Thomas E. Tierney

Laser-launched flyer plates for shock physics experiments

The TRIDENT laser was used to launch Cu, Ga, and NiTi flyers from poly(methylmethacrylate) (PMMA) substrates, coated with thin (∼micron) layers to absorb the laser energy, confine the plasma, and insulate the flyer. The laser pulse was ∼600ns long, and the flyers were 50 to 250μm thick and 4 mm in diameter. With an energy of 10–20 J, speeds of several hundred meters per second were obtained. Simulations were performed of the flyer launch process, using different models. The simulations reproduced the magnitude of the flyer speed and qualitative variations with drive energy and design parameters, but systematically overpredicted the flyer speed. The most likely explanation is that some of the laser energy was deposited in the transparent substrate, reducing the amount available for acceleration. The deceleration of the flyer was measured on impact with a PMMA window. Given the equation of state and optical properties of PMMA, the deceleration allowed points to be deduced on the principal Hugoniot of Cu. Th...

Laser-induced shock waves in condensed matter: some techniques and applications

Laser-induced shock waves in condensed matter have important applications in dynamic material studies and high pressure physics. We briefly review some techniques in laser-induced shock waves, including direct laser drive, laser-launched flyer plate, quasi-isentropic loading, point and line imaging velocity interferometry, transient X-ray diffraction, spectroscopy and shock recovery, and their applications to study of equation of state, spallation, and phase transitions.

Blast wave radiation source measurement experiments on the Z Z-pinch facility

The Dynamic Hohlraum (DH) radiation on the Z facility at Sandia National Laboratories [R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceeding of the 11th International Pulsed Power Conference, Baltimore, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709] is a bright source of radiant energy that has proven useful for high energy density physics experiments. But the radiation output from a DH on Z needs to be well known. In this paper, a new method is presented for measuring the radiation fluence deposited in an experiment, specifically, an experiment driven by a Z DH. This technique uses a blast wave produced in a SiO2 foam, which starts as supersonic but transitions to subsonic, producing a shock at the transition point that is observable via radiography. The position of this shock is a sensitive measure of the radiation drive energy from the Z DH. Computer simulations have been used to design and analyze a Z foam blast wave experiment.

Dynamic response of materials on subnanosecond time scales, and beryllium properties for inertial confinement fusion

During the past few years, substantial progress has been made in developing experimental techniques capable of investigating the response of materials to dynamic loading on nanosecond time scales and shorter, with multiple diagnostics probing different aspects of the behavior. These relatively short time scales are scientifically interesting because plastic flow and phase changes in common materials with simple crystal structures—such as iron—may be suppressed, allowing unusual states to be induced and the dynamics of plasticity and polymorphism to be explored. Loading by laser-induced ablation can be particularly convenient: this technique has been used to impart shocks and isentropic compression waves from ∼1to200GPa in a range of elements and alloys, with diagnostics including line imaging surface velocimetry, surface displacement (framed area imaging), x-ray diffraction (single crystal and polycrystal), ellipsometry, and Raman spectroscopy. A major motivation has been the study of the properties of be...

Late-Time Radiography of Beryllium Ignition-Target Ablators in Long-Pulse Gas-Filled Hohlraums

A multiple-laboratory campaign is underway to qualify beryllium as a fusion capsule ablator for the National Ignition Facility [Moses and Wuest, Fusion Sci. Technol. 43, 420 (2003)]. Although beryllium has many advantages over other ablator materials, individual crystals of beryllium have anisotropic properties, e.g., sound speed, elastic constants, and thermal expansion coefficients, which may seed hydrodynamic instabilities during the implosion phase of ignition experiments. Experiments based on modeling have begun at the OMEGA laser [Boehly, McCrory, Verdon et al., Fusion Eng. Design 44, 35 (1999)] to create a test bed for measuring instability growth rates with face-on radiography of perturbed beryllium samples with the goal of establishing a specification for microstructure in beryllium used as an ablator. The specification would include the size and distribution of sizes of grains and voids and the impurity content. The experimental platform is a 4kJ laser-heated (for ∼6ns) hohlraum that is well mod...

