Richtmyer–Meshkov (RM) mixing seeded by multimode initial surface perturbations in a convergent, compressible, miscible plasma system is measured on the OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system. A strong shock (Mach 12–20), created by 50 laser beams, is used to accelerate impulsively a thin aluminum shell into a lower density foam. As the system converges, both interfaces of the aluminum are RM unstable and undergo mixing. Standard x-ray radiographic techniques are employed to survey accurately the zero-order hydrodynamics, the average radius and overall width, of the marker. LASNEX [G. B. Zimmerman et al., Comments on Plasma Physics 2, 51 (1975)] simulations are consistent with the zero-order behavior of initially smooth markers. In experiments with smooth aluminum markers, the measured marker width shortly after shock passage behaves incompressibly and thickens due to Bell–Plesset effects. At high convergence (>4), the marker begins to compress as the rebounding shock passes back through the marker. When an initial multimode perturbation is introduced to the outer surface of the marker, the measured marker width is observed to increase by 10–15 μm, and is substantially smaller than as-shot simulations using RAGE [R. M. Baltrusaitis et al., Phys. Fluids 8, 2471 (1996)] would predict.
Strongly shocked cylindrically convergent implosions were conducted on the OMEGA laser. The directly driven targets consist of a low-density foam core and an embedded aluminum shell covered by an epoxy ablator. The outer surface of the aluminum shell has imposed single-mode perturbations with wave numbers k ϭ 0.25, 0.7, 1.05, and 2.5~rad0mm! and initial amplitudes h 0 0l ϭ 0.04, 0.11, 0.33, and 0.4. In our convergent geometry, perturbation growth without evidence of saturation, for h0l as large as 4.5 is observed for k , 1. For k . 1 growth rate scaling with wavenumber breaks down and transition to turbulence is suggested.
Temporally, spatially, and spectrally resolved x-ray image data from direct-drive implosions on OMEGA were interpreted with the aid of radiation-hydrodynamic simulations. Neither clean calculations nor those using a turbulent mix model can explain fully the observed migration of shell-dopant material (titanium) into the core. Shell-dopant migration was observed via time-dependent, spatially integrated spectra, and spatially and spectrally resolved x-ray images of capsule implosions and resultant dopant emissions. The titanium emission was centrally peaked in narrowband x-ray images. In post-processed clean simulations, the peak titanium emission forms in a ring in self-emission images as the capsule implodes. Post-processed simulations with mix reproduce trends in time-dependent, spatially integrated spectra, as well having centrally peaked Ti emission in synthetic multiple monochromatic imager. However, mix simulations still do not transport Ti to the core as is observed in the experiment. This suggests that phenomena in addition to the turbulent mix must be responsible for the transport of Ti. Simple diffusion estimates are unable to explain the early Ti mix into the core. Mechanisms suggested for further study are capsule surface roughness, illumination non-uniformity, and shock entrainment.
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