Scientists have used lasers to uncover the secrets of stellar material at extreme pressure in the laboratory for the first time

A research team, including scientists from the University of Rostock and the Helmholtz Center Dresden-Rosendorf (HZDR), conducted laboratory experiments at Lawrence Livermore National Laboratory (LLNL) to provide new insights into the complex process of pressure-driven ionization in giants of planets and stars. Their research, published May 24 in the journal Nature, reveals the material properties and behavior of matter under extreme compression and has important implications for astrophysics and nuclear fusion research.

The international team of researchers used the world's largest and highest-energy laser, the National Ignition Facility (NIF), to generate the extreme conditions required for pressure-driven ionization. The team used 184 laser beams to heat the inside of the cavity, converting laser energy into X-rays that heated a 2-millimeter-diameter beryllium shell placed in the center. As the outer part of the shell expands rapidly due to heating, the interior accelerates inward to reach temperatures of about 2 million Kelvin and pressures of up to 3 billion atmospheres, producing a small chunk of material found in a dwarf star for a few nanoseconds in the laboratory.

X-ray Thomson scattering is used to probe highly compressed beryllium samples up to 30 times the density of their ambient solids to infer their density, temperature, and electronic structure. The results show that after intense heating and compression, at least three-quarters of the electrons in beryllium are converted to a conductive state. In addition, the study found unexpectedly weak elastic scattering, indicating a reduced degree of localization of the remaining electrons.

The material inside giant planets and some relatively cool stars is highly compressed by the weight of the layers above. At such high pressures generated by high compression, the proximity of atomic nuclei leads to interactions between the electron-bound states of neighboring ions and eventually causes them to fully ionize. While ionization in burning stars is primarily determined by temperature, pressure-driven ionization predominates in cooler objects.

"The degree of ionization of the atoms inside a star is critical to the efficiency with which energy is transferred from the center to the outside by radiation. If this restriction is too strict, it becomes turbulent in the celestial body, similar to a pan, "explains Dominik Kraus, who was still working in California when the project began and is now a physics professor and HZDR team leader at the University of Rostock. "If it's too turbulent, life as we know it may not be able to travel in close orbits around small stars."

Despite its importance to the structure and evolution of celestial bodies, pressure ionization as a pathway for highly ionized matter is not well understood in theory. In addition, the LLNL physicist who led the project and is an alumnus of Tilo Doppner at the University of Rostock, said that the extreme states of matter required are difficult to create and study in the lab. "Our work opens up new ways to study and model the behavior of matter under extreme compression. Ionization in a dense plasma is a key parameter because it affects the equation of state, thermodynamic properties, and the transport of radiation through opacity."

The study also has important implications for NIF's inertial confinement fusion experiments, where X-ray absorption and compressibility are key parameters for optimizing high-performance fusion experiments. Doppner added that a comprehensive understanding of pressure - and temperature-driven ionization is essential to simulate compressed materials and, eventually, to develop abundant carbon-free energy sources through laser-driven nuclear fusion.

"The pioneering results have also been made possible by the hard work of PhD students at the University of Rostock and Helmholtz-Zentrum Dresden-Rossendorf, some of whom have completed research at NIF in California," Professor of Physics Ronald Redmer reports to the University of Rostock and is an expert in the theoretical description of dense astrophysical plasmas. "The evaluation of the results of the complex experimental setup and the modeling of the state of the plasma under study are very complex and require a lot of computational power. It has taken years to reach the current understanding of the experimental data."

The researchers also hope to learn more about substances under billions of atmospheric pressures at a facility in Germany. With the help of the Helmholtz International Extreme Field Beam (HIBEF) at the Schonefeld European XFEL, scientists at the University of Rostock and the Helmholtz Center in Dresden hope to achieve similar conditions on a much smaller scale. This will enable more experiments than NIF is currently likely to conduct.

Source: Laser Network