Livermore computer simulations exploring the effects of shock waves on crystalline HMX were featured on the cover of the May 14, 2015, issue of Journal of Applied Physics. The work, performed by staff scientist Ryan Austin and a team of Laboratory researchers, is part of a project to better understand the safety and performance of high explosives, such as HMX, which are used in the nuclear stockpile and by the Department of Defense.
The HMX crystals contain defects in the form of pores or bubbles. When shocked, the pores collapse and form “hot spots,” which can produce small burning regions that propagate in a self-sustained manner. The interaction of burn and pressure fronts from many hot spots initiates the path toward detonation. Austin says, “From a mechanistic viewpoint, we still don’t understand precisely how these burning reactions are initiated or how the transition to detonation occurs.”
Austin employed state-of-the-art material models that put together most of the important physical processes. A significant finding of the simulations was prominent shear banding (melt cracking) around a collapsing pore—an effect derived from the strength response of the crystal. This behavior contrasts predictions from conventional strength models, which typically do not account for the anisotropic and viscoplastic nature of crystal deformation. The simulations show reactions can be initiated within the shear bands (extended hot spots) on a nanosecond timescale, indicating that dynamic flow strength is an important consideration.
Contact: Ryan Austin (925) 423-1445 (austin28@llnl.gov).
In recent laser wakefield acceleration experiments, a team of scientists from Livermore and the University of California, Los Angeles (UCLA), revealed new electron ring formations in addition to the typically observed beams. Using the ultrashort-pulse Callisto laser system in the Laboratory’s Jupiter Laser Facility, the team produced a plasma in a low-density gas-cell target. The research appeared in the July 31, 2015, issue of Physical Review Letters.
The interaction of the high-intensity laser with the gas created a relativistic plasma wave that then accelerated some of the electrons in the plasma to more than 100-megaelectronvolt energies. These electron beams are usually directed along the laser axis and have fairly low divergence. In certain cases, they were also accompanied by a second, off-axis beam that had a ring-like shape—a feature that has never been previously reported. UCLA collaborators performed computationally intensive three-dimensional calculations of the experimental conditions to determine the feature’s origin. “The dynamics of the plasma wave are often calculated in simulations, but the small spatial scale and fast timescale of the wakefield process has made direct measurements of many effects difficult or impractical,” says lead author Brad Pollock. “The discovery of new features allows us to confidently compare simulations with experiments.” This work was partially funded by the Laboratory Directed Research and Development program.
Contact: Brad Pollock (925) 424-2753 (pollock6@llnl.gov).
Three technologies developed by Livermore researchers and their collaborators received R&D 100 awards from R&D Magazine in its annual competition to honor top scientific and engineering technologies with commercial potential. This year’s award winners are as follows:
Determining the chemical abundance pattern left by the earliest stars in the cosmos is important to understanding the evolution of the universe. An international team led by Livermore’s Brian Bucher has made an important contribution to predicting the unique chemical signature left by early stars—which formed approximately 13.8 billion years ago—with the first direct measurement under stellar conditions of a nuclear reaction. This work appeared in the June 26, 2015, issue of Physical Review Letters. Bucher says, “Verifying the predicted composition of stellar ashes by comparing them to observational data is vital to our understanding the properties of the first stars and the formation of the first galaxies.”
To accurately determine early stars’ abundance signatures requires proper modeling of the stars and their nuclear reactions. One reaction that largely influences some key properties of the abundance pattern is the fusion of two carbon nuclei into a magnesium nucleus and one neutron. However, measuring stellar reaction rates in the laboratory is challenging because the likelihood of producing a reaction is rare. The team successfully measured the carbon-fusion reaction at stellar energies using a Laboratory accelerator. With this measurement, the team has significantly improved the precision of this rate for stellar modeling. Bucher says, “We’ve studied its impact on the resulting stellar abundance pattern predictions, helping to identify the signature of the universe’s elusive first generation of stars and their supernovae.”
