World’s most powerful laser reveals secrets of pressure-driven ionization in stars and nuclear fusion

Pressure driven ionization

Scientists conducted laboratory experiments at the Lawrence Livermore National Laboratory’s National Ignition Facility that generated the extreme compressions needed for pressure-driven ionization. Their research provides new insights into atomic physics at gigabar pressures, which benefits astrophysics and nuclear fusion research. Credits: Graphic illustration by Greg Stewart/SLAC National Accelerator Laboratory; insert by Jan Vorberger/Helmholtz-Zentrum Dresden-Rossendorf

Scientists at Lawrence Livermore National Laboratory have successfully used the world’s most powerful laser to simulate and study pressure-driven ionization, a process vital to understanding the structure of planets and stars. The research revealed unexpected properties of highly compressed matter and has significant implications for both astrophysics and nuclear fusion research.

Scientists conducted laboratory experiments at the Lawrence Livermore National Laboratory (LLNL) that provide new insights into the complex process of pressure-driven ionization in giant planets and stars. Their research, published May 24 in Natureunveils material properties and the behavior of matter under extreme compression, offering important implications for astrophysics and nuclear fusion research.

If you can recreate the conditions that occur in a stellar object, then you can actually find out what’s going on inside it, said collaborator Siegfried Glenzer, director of the High Energy Density Division at the Department of Energy’s SLAC National Accelerator Laboratory. It’s like putting a thermometer in the star and measuring how hot it is and what those conditions do to the atoms within the material. It can teach us new ways to manipulate matter for fusion energy sources.

The international research team used the world’s largest and most energetic laser, the National Ignition Facility (NIF), to generate the extreme conditions needed for pressure-driven ionization. Using 184 laser beams, the team heated the inside of a cavity, converting the laser energy into X-rays that heated a 2mm diameter beryllium shell in the centre. As the exterior of the shell rapidly expanded due to heating, the interior accelerated inward, reaching temperatures of around two million kelvins and pressures of up to three billion atmospheres, and creating a tiny piece of matter like the one that it is found in dwarf stars for a few nanoseconds in the laboratory.

The highly compressed beryllium sample, up to 30 times its ambient solid density, was probed using Thomson X-ray scattering to infer its density, temperature and electronic structure. The results revealed that, upon intense heating and compression, at least three out of four electrons in beryllium switched to conducting states. Furthermore, the study discovered unexpectedly weak elastic scattering, indicating reduced localization of the remaining electron.

The matter inside the giant planets and some relatively cold stars is strongly compressed by the weight of the overlying layers. At such high pressures, generated by high compression, the proximity of atomic nuclei leads to interactions between bonded electronic states of neighboring ions and, finally, to their complete ionization. While ionization in burning stars is driven primarily by temperature, pressure-driven ionization dominates in cooler objects.

Despite its importance to the structure and evolution of celestial objects, pressure ionization as a pathway to highly ionized matter is not well understood in theory. Furthermore, the required extreme states of matter are very difficult to create and study in the laboratory, said LLNL physicist Tilo Dppner, who led the project.

By recreating extreme conditions similar to those inside giant planets and stars, we were able to observe changes in material properties and electron structure that are not captured by current models, Dppner said. Our work opens up new avenues for studying and modeling the behavior of matter under extreme compression. Ionization in dense plasmas is a key parameter as it affects the equation of state, thermodynamic properties and the transport of radiation through the opacity.

The research also has significant implications for inertially confined fusion experiments at NIF, where X-ray absorption and compressibility are key parameters for optimizing high-performance fusion experiments. A comprehensive understanding of pressure- and temperature-driven ionization is essential for modeling compressed materials and ultimately developing an abundant, carbon-free energy source using laser-guided nuclear fusion, Dppner said.

The National Ignition Facility’s unique capabilities are unrivaled. There is only one place on Earth where we can create the extreme compressions of planetary cores and stellar interiors in the laboratory, study and observe them, and that is the world’s largest and most energetic laser, said Bruce Remington, NIF program leader. Discovery Science. . Building on the foundation of previous research at the NIF, this work is expanding the frontiers of laboratory astrophysics.

Reference: Observation of the onset of pressure-driven K-shell delocalization by T. Dppner, M. Bethkenhagen, D. Kraus, P. Neumayer, DA Chapman, B. Bachmann, RA Baggott, MP Bhme, L. Divol, RW Falcone , LB Fletcher, OL Landen, MJ MacDonald, AM Saunders, M. Schrner, PA Sterne, J. Vorberger, BBL Witte, A. Yi, R. Redmer, SH Glenzer, and DO Gericke, 24 May 2023, Nature.
DOI: 10.1038/s41586-023-05996-8

Led by Dppner, the LLNL research team included co-authors Benjamin Bachmann, Laurent Divol, Otto Landen, Michael MacDonald, Alison Saunders and Phil Sterne.

The groundbreaking research was the result of an international collaboration to develop Thomson X-ray scattering at NIF as part of LLNL’s Discovery Science programme. Collaborating scientists from the SLAC National Accelerator Laboratory, University of California Berkeley, University of Rostock (Germany),[{” attribute=””>University of Warwick (U.K.), GSI Helmholtz Center for Heavy Ion Research (Germany), Helmholtz-Zentrum Dresden-Rossendorf (Germany), University of Lyon (France), Los Alamos National Laboratory, Imperial College London (U.K.) and First Light Fusion Ltd. (U.K.).

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