Search

The National Ignition Facility (NIF) is capable of creating new and novel high-energy-density (HED) systems relevant to astrophysics. Specifically, a system could be created that studies the effects of a radiative shock on a hydrodynamically unstable interface. These dynamics would be relevant to the early evolution after a core-collapse supernova of a red supergiant star. Prior to NIF, no HED facility had enough energy to perform this kind of experiment. The experimental target will include a 340 μm predominantly plastic ablator followed by a low-density SiO2 foam. The interface will have a specific, machined pattern that will seed hydrodynamic instabilities. The growth of the instabilities in a radiation-dominated environment will be observed. This experiment requires a ≥300 eV hohlraum drive and will be diagnosed using point projection pinhole radiography, which have both been recently demonstrated on NIF.

A laser-driven experiment produces images of dense shocked material by x-ray transmission. The post-shock material is sufficiently dense that no significant signal passes through the dense layer, and therefore the shock compression cannot be directly measured by comparing transmitted intensities. One could try to determine the shock compression ratio by observing the ratio of the total distance travelled by the shock to the dense post-shock layer width, but small deviations of the angle of the shock with respect to the angle of imaging create large asymmetric errors in observation. A statistical approach to recovering shock compression by appropriately combining data from several experiments is developed, using fits to a simple model for the shock and shock tube geometry.

This paper discusses high-energy-density laboratory astrophysics experiments exploring the Rayleigh-Taylor instability under conditions similar to the blast wave driven, outermost layer in a core-collapse supernova. The planar blast wave is created in an experimental target using the Omega laser. The blast wave crosses an unstable interface with a seed perturbation machined onto it. The perturbation consists of a 3D “egg crate” pattern and, in some cases, an additional longer wavelength mode is added to this 3D, single-mode pattern. The main diagnostic of this experiment is x-ray radiography. This paper explores an image processing technique to improve the identification and characterization of structure in the radiographic data.

We discuss the design of jet-driven, radiative-blast-wave experiments for a 10 kJ class pulsed laser facility. The astrophysical motivation is the fact that jets from Young Stellar Objects are typically radiative and that the resulting radiative bow shocks produce complex structure that is difficult to predict. To drive a radiative bow shock, the jet velocity must exceed the threshold for strong radiative effects. Using a 10 kJ class laser, it is possible to produce such a jet that can drive a radiative bow shock in gas that is dense enough to permit diagnosis by x-ray radiography. We describe the design and simulations of such experiments. The basic approach is to shock the jet material and then accelerate it through a collimating hole and into a Xe ambient medium. We identify issues that must be addressed through experimentation or further simulations in order to field successful experiments.

We present data from recent high-energy-density laboratory experiments designed to explore the Rayleigh-Taylor instability under conditions relevant to supernovae. The Omega laser is used to create a blast wave structure that is similar to that of the explosion phase of a core-collapse supernova. An unstable interface is shocked and then decelerated by the planar blast wave, producing Rayleigh-Taylor growth. Recent experiments were performed using dual, side-on, x-ray radiography to observe a 3D “egg crate” mode and an imposed, longer-wavelength, sinusoidal mode as a seed perturbation. This paper explores the method of data analysis and accurately estimating the position of important features in the data.

The mechanics of a radiative shock which has “collapsed,” or been compressed to high density, via radiative cooling is discussed. This process is relevant to an experiment in xenon gas that produced a driven, radiatively collapsed shock, and also to a simulation of the supernova 1987A shock wave passing through the outer layers of the star and into the low-density circumstellar material.

Recent results from an ongoing series of Rayleigh-Taylor instability experiments being conducted on the Omega Laser are described. The goal of these experiments is to study, in a controlled laboratory setting, the mixing that occurs at an unstable interface subjected to an acceleration history similar to the explosion phase of a core-collapse supernova. In a supernova, the Reynolds number characterizing this mixing is extremely large (Re > 1010) and is more than sufficient to produce a turbulent flow at the interface. In the laboratory experiment, by contrast, the spatial scales are much smaller, but are still sufficiently large (Re > 105) to support a turbulent flow and therefore recreate the conditions relevant to the supernova problem. The data from these experiments will be used to validate astrophysical codes as well as to better understand the transition to turbulence in such high energy density systems. The experimental results to date using two-dimensional initial perturbations demonstrate a clear visual transition from a well-ordered perturbation structure consisting of only a few modes to one with considerable modal content. Analysis of these results, however, indicates that while a turbulent spectrum visually appears to be forming, the layer has not yet reached the asymptotic growth rate characteristic of a fully turbulent layer. Recent advances in both target fabrication and diagnostic techniques are discussed as well. These advances will allow for the study of well-controlled 3D perturbations, increasing our ability to recreate the conditions occurring in the supernova.

In recent years, we have carried out experiments at the University of Rochester’s Omega laser in which supersonic, dense-plasma jets are formed by the interaction of strong shocks in a complex target assembly (Foster et al., Phys. Plasmas 9 (2002) 2251). We describe recent, significant extensions to this work, in which we consider scaling of the experiment, the transition to turbulence, and astrophysical analogues. In new work at the Omega laser, we are developing an experiment in which a jet is formed by laser ablation of a titanium foil mounted over a titanium washer with a central, cylindrical hole. Some of the resulting shocked titanium expands, cools, and accelerates through the vacuum region (the hole in the washer) and then enters a cylinder of low-density foam as a jet. We discuss the design of this new experiment and present preliminary experimental data and results of simulations using AWE hydrocodes. In each case, the high Reynolds number of the jet suggests that turbulence should develop, although this behaviour cannot be reliably modelled by present, resolution-limited simulations (because of their low-numerical Reynolds number).