[en] Strong shock waves produced by illumination of a CH target by laser produced x-rays were driven through a copper sphere. The motion and deformation of the sphere were measured using radiographs generated by backlighting the sphere with a large area backlighter. The sphere became non-spherical after the passage of the shock, having a complicated down-stream structure. This was an instability-induced structure that was predicted by calculations. The experiment is a convenient laboratory model of the complicated interactions occurring in much larger systems such as in astrophysics in the interaction of shocks formed in the interstellar medium with various types of clouds. In particular, the experiment is a useful tool for checking the computational ability of the new generation ASCI computers, as it requires three-dimensional modeling. This experiment has shown that three dimensional calculations seem to be necessary to describe major features observed in the experiment. Any attempt to explain hydrodynamic behavior with similar instabilities must take into account these three dimensional effects

[en] We have demonstrated a technique for diagnosing the growth of subresolution-scale perturbations by the appearance of longer-wavelength, coupled modes once the growth has proceeded into the nonlinear regime. Comparison of the growth rate of this larger scale feature with numerical simulations can then be used to infer the growth rates of the initial perturbations. This experiment was conceived as an analog of large-scale computer simulations where the large eddy approximation is applied. There a subgrid-scale model is used to represent the effects of small scales on large-scale motion, which is directly numerically simulated

[en] The evolution of the Rayleigh-Taylor (RT) instability at an embedded, or classical, interface is examined in a series of experiments at the Nova laserfacility .[reference for Nova] These experiments focused on the transition from the linear to nonlinear regimes for both single- and multimode initialperturbations. The development of a single mode at the embedded interface is compared to its evolution at an ablation front and the effect of ablativestabilization is experimentally demonstrated. The multimode experiments have shown evidence of the process of bubble competition, whereinneighboring structures either continue to rise or are washed downstream in the flow depending upon their relative size. The experiments with simulations performed with either the LASNEX are comparedcode [G. B.Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2,51 (1975).], a two-dimensional Lagrangian radiation-hydrodynamics code, or CALE [R. Tipton, reference for CALE], a two-dimensional arbitrary Lagrange-Eulerian radiation-hydrodynamics code

[en] We examined the progression of the Rayleigh-Taylor (RT) instability from an initial multimode perturbation. The RT experiments focused on the transition from the linear to non-linear regimes for perturbation growth at an embedded, or classical, interface. The multimode experiments have attempted to observe the process of bubble competition wherein neighboring structures either continue to rise or are washed downstream in the flow depending upon; their relative size. This competition is predicted to result in an inverse cascade at late times where progressively larger structures will begin to dominate the flow. Experiments to date have shown evidence of coupled modes arising, but have not yet accelerated the interface long enough to produce the several generations of coupling required for a true inverse cascade

[en] The evolution of the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities in the nonlinear regime of growth was investigated in indirect-drive experiments on the Nova laser. The RT experiments investigated the evolution of both single- and multimode perturbations at an embedded interface, isolated from the effects of ablation. This ''classical'' geometry allows short wavelength (λ ∼ 10-20 microm) perturbations to grow strongly, in marked contrast to prior results at an ablation front. The RM experiments studied singly- and doubly-shocked perturbed interfaces in both face-on and side-on geometries. (U)

[en] We review experiments and calculations of the compressible Richtmyer-Meshkov instability from a single-mode, nonlinear initial perturbation. These experiments were performed using the Nova laser. Measurements of the time-evolution of the mixing region were reported previously. We compared the experimental measurements with numerical simulations ,2 We found both experiment and simulation to be in good agreement with recent theories for the nonlinear evolution of the instability ,4 Experimental results beyond those previously presented provide additional support for the use of two phase flow models to describe the flow in the nonlinear regime. These experiments include measurement of the mixing region at additional times, including times earlier in the evolution of the instability than previously reported. We have also carried out experiments to examine the difference in the evolution of the instability from initial perturbations consisting of circular sawtooth grooves as well as rectilinear sawteeth. Our previous two-dimensional numerical simulations approximated the experimental linear grooves as circular grooves. We reasoned that the difference between the two cases would be small, based on scaling arguments, and limited to a very small region near the centerline. New experimental and numerical results confirm this. Finally, we discuss some additional issues in the derivation of the two-phase flow model used previously in describing the growth of the Richtmyer-Meshkov instability in the nonlinear phase relevant to other work presented at this meeting ,6

