This award supports a study of the fundamental properties of warm dense matter (WDM), a poorly understood plasma state of matter made of electrons and ions at high density and relatively low temperature. The focus of the study will be on WDM formed under the influence of unusually energetic “non-thermal” electrons. These electrons travel at nearly the speed of light, penetrate deep into dense plasma, and change how energy is transported and how matter is heated under extreme conditions. A better understanding of these processes is important for both natural and laboratory systems, including solar and stellar plasmas and the efforts to develop sources of fusion energy here on Earth. Progress has been limited because WDM is difficult to model and to investigate experimentally as the material enters a poorly understood state between an ideal plasma and condensed matter. Existing theories do not yet fully describe this regime, and precise experimental data remain limited. To address these limitations, non-thermal-electron-driven WDM will be created with a high-power, short-pulse laser and probed using ultrashort hard X-ray pulses from X-ray free-electron lasers (XFELs). The project will provide training opportunities for graduate and undergraduate students at cutting-edge XFEL facilities in the United States, Japan, and Germany. The goals of this project are to determine the transient plasma conditions in non-thermal-electron-driven warm dense matter through spatially and tem