A research project undertaken by the Center for Advanced Systems Understanding (CASUS) at Helmholtz-Zentrum Dresden Rossendorf (HZDR) aims to overhaul an analytical tool used to evaluate laser fusion. The work is being funded by the European Union via the Just Transition Fund and by the Free State of Saxony. Scientists on the “X-ray laser optimization of laser fusion” project (Röntgenlaser-Optimierung der Laserfusion or ROLF), led by Tobias Dornheim of CASUS, are to develop machine-learning methods to enable a reliable theoretical description of warm dense matter (WDM). “One major issue of laser fusion is achieving stable compression with the laser blast,” said Dornheim. “It is imperative that the fuel capsule implodes as evenly as possible, i.e., without any instabilities, to ensure that as much fuel as possible is fused and a corresponding amount of usable energy is released. To achieve this, we must first improve our understanding of how WDM behaves.” The Helmholtz International Beamline for Extreme Fields (HIBEF) at the European X-ray Free-Electron Laser (XFEL) provides insights into the structure of materials and into very fast natural processes, such as those that take place in samples of WDM. Courtesy of HZDR Science Communication Lab. Warm dense matter, as can be found, for example, in the cores of planets and stars, is researched experimentally at large-scale research facilities such as the Helmholtz International Beamline for Extreme Fields at the European X-ray Free-Electron Laser (XFEL), in Schenefeld, Germany, and the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in the U.S. At these facilities, WDM can be generated for fractions of a second using powerful laser flashes. Dornheim's team is cooperating with both institutions. An important experimental method for analyzing laser fusion is x-ray Thomson scattering (XRTS), which is where the ROLF project comes in. Until now, the evaluation of the measured data was primarily based on a series of uncontrolled approximations. However, a year ago, the CASUS team demonstrated that precise data evaluation is possible without using any simulations or models. Dornheim and his team are using a fundamental mathematical method, namely the Laplace transform. Within ROLF, the researchers plan to create an open-source software package to make this evaluation method accessible to all laser fusion experts. Additionally, they intend to develop the method further to facilitate applications beyond model-free, high-precision temperature determination using XRTS measurements. In the future, it should also be possible to determine other relevant variables, such as the density or the degree of ionization of WDM. The team then intends to use the newly designed software to analyze existing XRTS data, for example, from the European XFEL, to develop and experimentally test new measurement methods for x-ray scattering. Once the XRTS diagnostics have a solid foundation, the findings derived from x-ray scattering will be incorporated into laser fusion simulations. “We assume that the parameters derived from these simulations will enable significantly better compression of the capsule and usher in a whole new generation of fusion experiments,” Dornheim said.