[en] We have conducted an investigation of the spatial resolution of a new gated x-ray pinhole camera (FXI). The spatial resolution, or its Fourier transform the modulation transfer function (MTF), is critical for quantitative interpretation of recent hydrodynamic instability data taken on the Nova laser. We have taken data corresponding to backlit straight edges, pinholes, and grids, both on the bench and in situ on Nova. For both the pinhole and edge data, the MTF at all wavelengths of interest can be deduced from a single image. Grids are of more limited usefulness, giving the value of the MTF only at the spatial period of the grid. These different techniques for characterizing the MTF of gated x-ray pinhole cameras will be discussed, with results specific to the FXI presented. copyright 1997 American Institute of Physics

[en] The evolution of the Rayleigh-Taylor (RT) instability in a compressible medium has been investigated at an accelerating embedded interface and at the ablation front in a series of experiments on the Nova laser. The x-ray drive generated in a gold hohlraum ablatively accelerated a planar target consisting of a doped plastic pusher backed by a higher density titanium payload with perturbations placed at the plastic-Ti interface. The targets were diagnosed by face-on and side-on radiography. In previous work focusing on single mode perturbations, wavelengths as short as 10 m have been observed to grow strongly at the embedded interface. Here multimode perturbations consisting of either 2, 10 or 20 modes superposed in phase have been investigated

[en] The Rayleigh-Taylor (RT) instability, which occurs when a lower-density fluid accelerates a higher-density layer, is common in nature. At an ablation front a sharp reduction in the growth rate of the instability at short wave-lengths can occur, in marked contrast to the classical case where growth rates are highest at the shortest wavelengths. Theoretical and numerical investigations of the ablative RT instability are numerous and differ considerably on the level of stabilization expected. We present here the results of a series of laser experiments designed to probe the roll-over and cutoff region of the ablation-front RT dispersion curve in indirect drive. Aluminum foils with imposed sinusoidal perturbations ranging in wavelength from 10 to 70 pm were ablatively accelerated with a radiation drive generated in a gold cylindrical hohlraum. A strong shock wave compresses the package followed by an ∼2 ns period of roughly constant acceleration and the experiment is diagnosed via face-on radiography. Perturbations with wavelengths (ge) 20 (micro)m experienced substantial growth during the acceleration phase while shorter wavelengths showed a sharp drop off in overall growth. These experimental results compared favorably to calculations with a 2-D radiation-hydrodynamics code, however, the growth is significantly affected by the rippled shock launched by the drive. We performed numerical simulations to elucidate the influence of the rippled shock wave on the eventual growth of the perturbations, allowing comparisons to the analytic model developed by Betti et al. This combination of experiments, simulations and analytic modeling illustrates the qualitative simplicity yet quantitative complexity of the compressible RT instability. We have measured the Rayleigh-Taylor (RT) dispersion curve for a radiatively-driven sample in a series of experiments on the Nova laser facility. Planar aluminum foils were ablatively-accelerated and the subsequent perturbation growth was diagnosed via x-ray radiography. These measurements unambiguously map out the linear regime dispersion curve, including the observation of stabilization at short wavelengths. The data are compared favorably to two-dimensional simulations. Due to the influence of the rippled shock transit phase of the experiment, direct comparison to the ablation front RT theory of R. Betti was difficult. Instead, a numerical ''experiment'' was constructed that minimized the influence of the shock and this was compared to the Betti model showing quite good agreement

[en] The x-ray driven ablation front hydrodynamic instability experiments at Nova span 1988-present, and can be divided into three generations. The 1st generation experiments consisted of planar foils with perturbations of the form k = k_x imposed on the drive side of the foil. A variety of drive pulse shapes, foil materials, and perturbation wavelengths and amplitudes were investigated, with growth factors of up to 80 being observed. The 2nd generation experiments investigated mode-mode interactions with imposed perturbations corresponding to the superposition of modes. They have done experiments with two-mode and eight-mode foils. In the linear regime, the modes grow independently with their own respective growth rates. In the nonlinear regime, in addition to the higher harmonics of the pre-existing modes, coupled terms k_i ± k_j occur. The 3rd generation experiments focus on 3D Rayleigh-Taylor growth. They have recently done experiments with an imposed 3D single-mode perturbation of the form k = (k_x,k_y), with k_x = k_y. In the linear regime, this perturbation grows exponentially with wave vector k = (k_x² + k_y²)^1/2. In the nonlinear regime, the perturbations evolve into broad bubbles surrounded on four corners by very dense, localized spikes with archways or saddle points in between. Simulations suggest that this 3D square mode grows larger than the corresponding 2D perturbation with the same magnitude wavevector and initial amplitude