During experiments at the National Ignition Facility (NIF), 192 laser beams converge onto a tiny target at the center of the 10-meter-diameter, spherical target chamber in support of stockpile stewardship and fusion-ignition physics. Livermore scientists use the data from these experiments to certify the safety and security of the nation’s nuclear stockpile and make progress toward achieving fusion ignition and energy gain. With NIF, researchers can also explore the physics of material conditions at the centers of gas giant planets and stars, as well as other extreme environments.
Researchers conduct approximately 400 shots a year at NIF. The facility relies on a small army of personnel to keep it operational 24 hours a day nearly year round. One of the many critical teams is the Debris and Shrapnel Working Group (DSWG). Its mission is to evaluate risks to NIF equipment from target debris and shrapnel produced during experiments. Through computer modeling, risk assessment analysis, and experience, the team helps ensure that debris and shrapnel generated by laser shots does not damage optics, diagnostics, or other components that are expensive and difficult to replace.
DSWG is one of three groups within TaLIS (Target–Laser Interaction Sphere), an umbrella group led by Dean LaTray whose purpose is assessing the potential damage to NIF from the interaction of the laser with the target. A second group within TaLIS examines the danger posed by laser–plasma interactions, which can cause laser light to pass back through the beamlines, damaging mirrors and laser glass. A third group performs configuration reviews, examining the interface between laser light, targets, diagnostics, and the rest of the target chamber. “The service TaLIS provides is unique because NIF is unique,” says LaTray. “We offer capabilities that no other facility can provide.”
Debris and shrapnel risks stem from three sources: x-ray loads emitted from the target when hit by the laser, debris wind (vaporized material, low-density particles, or molten spray traveling at high velocity), and unvaporized target components. In the first case, laser energy heats up the target and the emitted x rays deposit energy onto diagnostics that cause materials on exposed surfaces to ablate—generating a damage-inducing pressure pulse. Debris wind poses a risk as a distributed, blast-like pressure wave. These two sources primarily threaten diagnostics in close proximity to the targets. Finally, when deposited laser energy is not sufficient to fully vaporize the target, chunks of molten or solid material driven at up to 10 kilometers per second can impact target chamber components, including optics and diagnostics.
To mitigate these risks, DSWG runs computer simulations of shots to model the deposition of the laser energy and the subsequent response of targets and diagnostics using several Livermore-developed programs, including ARES (for laser–matter interaction and radiation hydrodynamics) and ALE3D, Dyna3D, or LS-DYNA (for shrapnel impact and structural deformation). The models follow the evolution in time as far forward as possible to identify and characterize the risks. “We protect the facility and facilitate the science,” says DSWG leader Nathan Masters. “We ensure minimal risk to the facility. Our work is successful when nobody at NIF hears about us.”
Although experimental teams may only need data from the first 20 to 50 nanoseconds (ns) of a shot, Masters’s group tries to model the effects up to several microseconds and then extrapolate where the energy and debris are directed. Based on the simulation results and previous experience with similar experiments, Masters and the other members of his group, Rosita Cheung, Michelle Oliveira, Andrew Thurber, and Jae Chung, determine whether the damage risk is significant. If so, the group recommends changes to the experimental design that will reduce risk to acceptable levels. In the more than 10 years since the effort began, the team has analyzed hundreds of NIF experiments. “Through a combination of empirical and quantitative assessment,” says Dan Kalantar, target area senior scientist and advisor to the group, “we can say whether a configuration is safe based on our experience and our evaluation of risks.”