Observing the onset of pressure-driven K-shell delocalization

The gravitational pressure in many astrophysical objects exceeds one Gigabar (1 billion atmospheres) for a large part of their interior. At theses extreme conditions, matter is compressed to a state where the distance between nuclei becomes as small as the K-shell, containing the most tightly bound electrons, of light elements. These strong interactions of neighbouring particles modify existing bound states and, above a certain pressure, drive the electrons into a delocalised, conducting state. Both modified bound states and increased ionisation significantly affect the equation of state and radiation transport which, in turn, determine the evolution and structure of these objects. Still, our understanding of this transition is far from satisfying and, up to now, experimental data are sparse due to the extreme conditions required. Here, we report on an experiment that creates and diagnoses matter at pressures above 3 Gigabar by utilising the full capabilities of the National Ignition Facility where 184 laser beams were used to implode a beryllium shell, generating highly compressed states. Bright X-ray flashes enable precision radiography and X-ray Thomson scattering measurements revealing both the macroscopic and the microscopic state of the highly compressed beryllium. The inelastic scattering component shows clear signs of quantum degenerate electrons with the density reaching up to 30 times compression, and a temperature of around 2 million Kelvin. At the most extreme conditions, we also observe strongly reduced elastic scattering, which mainly originates from bound K-shell electrons. We attribute this reduction to the onset of delocalisation of the remaining K-shell electron. With this interpretation, the ion charge inferred from the scattering data agrees well with ab initio simulations, but it is significantly higher than widely used models predict. Our results yield a profound understanding of matter in the interior of brown and white dwarfs and will enhance their evolutionary models required to accurately determine the age of stellar populations. They are also imperative for improving the predictive capabilities supporting inertial confinement fusion experiments, ultimately paving the way to an abundant, carbon-free source of energy.