B. E. Blue

High-Energy-Density, Laboratory-Astrophysics Studies of Jets and Bow Shocks

Large-scale directional outflows of supersonic plasma, also known as jets, are ubiquitous phenomena in astrophysics [1]. The interaction of such jets with surrounding matter often results in spectacular bow shocks, and intense radiation from radio to gamma-ray wavelengths. The traditional approach to understanding such phenomena is through theoretical analysis and numerical simulations. However, such numerical simulations have limited resolution, often assume axial symmetry, do not include all relevant physical processes, and fail to scale correctly in Reynolds number and perhaps other key dimensionless parameters. Additionally, they are frequently not tested by comparison with laboratory experiments. Recent advances in high-energy-density physics using large inertial-confinement-fusion devices now allow controlled laboratory experiments on macroscopic volumes of plasma of direct relevance relevant to astrophysics [2]. In this Letter we report the first results of experiments designed to study the evolution of supersonic plasma jets and the bow shocks they drive into a surrounding medium. Our experiments reveal both regular and highly complex flow patterns in the bow shock, thus opening a new window--complementary to computer simulations--into understanding the nature of three-dimensional astrophysical jets.

Dual, orthogonal, backlit pinhole radiography in OMEGA experiments

Backlit pinhole radiography used with ungated film as a detector creates x-ray radiographs with increased resolution and contrast. Current hydrodynamics experiments on the OMEGA Laser use a three-dimensional sinusoidal pattern as a seed perturbation for the study of instabilities. The structure of this perturbation makes it highly desirable to obtain two simultaneous orthogonal backlighting views. We accomplished this using two backlit pinholes each mounted 12mm from the target. The pinholes, of varying size and shape, were centered on 5mm square foils of 50μm thick Ta. The backlighting is by K-alpha emission from a 500μm square Ti or Sc foil mounted 500μm from the Ta on a plastic substrate. Four laser beams overfill the metal foil, so that the expanding plastic provides radial tamping of the expanding metal plasma. The resulting x-rays pass through the target onto (ungated) direct exposure film (DEF). Interference between the two views is reduced by using a nose cone in front of the DEF, typically with a...

Transition to turbulence and effect of initial conditions on three-dimensional compressible mixing in planar blast-wave-driven systems

Perturbations on an interface driven by a strong blast wave grow in time due to a combination of Rayleigh–Taylor, Richtmyer–Meshkov, and decompression effects. In this paper, results from three-dimensional (3D) numerical simulations of such a system under drive conditions to be attainable on the National Ignition Facility [E. M. Campbell, Laser Part. Beams 9, 209 (1991)] are presented. Using the multiphysics, adaptive mesh refinement, higher order Godunov Eulerian hydrocode, Raptor [L. H. Howell and J. A. Greenough, J. Comput. Phys. 184, 53 (2003)], the late nonlinear instability evolution, including transition to turbulence, is considered for various multimode perturbation spectra. The 3D post-transition state differs from the 2D result, but the process of transition proceeds similarly in both 2D and 3D. The turbulent mixing transition results in a reduction in the growth rate of the mixing layer relative to its pretransition value and, in the case of the bubble front, relative to the 2D result. The post...

Development of intense point x-ray sources for backlighting high energy density experiments (invited)

High-energy-density (HED) experiments are often diagnosed using x-ray backlighting. Recently, experiments have been designed and fielded that require x-ray backlighting having large fields of view and high x-ray energies. These types of experiments will be even more prevalent on the National Ignition Facility laser. Point backlighting offers the potential to obtain higher-energy x rays using less laser energy while giving a large, uniform field of view (millimeters). We present recent results from Los Alamos National Lab, Lawrence Livermore National Lab, and the University of Rochester’s Laboratory for laser energetics obtained on the OMEGA laser at the University of Rochester on the development of such bright sources. We include discussion of the challenges and successes to date.

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.

