In a paper published in the April 11, 2016, online issue of Nature Physics, researchers at Lawrence Livermore’s National Ignition Facility (NIF) analyzed “high-foot” inertial confinement fusion (ICF) experiments that reached the highest levels of alpha-particle heating, or self-heating, achieved by any laser facility. Alpha-particle heating within deuterium–tritium (DT) targets is a key step on the path to ignition.
In the high-foot experiments, the early-time foot of the drive—the initial “picket” of the laser pulse—was approximately doubled as compared to the low-foot drive, thus launching a stronger and faster first shock and substantially reducing the implosion instabilities associated with low-foot experiments. Recent three-dimensional simulations of the fusion targets used in both high- and low-foot experiments have shown reasonable agreement with the experimental results, indicating an improved understanding of the implosions that can be used to guide future work toward ignition. “We have obtained a factor-of-two yield amplification from alpha heating, but more importantly, we see an array of evidence that symmetry control and engineering features are limiting further progress,” says Omar Hurricane, ICF program chief scientist and lead author of the paper.
NIF provides insights and data for the National Nuclear Security Administration’s science-based Stockpile Stewardship Program. Advances in ignition physics and performance also play a key role in fundamental science and potential energy applications.
Contact: Omar Hurricane (925) 424-2701 (hurricane1@llnl.gov).
Hydrogen is the most abundant element found in the universe, making up nearly three-quarters of all matter, but many questions about the element remain. In a paper published in the April 15, 2016, edition of Nature Communications, a team of researchers, including scientists from Lawrence Livermore, the University of California (UC) at Berkeley, UC at Los Angeles, SLAC National Accelerator Laboratory, the University of Rostock in Germany, and Sandia National Laboratories, describe what happens to hydrogen at high pressure.
The team used an x-ray scattering technique to detect free electrons that appear in high-pressure shock waves formed when hydrogen is subjected to a high-energy laser beam. The experiments were conducted at Livermore’s Jupiter Laser Facility using the two-beamed Janus laser. One beam launched a shock wave into the deuterium target, and a second beam created x rays that scattered off the shocked hydrogen. A crystal spectrometer spread the scattered x rays into a spectrum that was then compared to theoretical calculations. By conducting the same experiment at several pressures, the team established the specific pressure at which hydrogen turned from an insulator to a metal.
The researchers determined the hydrogen content at a variety of shock conditions, calculating how many deuterium molecules turned into single atoms—a process called dissociation. They found that the pressures at which their x-ray measurements indicated the appearance of free electrons (ionization) coincided with where dissociation occurred. Former Livermore graduate student and lead author of the paper, Paul Davis, says, “The details of how hydrogen dissociates under pressure and becomes electrically conductive are important for scientists seeking to understand planetary interiors and the dynamo action that causes their magnetic fields.”
Contact: Rip Collins (925) 423-2204 (collins7@llnl.gov).
A study conducted by Lawrence Livermore material scientists found that foams produced through three-dimensional (3D) printing are more durable and offer longer mechanical performance than standard cellular solids. The research appears in the April 27, 2016, edition of Scientific Reports.
Foams are an important class of materials with applications ranging from thermal insulation and shock-absorbing support cushions to lightweight structural and floatation components. Conventional foams are created by processes that result in a nonuniform structure with significant variations in the size, shape, thickness, connectedness, and topology of constituent cells. As an improved alternative to conventional foam products, Lawrence Livermore scientists recently demonstrated how direct ink writing could be used to print 3D foams with uniform structures. “These foams offer tremendous flexibility in creating programmable architectures, customizable shape, and tunable mechanical response,” says lead author Amitesh Maiti.
As part of the work, Lawrence Livermore researchers tested the new material’s stability by conducting accelerated aging experiments in which samples of both conventional and 3D-printed foam were subjected to elevated temperatures under constant compressive stress. The stress condition, mechanical response, and permanent structural deformation of each sample were monitored for one year. The team then modeled the evolution of these properties over decades under ambient conditions. Maiti says, “This work strongly indicates the superior long-term stability and performance of the printed material and may result in 3D-printed foam replacing traditional foam in specific future applications.”
Contact: Amitesh Maiti (925) 422-6657 (maiti2@llnl.gov).