Using a hohlraum shaped like a rugby ball—rather than a typical straight cylinder—scientists conducting inertial confinement fusion (ICF) experiments at the National Ignition Facility (NIF) have significantly boosted the amount of laser-induced energy absorbed by the ICF fuel capsule. The research, which appears in the October 29, 2018, edition of Nature Physics, detailed how scientists from Lawrence Livermore and Los Alamos national laboratories increased the level of laser-induced energy absorption to about 30 percent, nearly double what was achieved with cylindrical hohlraums.
The research team used a single-shell, aluminum target capsule inside a gold rugby hohlraum, which is wider in the center and tapered toward the ends. The experiment was driven by 1-megajoule laser shots, which coupled about 300 kilojoules with the capsule. The hohlraum shape allowed for a capsule with a radius about 50 percent larger than usual, exposing more surface area to capture energy. In addition, the hohlraum’s curved inner walls better directed x-ray energy to the capsule.
Spherical capsules remain symmetrical during implosion, but perturbations and other imperfections degrade the fusion process and inhibit ignition—when more energy is produced than the amount of energy deposited by the lasers in the hohlraum. “If we have more energy available, then we can tolerate these defects better, and we will have more margin of error to achieve ignition,” says Livermore physicist and lead author of the paper Yuan Ping. The team plans to gradually step up the total amount of laser energy while finding the right mix of energy, laser pulse shape, capsule size, hohlraum shape, and implosion symmetry to optimize energy delivery to the fusion fuel—providing a new path to ignition.
Contact: Yuan Ping (925) 422-7052 (ping2@llnl.gov).
Cellular membranes serve as an ideal example of a multifunctional, tunable, precise, and efficient biological system. However, until recently, these membranes have been difficult to reproduce in a laboratory setting. In research that appears on the cover of the December 17, 2018, online edition of Advanced Materials, Lawrence Livermore scientists have now created polymer-based membranes with 1.5-nanometer carbon nanotube pores that mimic the architecture of cellular membranes.
The inner channel of a carbon nanotube is narrow, hydrophobic, and extremely smooth—all properties that mirror those of biological pores. Carbon nanotube porins (CNTPs) are short segments of carbon nanotubes (5 to 15 nanometers) that can transport protons, water, and macromolecules, including DNA. “CNTPs are unique among biomimetic nanopores because carbon nanotubes are robust and highly chemically resistant, which make them amenable for use in a wider range of separation processes, including those requiring harsh environments,” says Aleksandr Noy, a Livermore materials scientist and senior author on the paper.
The team integrated CNTP channels into polymer membranes, mimicking the structure, architecture, and basic functionality of biological membranes in an all-synthetic architecture. Proton and water transport measurements showed that the CNTPs maintain their high permeability in the polymer membrane environment. Jeremy Sanborn, a Lawrence Scholar and co-author on the paper, says, “This development opens new opportunities for delivery of molecular reagents to vesicular compartments to initiate confined chemical reactions and mimic the sophisticated transport-mediated behaviors of biological systems.”
Contact: Aleksandr Noy (925) 423-3396 (noy1@llnl.gov).
Spring snowpack in the mountains of the western United States has not declined substantially since the 1980s, despite an increase in temperature of 1°C during the same period. In research that appears in the December 17, 2018, edition of the journal Geophysical Research Letters, scientists at Lawrence Livermore, Oregon State University, and the University of Washington describe how the snowpack’s apparent insensitivity to warming results from changes in atmospheric circulation caused by natural swings in the sea surface temperature over the Pacific Ocean.
For the study, the team looked at trends in sea surface temperature over a 35-year period and used a computational method called “dynamical adjustment” to quantify the influences of both natural variability and human-induced warming on snowpack changes. “Our results indicate that the contribution of global warming to western U.S. snowpack loss has in reality been large and widespread since the 1980s, but mostly offset by natural variability in the climate system,” says Livermore scientist Stephen Po-Chedley, who co-authored the paper.
The results point to a faster rate of snowpack loss in coming decades as the phase of natural variability becomes less favorable for snowpack accumulation. Since 1950, the snowpack on April 1 (the typical peak in annual snowpack) has decreased by 15 percent over much of the western United States, as warmer temperatures have caused a shift from snow to rain, particularly at low elevations. Climate models indicate a further decrease in winter snowpack of approximately 60 percent by 2050, leading to a dramatic reduction in summer stream flows.
Contact: Stephen Po-Chedley (925) 422-3421 (pochedley1@llnl.gov).