The effect of a short wavelength mode on the evolution of a long wavelength perturbation driven by a strong blast wave

Shock-accelerated material interfaces are potentially unstable to both the Richtmyer–Meshkov and Rayleigh–Taylor (RT) instabilities. Shear that develops along with these instabilities in turn drives the Kelvin–Helmholtz instability. When driven by strong shocks, the evolution and interaction of these instabilities is further complicated by compressibility effects. This paper details a computational study of the formation of jets at strongly driven hydrodynamically unstable interfaces, and the interaction of these jets with one another and with developing spikes and bubbles. This provides a nonlinear spike-spike and spike-bubble interaction mechanism that can have a significant impact on the large-scale characteristics of the mixing layer. These interactions result in sensitivity to the initial perturbation spectrum, including the relative phases of the various modes, that persists long into the nonlinear phase of instability evolution. Implications for instability growth rates, the bubble merger process, ...

Improved pinhole-apertured point-projection backlighter geometry

Pinhole-apertured point-projection x-ray radiography is an important diagnostic technique for obtaining high resolution, high contrast, and large field-of-view images used to diagnose the hydrodynamic evolution of high energy density experiments. In this technique, a pinhole aperture is placed between a laser irradiated foil (x-ray source) and an imaging detector. Future high energy density experiments that utilize more opaque materials will require backlighters with improved contrast as compared to what is currently used. In this article, we present an improved backlighter geometry that utilizes a tilted pinhole for debris mitigation and a front-side illuminated backlighter foil for improved photon statistics.

Debris mitigation in pinhole-apertured point-projection backlit imaging

Pinhole-apertured point-projection x-ray radiography is an important diagnostic technique for obtaining high resolution, high contrast, and large field-of-view images used to diagnose the hydrodynamic evolution of high energy density experiments. In this technique, a pinhole aperture is placed between a laser irradiated foil (x-ray source) and an imaging detector. In the present geometry, the x rays that are not transmitted through the pinhole aperture, ablate the pinhole substrate’s surface, and turn it into a flyer plate. The pinhole substrate then breaks apart into shrapnel, and that shrapnel can damage diagnostics inside the target chamber. In this letter, we present a technique on mitigating the debris by using a tilted pinhole.

Understanding reconstructed Dante spectra using high resolution spectroscopy

The Dante is an 18 channel filtered diode array used at the National Ignition Facility (NIF) to measure the spectrally and temporally resolved radiation flux between 50 eV and 20 keV from various targets. The absolute flux is determined from the radiometric calibration of the x-ray diodes, filters, and mirrors and a reconstruction algorithm applied to the recorded voltages from each channel. The reconstructed spectra are very low resolution with features consistent with the instrument response and are not necessarily consistent with the spectral emission features from the plasma. Errors may exist between the reconstructed spectra and the actual emission features due to assumptions in the algorithm. Recently, a high resolution convex crystal spectrometer, VIRGIL, has been installed at NIF with the same line of sight as the Dante. Spectra from L-shell Ag and Xe have been recorded by both VIRGIL and Dante. Comparisons of these two spectroscopic measurements yield insights into the accuracy of the Dante reconstructions.

Measurement and simulation of jet mass caused by a high-aspect ratio hole perturbation

Inertial confinement fusion (ICF) capsule performance can be negatively impacted by the presence of hydrodynamic instabilities. To perform a gas fill on an ICF capsule current plans involve drilling a small hole and inserting a fill tube to inject the gas mixture into the capsule. This introduces a perturbation on the capsule, which can seed hydrodynamic instabilities. The small hole can cause jetting of the shell material into the gas, which might adversely affect the capsule performance. We have performed simulations and experiments to study the hydrodynamic evolution of jets from high-aspect ratio holes, such as the fill tube hole. Although simulations using cold materials over predict the amount of mass in the jet, when a reasonable amount of preheat (< 1 eV) is introduced, the simulations are in better agreement with the experiment.