The first hydrodynamic instability growth measurements with three-dimensional (3D) surface-roughness modulations were performed on CH shell spherical implosions at the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 43, 2841 (2004)]. The initial capsule outer-surface amplitudes were increased approximately four times, compared with the standard specifications, to increase the signal-to-noise ratio, helping to qualify a technique for measuring small 3D modulations. The instability growth measurements were performed using x-ray through-foil radiography based on time-resolved pinhole imaging. Averaging over 15 similar images significantly increased the signal-to-noise ratio, making possible a comparison with 3D simulations. At a convergence ratio of ${\sim}2.4$, the measured modulation levels were ${\sim}3$ times larger than those simulated based on the growth of the known imposed initial surface modulations. Several hypotheses are discussed, including increased instability growth due to modulations of the oxygen content in the bulk of the capsule. Future experiments will be focused on measurements with standard 3D ‘native-roughness’ capsules as well as with deliberately imposed oxygen modulations.

L'aérodynamique étudie le mouvement des corps dans un fluide. Cette science pour l'ingénieur regroupe autour d'une application commune, la locomotion, un ensemble de questions qui s'adressent à recherche fondamentale en mécanique des fluides. L'objectif de cet article est de rappeler ces interrogations fondamentales au cœur desquelles on trouve la dynamique des tourbillons, les instabilités hydrodynamiques et la turbulence.

Recent direct drive implosion experiments, performed on the OMEGA laser, have been analyzed by comparing full two-dimensional (2D) and one-dimensional (1D) numerical simulations. The 2D simulations result in a fusion yield higher than experimental results. A simple full-mixing model, leaving only the clean region, overestimates yield degradation. Fully turbulent mixing is expected to develop in most of the mixing region; however regions slightly beyond the radius of the most penetrating spike are expected to remain clean and to contribute to fusion yield. One can correct the mixing model by redefining the clean region. Accounting for this unmixed region results in improved agreement with experimental results. Differences in central pressure, density, temperature, and fusion rate in implosions dominated by low mode number perturbations imply that mix effects might not be limited to the mix region, and that 2D simulations are necessary to describe the large scale flow affecting the central region. The same analysis has been undertaken for implosions with different convergence ratios, but with similar initial perturbation spectra. These implosions should be compared to implosions dominated by high mode number perturbations, which might be described by models based on 1D simulations.