H. Louis

An experimental testbed for the study of hydrodynamic issues in supernovae

More than a decade after the explosion of supernova 1987A, unresolved discrepancies still remain in attempts to numerically simulate the mixing processes initiated by the passage of a very strong shock through the layered structure of the progenitor star. Numerically computed velocities of the radioactive 56Ni and 56Co, produced by shock-induced explosive burning within the silicon layer, for example, are still more than 50% too low as compared with the measured velocities. To resolve such discrepancies between observation and simulation, an experimental testbed has been designed on the Omega Laser for the study of hydrodynamic issues of importance to supernovae (SNe). In this paper, results are presented from a series of scaled laboratory experiments designed to isolate and explore several issues in the hydrodynamics of supernova explosions. The results of the experiments are compared with numerical simulations and are generally found to be in reasonable agreement.

Effect of shock proximity on Richtmyer–Meshkov growth

Experiments conducted on the Omega laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] and simulations show reduced Richtmyer–Meshkov growth rates in a strongly shocked system with initial amplitudes kη0⩽0.9. The growth rate at early time is less than half the impulsive model prediction, rising at later time to near the impulsive prediction. An analytical model that accounts for shock proximity agrees with the results.

Nonlinear mixing behavior of the three-dimensional Rayleigh–Taylor instability at a decelerating interface

Results are reported from the first experiments to explore the evolution of the Rayleigh–Taylor (RT) instability from intentionally three-dimensional (3D) initial conditions at an embedded, decelerating interface in a high-Reynolds-number flow. The experiments used ∼5 kJ of laser energy to produce a blast wave in polyimide and/or brominated plastic having an initial pressure of ∼50 Mbars. This blast wave shocked and then decelerated the perturbed interface between the first material and lower-density C foam. This caused the formation of a decelerating interface with an Atwood number ∼2/3, producing a long-term positive growth rate for the RT instability. The initial perturbations were a 3D perturbation in an “egg-crate” pattern with feature spacings of 71 μm in two orthogonal directions and peak-to-valley amplitudes of 5 μm. The resulting RT spikes appear to overtake the shock waves, moving at a large fraction of the predeceleration, “free-fall” velocity. This result was unanticipated by prior simulations...

Three-Dimensional Hydrodynamic Experiments on the National Ignition Facility

The production of supersonic jets of material via the interaction of a strong shock wave with a spatially localized density perturbation is a common feature of inertial confinement fusion and astrophysics. The behavior of two-dimensional (2D) supersonic jets has previously been investigated in detail [J. M. Foster et. al, Phys. Plasmas 9, 2251 (2002)]. In three-dimensions (3D), however, there are new aspects to the behavior of supersonic jets in compressible media. In this paper, the commissioning activities on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] to enable hydrodynamic experiments will be presented as well as the results from the first series of hydrodynamic experiments. In these experiments, two of the first four beams of NIF are used to drive a 40 Mbar shock wave into millimeter scale aluminum targets backed by 100 mg/cc carbon aerogel foam. The remaining beams are delayed in time and are used to provide a point-projection x-ray backlighter source for diagnosing the three-dimensional structure of the jet evolution resulting from a variety of 2D and 3D features. Comparisons between data and simulations using several codes will be presented.

Turbulent hydrodynamics experiments using a new plasma piston

A new method for performing compressible hydrodynamic instability experiments using high-power lasers is presented. A plasma piston is created by supersonically heating a low-density carbon based foam with x-rays from a gold hohlraum heated to ∼200 eV by a ∼1 ns Nova laser pulse [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)]. The piston causes an almost shockless acceleration of a thin, higher-density payload consisting of a layer of gold, initially 1/2 μm thick, supported on 10 μm of solid plastic, at ∼45 μm/ns2. The payload is also heated by hohlraum x-rays to in excess of 150 eV so that the Au layer expands to ∼20 μm prior to the onset of instability growth. The Atwood number between foam and Au is ∼0.7. Rayleigh–Taylor instability, seeded by the random fibrous structure of the foam, causes a turbulent mixing region with a Reynolds number >105 to develop between piston and Au. The macroscopic width of the mixing region was inferred from the change in Au layer width, which was recorded via tim...

Fabrication and characterization of Z-pinch foam targets

This article describes the fabrication and characterization of targets produced for Z-pinch physics experiments. Low density foams with densities as low as 1 mg/cc were made. Targets fabricated include 1 cm×1 cm solid cylinders of SiO2 aerogels at various densities, 1 cm×1 cm solid cylinders of 10 mg/cc agar, and 10 mg/cc agar annuli with 10 mm o.d., 9 mm i.d., and 1 cm length. Target geometry and density uniformity were characterized by ion microtomography (IMT) which can nondestructively measure three-dimensional density variations to 1% with micron-scale resolution. The results of IMT measurements on several targets are reported and a discussion of factors affecting the quantitative microanalysis of these materials is presented.

A novel method for diagnosing the growth of subresolution-scale perturbations

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.