Novel crystalline carbon-cage structure synthesized from laser-driven shock wave loading of graphite

We report a novel crystalline carbon-cage structure synthesized from laser-driven shock wave loading of a graphite-copper mixture to about 14+/-2 GPa and 1000 +/- 200 K. Quite unexpectedly, it can be structurally related to an extremely compressed three-dimensional C60 polymer with random displacement of C atoms around average positions equivalent to those of distorted C60 cages. Thus, the present carbon-cage structure represents a structural crossing point between graphite interlayer bridging and C60 polymerization as the two ways of forming diamond from two-dimensional and molecular carbon.

Microstructure morphology of shock-induced melt and rapid resolidification in bismuth

With the growing importance of nanotechnology, there is increased emphasis on rapid solidification processing to produce materials microstructures with a finer length scale. However, few studies have focused on the question of how a material restructures itself on the microstructural scale when it refreezes at very high cooling rates. Here we report on the development of microstructures in pure bismuth metal as it is subjected to rapid shock-driven melting and subsequent resolidification (on release of pressure), where the estimated effective undercooling rates are on the order of 1010K∕s, orders of magnitude faster than any achieved before in bulk material. Microscopic examination of the recovered material indicates that the melting transformation was far from homogeneous, and substantial morphological changes are observed compared to the starting microstructure.

Kilovolt radiation from a laser-produced Al plasma

We have characterized kilo-electron-volt (keV) emission from Al plasmas for various laser illuminations at the OMEGA laser with the goal of optimizing the ability to backlight low-atomic-number materials such as beryllium for fusion ignition studies. The plasma is diagnosed by spectral-measurement comparisons to detailed theoretical atomic physics models. It is found that a significant fraction of the radiation is due to the x-ray continuum, that the electron temperature Te depends weakly on laser energy and power, and that the conversion efficiency to Lyman α (Lα, the N=2→1 transition in H-like Al) at 1.73 keV is reduced as laser energy is increased. As the number of beams is increased, the extra laser energy goes into a larger, higher-density plasma in which He-like and H-like ions are more effectively ionized.

Role of spall in microstructure evolution during laser-shock-driven rapid undercooling and resolidification

We previously reported [Colvin et al., J. Appl. Phys. 101, 084906 (2007)] on the microstructure morphology of pure Bi metal subjected to rapid laser-shock-driven melting and subsequent resolidification upon release of pressure, where the estimated effective undercooling rates were of the order of 109–1010 K/s. More recently, we repeated these experiments, but with a Bi/Zn alloy (Zn atomic fraction of 2%–4%) instead of elemental Bi and with a change in target design to suppress spall in the Bi/Zn samples. We observed a similar microstructure morphology in the two sets of experiments, with initially columnar grains recrystallizing to larger equiaxed grains. The Bi samples, however, exhibited micron-scale dendrites on the spall surfaces, whereas there were no dendritic structures anywhere in the nonspalled Bi/Zn, even down to the nanometer scale as observed by transmission electron microscopy. We present the simulations and the interferometry data that show that the samples in the two sets of experiments fol...

Gold wall ablation and hohlraum filling measurements of vacuum and gas-filled hohlraums

An understanding of the timing and dynamics of hohlraum filling by laser-induced gold wall ablation is critical to the performance of indirectly-driven fusion ignition designs for the National Ignition Facility [E. Moses and C. Wuest, Fusion Science and Technology, 43, 420 (2003)]. Hohlraum wall ablation negatively affects ignition hohlraum performance by (1) reducing laser coupling by increasing backscatter by laser plasma instabilities, e.g., stimulated Brillouin scattering, (2) altering where lasers couple by moving the critical surface away from the walls and changing the refractive index, and (3), in the case of vacuum hohlraums, ablating directly into contact with the ablation layer of the fuel capsule. We report on measurements of gold-filling of hohlraums from a series of OMEGA laser [T.R. Boehly, R.L. McCrory, C.P. Verdon et al., Fusion Engineering and Design, 44, 35 (1999)] experiments involving vacuum and gas-filled hohlraums. On-axis x-ray imaging of gold self-emission shows delayed filling for gas-filled hohlraums, as expected. In addition, we present data on the hohlraum temperature penalty incurred with the use of a 1-atmosphere methane-fill. We discuss data and simulation predictions for 1-atmosphere neopentane filled hohlraums driven with a modified laser pulse.