Two papers published in the February 5, 2014, edition of Physical Review Letters (Dittrich et al., Park et al.) and one published in the February 20, 2014, edition of Nature detail a series of National Ignition Facility (NIF) experiments that achieved an order of magnitude improvement in yield performance over past experiments. Ignition—the process of releasing fusion energy equal to or greater than the amount of energy used to confine the fuel—has been a long-term goal of inertial confinement fusion science. A key step along the path to ignition is to have “fuel gains” greater than unity, where the energy generated through fusion reactions exceeds the amount of energy deposited into the fusion fuel. Although ignition remains the ultimate goal, the milestone of achieving fuel gains greater than unity has been reached for the first time ever on any facility.
“What’s really exciting is that we are seeing a steadily increasing contribution to the yield coming from the alpha-particle self-heating, as we push the implosion a little harder each time,” says Livermore physicist Omar Hurricane. In this process, alpha particles—helium nuclei produced in the deuterium–tritium (DT) fusion process—deposit their energy in the DT fuel, rather than escaping. The alpha particles further heat the fuel, increasing the rate of fusion reactions, thus producing more alpha particles. This feedback process is the mechanism that leads to ignition. The process has been demonstrated in a series of experiments in which the fusion yield has been systematically increased by more than a factor of 10 over previous approaches.
The experimental series was designed to limit mixing of the target’s plastic shell with the DT fuel as it is compressed. It was hypothesized that this mixing was the source of degraded fusion yields observed in previous experiments. To suppress the instability that causes mixing, researchers modified the laser pulse used to compress the fuel. The higher yields that were obtained affirmed the hypothesis. The experimental results have matched computer simulations much better than previous experiments, providing an important benchmark for the models used to predict the behavior of matter under conditions similar to those generated during a nuclear explosion, a primary goal for NIF.
Contacts: Thomas Dittrich (925) 422-4706 (dittrich1@llnl.gov), Omar Hurricane (925) 424-2701 (hurricane1@llnl.gov), or Hye-Sook Park (925) 422-7062 (park1@llnl.gov).
For the first time, an international team of astrophysicists, including Lawrence Livermore scientists, have unraveled how stars blow up in supernova explosions. Using NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR)—a high-energy x-ray observatory—the international collaboration created the first-ever map of radioactive material in a supernova remnant, named Cassiopeia A, or Cas A. (See image below of Cas A captured by NuSTAR.) “One of NuSTAR’s science goals is to map recently synthesized material in young supernova remnants, and Cas A is one of the youngest supernova remnants we know of,” says Mike Pivovaroff, a Livermore physicist and coauthor of a paper published in the February 20, 2014, issue of Nature.
While small stars such as our Sun die less violent deaths, stars with more than eight times the mass of our Sun blow up in core-collapse supernova explosions and create remnants such as Cas A. Because these explosions transform lighter elements into elements heavier than iron, the debris clouds are uniquely responsible for seeding the universe with many heavy elements that are prerequisites for the formation of life on Earth. NuSTAR is the first telescope capable of producing maps of radioactive material in supernova remnants. In the Cas A study, the material is titanium-44, an atom with an unstable nucleus produced at the heart of the exploding star. “Cas A was a mystery for so long, but now with the map of radioactive material, we’re getting a more complete picture of the core of the explosion,” says Bill Craig, a former Lawrence Livermore scientist now at the University of California at Berkeley and coauthor of the paper.
The NuSTAR map of Cas A, which shows the titanium concentrated in clumps at the remnant’s center, points to a possible solution to the mystery of how the star met its demise. When researchers have performed computer simulations of supernova blasts, the main shock wave stalls out, and the star fails to shatter. “These latest findings strongly suggest the exploding star literally sloshed around, reenergizing the stalled shock wave and allowing the star to blast off its outer layers,” says Pivovaroff, who is part of the optics team along with Livermore’s Julia Vogel and Todd Decker. The optics principles and the fabrication approach for the x-ray optics in NuSTAR are based on those developed for Livermore’s High Energy Focusing Telescope. (See S&TR, March 2006, Floating into Thin Air.)
Contact: Mike Pivovaroff (925) 422-7779 (pivovaroff1@llnl.gov).