Studying astrophysical objects is a significant challenge due to the extreme conditions of high temperatures and immense densities. Similarly, these conditions are present during the implosion phase of inertial confinement fusion capsules. At the High Energy Density (HED) instrument of European XFEL, these conditions can be replicated using the powerful drivers provided by the HiBEF consortium (Helmholtz International Beamline for Extreme Fields). Combined with the brilliant X-ray flashes of the European XFEL, scientists can now study this exotic state of matter more closely than ever before.
Warm Dense Matter: A Unique Phenomenon
Typically, matter on Earth exists as solid, liquid, or gas. In space, plasma-a hot, ionized gas-is also found. However, at high temperatures and immense densities, such as in stars or during meteor impacts, matter cannot be easily classified as solid or plasma and is referred to as warm dense matter. This state of matter exists at temperatures from 5,000 to several hundred thousand Kelvin and pressures hundreds of thousands of times greater than atmospheric pressure.
Discovery Through Ultra-High-Resolution X-ray Thomson Scattering
A team led by Thomas Preston from the HED instrument at European XFEL has investigated the structure and properties of plasmons in ambient aluminum. Plasmons, which are collective oscillations of electrons, play a crucial role in the optical properties of metals, semiconductors, and warm dense matter.
X-ray Thomson scattering is a key method for investigating excitations in solids and warm dense matter. In this process, an X-ray photon is scattered in the material, losing energy and momentum by exciting a plasmon. Scientists can identify these energy-loss photons with a spectrometer.
Unlike previous studies that could only measure these excitations with low-resolution X-rays, Preston's team and researchers from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and CASUS have now recorded ultra-high-resolution X-ray Thomson scattering spectra with an energy resolution improved more than tenfold, achieving a resolution of less than one hundred millielectronvolt.
The team's findings, published in the journal Physical Review B and honored as an "Editor's Suggestion," reveal detailed insights into the structure and properties of plasmons in aluminum. "We realized that we could repurpose an existing setup that was designed to make even higher resolution measurements of vibrations in solids, which have energy losses much smaller than scattering from a plasmon, in fact only a few tens of millielectronvolts," explains Preston.
"Through a clever choice of our X-ray energy, we can instead measure energy losses up to 40 electronvolts with similar resolution. The accuracy of our measurements made it possible to eliminate long-standing discrepancies between simulations and experimental observations," describes Preston. Future work will use this method to benchmark simulations for plasmons at higher temperatures and compressions.
Future Prospects
"These exciting new capabilities at the European XFEL allow for unprecedented insights into the behavior of matter at extreme conditions," explains lead author Thomas Gawne from the Young Investigator Group "Frontiers of Computational Quantum Many-Body Theory" led by Tobias Dornheim. Dornheim recently received funding from the European Union's Just Transition Fund for a laser fusion project. He plans to make X-ray Thomson scattering evaluation accessible beyond a small group of many-body system simulation experts. If successful, laser physicists at European XFEL and elsewhere could design their experiments more effectively.
Thomas Preston emphasizes the potential for collaboration with theoretical groups, particularly with CASUS scientists: "The unique combination of cutting-edge theory at CASUS and HZDR as well as the state-of-the-art experiments at the HED instrument at European XFEL opens up completely new possibilities for science. The relationship between measurement and simulation is critical to be able to drive and inform exciting new experiments."
This joint effort is significant not only for studying warm dense matter in astrophysical objects but also for research into inertial confinement fusion,
a promising approach to power generation based on nuclear fusion reactions that could become a climate-friendly and virtually inexhaustible energy source.
Research Report:Ultrahigh resolution x-ray Thomson scattering measurements at the European X-ray Free Electron Laser
