Kimberly Susan Budil

THE INTERACTION OF SUPERNOVA REMNANTS WITH INTERSTELLAR CLOUDS: EXPERIMENTS ON THE NOVA LASER

The interaction of strong shock waves, such as those generated by the explosion of supernovae with interstellar clouds, is a problem of fundamental importance in understanding the evolution and the dynamics of the interstellar medium (ISM) as it is disrupted by shock waves. Here we present the results of a series of scaled Nova laser high energy density experiments investigating the evolution of a high-density sphere embedded in a low-density medium after the passage of a strong shock wave, thereby emulating the supernova shock-cloud interaction. The Nova laser was utilized to generate a strong (~Mach 10) shock wave that traveled along a miniature beryllium shock tube, 750 μm in diameter, filled with a low-density plastic emulating the ISM. Embedded in the plastic was a copper microsphere (100 μm in diameter), emulating the interstellar cloud. The morphology and evolution as well as the shock wave trajectory were diagnosed via side-on X-ray radiography. We describe here experimental X-ray radiographic results of this interaction out to several cloud crushing times and compare them to detailed two- and three-dimensional radiation hydrodynamic simulations using both arbitrary Lagrangian and Eulerian hydrodynamics (ALE), as well as high-resolution adaptive mesh refinement (AMR) hydrodynamics. A key result is the first experimental evidence that the cloud is destroyed by a three-dimensional nonlinear bending-mode instability (Widnall instability), confirming earlier predictions with high-resolution three-dimensional calculations.

The flexible x‐ray imager

A new gated x‐ray framing camera has been developed at the Lawrence Livermore National Laboratory for use at the Nova laser facility. This diagnostic, the flexible x‐ray imager, has been designed as a modular unit that can be rapidly reconfigured to change the spectral response, magnification, sensitivity, and spatial and temporal resolutions of the instrument. The electrical gate pulse width may be varied from 200 ps to 2 ns depending upon whether the experimental emphasis is on temporal resolution or sensitivity. The long integration times are particularly useful in experiments where motional blurring occurs over even longer time scales. A detailed description of the instrument and its varied uses is presented.

Experiments to Produce a Hydrodynamically Unstable, Spherically Diverging System of Relevance to Instabilities in Supernovae

Results of the first spherically diverging, hydrodynamically unstable laboratory experiments of relevance to supernovae (SNe) are reported. The experiments are accomplished by using laser radiation to explode a hemispherical capsule, having a perturbed outer surface, which is embedded within a volume of low-density foam. The evolution of the experiment, like that of a supernova, is well described by the Euler equations. We have compared the experimental results to those of two-dimensional simulations using both a radiation-hydrodynamics code and a pure hydrodynamics code with front tracking.

Absolute measurements of the equations of state of low-Z materials in the multi-Mbar regime using laser-driven shocks

Although high intensity lasers offer the opportunity to explore the equations of state (EOSs) of materials under high energy density conditions, experimental difficulties have limited the application of laser-driven shocks to EOS measurements. However, we have recently performed absolute EOS measurements on the principal Hugoniot of liquid deuterium near one Mbar and of polystyrene from 10 to 40 Mbar. The D2 measurements were made with direct drive; the polystyrene experiments were indirectly driven. The data were sufficiently accurate to differentiate between existing EOS models and were surprising, particularly for D2. The results demonstrate that laser driven shocks can be used effectively to investigate high pressure EOSs.

High-pressure, high-strain-rate lattice response of shocked materials

Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the published Hugoniot Elastic Limit (HEL) for these materials. In situ x-ray diffraction has been used to directly measure the response of the shocked lattice during shock loading. Static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. In addition, experiments were conducted using a wide-angle detector to record x rays diffracted from multiple lattice planes simultaneously. These data showed uniaxial compression of Si (100) along the shock direction and three-dimensional compression of Cu (100). In the case of the Si diffraction, there was a multiple wave structure observed. This is evaluated to determine whether there is a phase transition occurring on the time scale of the experiments, or the HEL is much higher than previously reported. Results of the measurements are presented.