Contact: Brian Bucher (925) 422-7686 (bucher3@llnl.gov).
In a research paper featured on the cover of the August 7, 2015, issue of Physical Chemistry Chemical Physics, a Livermore team provides new insight into specific factors that determine the absorption characteristics of copper complexes. The results demonstrate that conventional interpretations based on the ligand field theory—a staple concept in inorganic chemistry—are insufficient for capturing the full characteristics of the absorption profile. Instead, the team matched computational simulation results with experimental spectroscopic data to identify how specific spectral characteristics are triggered by the dynamics of the surrounding chemical environment.
“These results are a first step toward creating optically tunable materials for filters and for energy-efficient ‘smart window’ technologies. They also could help us better understand the role of metal–ligand complexes in photobiology,” says Livermore’s Roger Qiu, lead author of the paper.
The new research also demonstrates the power of combining the Laboratory’s experimental and quantum chemistry simulation capabilities to tackle challenging scientific questions. This work is part of a project funded the Laboratory Directed Research and Development Program aimed at controlling the absorption characteristics of transition metal-ligand complexes for optical filter applications in high-power laser systems.
Contact: Roger Qiu (925) 422-1636 (qiu2@llnl.gov).
A team of scientists from Lawrence Livermore and the Russian Federal Nuclear Center-All-Russian Research Institute of Experimental Physics (RFNC-VNIIEF) has shown that at extreme conditions, beryllium has very little strength and most models overpredict its material strength. According to Marc Henry de Frahan, the lead author of a recent paper published on the cover of the Journal of Applied Physics, this finding has implications for scientists working with technology where beryllium is subject to extreme pressures and strain rates.
In the experiments, a piece of high explosive (HE) was detonated near the beryllium. The team imposed a sinusoidal ripple pattern on the beryllium samples. When the expanding HE products load up against the target, the target accelerates. The low-density gas pushes against the higher density metal, making the interface between the two materials Rayleigh–Taylor unstable, and the ripples grow in amplitude as the target accelerates. The ripple growth is limited by the strength of the beryllium. X-ray images of the side of the target showed the height of the ripples at some time after the HE loading occurred. Velocimetry measurements of the target showed its acceleration profile.
Because the researchers devised the initial ripple amplitude and measured the acceleration, they could infer the strength of the material using strength-model simulations. The experiment helped determine the effect of strength, which can refine the performance of various strength models.
Contact: Robert Cavallo (925) 422-0779 (cavallo1@llnl.gov).
Lawrence Livermore scientists, as part of an international team, have discovered the most Jupiter-like planet ever seen in a young star system. Using a new advanced adaptive-optics device on the Gemini Planet Imager (GPI)—located on the Gemini South Telescope in Chile—the team captured an image of the planet. Called 51 Eridani b, the planet could help scientists discover how Jupiter and other gas giants form and influence their planetary systems. Since a planet’s luminosity is a function of age, mass, and initial conditions, luminosity can provide insight into the planet’s formation, according to former Livermore researcher Bruce Macintosh of Stanford University, who was the lead author on a paper appearing in the August 14, 2015, edition of Science.
GPI—whose Livermore-developed adaptive optics are some of the most sophisticated in the world—was designed specifically for discovering and analyzing faint, young planets orbiting bright stars. The 51 Eridani b star is considered young—only 20 million years old and about twice the mass of Jupiter. When planets similar to 51 Eridani b coalesce, material falling into the planet releases energy and heats it up. Over the next 100 million years, these planets radiate that energy away, mostly as infrared light. Using GPI, astronomers observed the planet’s characteristics, which seem to suggest what Jupiter was like in its infancy. Livermore’s scientific involvement and technical support of GPI is led by S. Mark Ammons and funded by the Laboratory Directed Research and Development Program. Laboratory engineers Lisa Poyneer, David Palmer, and Brian Bauman made critical contributions to GPI’s design.
Contact: S. Mark Ammons (925) 422-2102 (ammons1@llnl.gov).