stagnated cores. An example of this is displayed in Fig. 66.1, where azimuthally averaged radial profiles of early OMEGA implosions (driven by 24 unsmoothed beams, circa 1986) are compared with results from our one-dimensional hydrodynamics code LILAC. [These profiles were produced from time-integrated images obtained with a Kirkpatrick-Baez (KB) microscope.] In four such implosion experiments, we found little agreement with the position and spatial extent of the stagnated shell as predicted by the simulations. All of the experimental results indicated a broad, center-peaked, and somewhat nebulous core, whereas the LILAC results predicted a well-defined stagnated shell. As a result, the YOC-convergence comparisons required the use of the calculated convergence ratios. Figure 66.2 shows a similar comparison for the 1986 data. Initial Shell Convergence Measurements Using OMEGA’s 60 Unsmoothed Beams