Equation of state measurements of hydrogen isotopes on Nova

High intensity lasers can be used to perform measurements of materials at extremely high pressures if certain experimental issues can be overcome. We have addressed those issues and used the Nova laser to shock-compress liquid deuterium and obtain measurements of density and pressure on the principal Hugoniot at pressures from 300 kbar to more than 2 Mbar. The data are compared with a number of equation of state models. The data indicate that the effect of molecular dissociation of the deuterium into a monatomic phase may have a significant impact on the equation of state near 1 Mbar.

SPATIAL RESOLUTION OF GATED X-RAY PINHOLE CAMERAS

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.

Interaction of Supernova Remnants with Interstellar Clouds: From the Nova Laser to the Galaxy

The interaction of strong shock waves, such as those generated by the explosion of supernovae with interstellar clouds, is a problem of fundamental importance in understanding the evolution and the dynamics of the interstellar medium (ISM) as it is disrupted by shock waves. The physics of this essential interaction sheds light on several key questions: (1) What is the rate and total amount of gas stripped from the cloud, and what are the mechanisms responsible? (2) What is the rate of momentum transfer to the cloud? (3) What is the appearance of the shocked cloud, its morphology and velocity dispersion? (4) What is the role of vortex dynamics on the evolution of the cloud? (5) Can the interaction result in the formation of a new generation of stars? To address these questions, one of us has embarked on a comprehensive multidimensional numerical study of the shock cloud problem using high-resolution adaptive mesh refinement (AMR) hydrodynamics. Here we present the results of a series of Nova laser experiments investigating the evolution of a high-density sphere embedded in a low-density medium after the passage of a strong shock wave, thereby emulating the supernova shock-cloud interaction. The Nova laser was utilized to generate a strong (~Mach 10) shock wave which traveled along a miniature beryllium shock tube, 750 μm in diameter, filled with a low-density plastic emulating the ISM. Embedded in the plastic was a copper microsphere (100 μm in diameter) emulating the interstellar cloud. Its morphology and evolution as well as the shock wave trajectory were diagnosed via side-on radiography. We describe here experimental results of this interaction for the first time out to several cloud crushing times and compare them to detailed two- and three-dimensional radiation hydrodynamic simulations using both arbitrary Lagrangian and Eulerian hydrodynamics (ALE) as well as high-resolution AMR hydrodynamics. We briefly discuss the key hydrodynamic instabilities instrumental in destroying the cloud and show the importance of inherently three-dimensional instabilities and their role in cloud evolution. We describe the relationship of these new experiments and calculations to recent ROSAT X-ray observations in the Cygnus Loop.

Equation of State and Material Property Measurements of Hydrogen Isotopes at the High-Pressure, High-Temperature, Insulator-Metal Transition

A high-intensity laser was used to shock compress liquid deuterium to pressures between 0.22 and 3.4 megabars (Mbar). Shock density, pressure, and temperature were determined using a variety of experimental techniques and diagnostics. This pressure regime spans the transformation of deuterium from an insulating molecular fluid to an atomic metallic fluid. Data reveal a significant increase in compressibility and a temperature inflection near 1 Mbar, both indicative of such a transition. Single-wavelength reflectivity measurements of the shock front demonstrated that deuterium shocked above {approx}0.5 Mbar is indeed metallic. (c) 2000 The American Astronomical Society.

Laser plasma diagnostics of dense plasmas

We describe several experiments on Nova that use laser-produced plasmas to generate x-rays capable of backlighting dense, cold plasmas ((rho) approximately 1-3 gm/cm3, kT approximately 5-10 eV, and areal density (rho) l approximately 0.01-0.05 g/cm2). The x-rays used vary over a wide range of hv, from 80 eV (x-ray laser) to 9 keV. This allows probing of plasmas relevant to many hydrodynamic experiments. Typical diagnostics are 100 ps pinhole framing cameras for long pulse backlighter and a time-integrated CCD camera for a short pulse backlighter.