It was a journey to the center of the Earth, if only for the briefest of moments.
But rather than tunneling thousands of miles from the Earth's surface, researchers from Lawrence Livermore National Laboratory (LLNL) and several universities used the National Ignition Facility (NIF) to recreate those extreme temperature and pressure conditions of the Earth's inner core. This enabled the first ever simultaneous measurement of iron's dynamic strength at relevant temperature and pressures.
"We study iron because it is a primary constituent of Earth's and other terrestrial planet cores, and how it functions under inner-core conditions is not well understood," said LLNL physicist and co-lead author Yong-Jae Kim. "These results provide important experimental benchmarks for iron rheology, the physics of the deformation and flow of material, at Earth inner-core conditions."
The findings were recently published in Nature Communications in a paper titled "Dynamic strength of iron under pressure-temperature conditions of Earth's inner core." In addition to the LLNL scientists, the team includes researchers from the University of California San Diego (UCSD), Universidad de Mendoza (Argentina), Universidad Politécnica de Madrid (Spain), and Stanford University.
"Until now, making measurements of iron rheology at inner-core pressure-temperature conditions has been extremely difficult, as no single laboratory technique could access this full parameter space for durations sufficient to allow material flow while enabling in situ diagnostics," said LLNL physicist and co-lead author Gaia Righi. "But NIF has this capability."
The world's most energetic laser system, NIF can create temperature and pressure conditions even more extreme than the sun or the Earth's inner core. NIF is critical to national security, as it provides the experimental basis for the National Nuclear Security Administration's stockpile modernization program. The capabilities developed for that mission - laser energy, pulse shaping, ultrafast in situ diagnostics and precise target fabrication - also enable experiments such as these through the NIF Discovery Science program. Results from NIF Discovery Science frequently contribute to national-security research applications.
The preparations for the experiments were painstaking. Righi simulated many target designs and pulse shapes to ensure the material would reach pressures of 3 million atmospheres and heat to 5,000 degrees Celsius but would not heat so quickly that it melted. That work is described in a 2022 Journal of Applied Physics paper.
Led by Kim, the campaign's responsible individual, the team conducted direct laser-driven experiments at NIF using the Rayleigh-Taylor (RT) instability, which occurs when a lighter material accelerates a denser one and induces unstable growth of interface perturbations. An everyday example is pouring vinegar over olive oil.
The lasers fired on a 5.35-millimeter square target composed of layers of different materials with a ripple pattern etched onto the iron surface. A precisely timed set of backlighter beams later fired on a thin metal foil, producing high-energy X-rays that image the target to evaluate the ripple growth at a different time in each experiment. Simultaneously, an optical laser, VISAR, tracked rear surface velocity to measure the pressure conditions achieved.
The research team then used state-of-the-art codes and high-performance computing to interpret the results from radiation hydrodynamic and molecular-dynamic simulations. The hydrodynamic simulations gave a big-picture view of what happened during the experiment, while molecular dynamics revealed the material response at the atomic level. Simulation results, constrained by the experimental data, provided the strength of iron at relevant conditions.
They also discovered an unpredicted effect of iron undergoing a pressure-induced phase transition - a re-arrangement of its atoms - which furthermore breaks up its microstructure into small grains. Both changes can also influence rheological behavior. The team found that high-pressure, ε-Fe derived from initially single crystal [001] α-Fe is consistently stronger than that from [111] α-Fe, contrary to the trend at ambient conditions. Large molecular dynamics simulations showed the same pattern, suggesting that the difference arises from how these two orientations of iron experience the low-pressure phase transition and later deformation in the ε phase.
"Understanding material strength and how it depends on microstructure in Earth's inner core is important because it may influence seismic anisotropy, or how earthquake waves travel through the inner core, which is linked to core dynamics and magnetic field history," Kim said.
The researchers hope to advance this research by studying the area of mixing between the Earth's inner and outer cores.
"This new, exciting result further deepens our understanding of the extreme flow behavior of phase-changing materials," Righi said. "I've been working on this project since I was a graduate student. I'm excited to be able to continue working in the field and see just how far we can push the NIF strength platform."
The project also marked an important step in Righi's career. As a graduate student working under UCSD professor and frequent discovery science collaborator Marc Meyers, she helped design and carry out the experiments as part of her Ph.D. thesis research. After earning her Ph.D., she became the inaugural Harold Brown LLNL postdoctoral fellow and joined LLNL as a staff scientist in 2025.
Kim and Righi are joined on the paper by LLNL researchers Thomas Lockard, Robert Rudd, Camelia Stan, Christopher Wehrenberg and Hye-Sook Park; Orlando Deluigi and Eduardo Bringa of Argentina's Universidad de Mendoza; Carlos Ruestes of the Universidad Politécnica de Madrid; Arianna Gleason of SLAC and Stanford University; and Marc Meyers of the University of California, San Diego.
