Study of Nonthermal Electron Driven Warm Dense Plasmas Using X-Ray Free Electron Lasers

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This project will explore fundamental properties of warm dense matter, a relatively low temperature but very high density plasma composed of electron and ions. In particular, the properties of so-called "non-thermal" electrons, a small but important population of high-energy electrons that are not in thermal equilibrium with the rest of the plasma, will be studied experimentally and computationally. Theoretical and numerical modeling of warm dense matter (WDM) is challenging because temperature and density ranges of WDM are too cold and dense to apply traditional plasma physics theory, but its temperature is too high to be treated with condensed matter physics techniques. In experiments, the high density and low temperature of WDM also prevent the use of conventional plasma diagnostics. To overcome these challenges, this project will use ultrashort hard x-ray pulses delivered by X-ray Free Electron Lasers (XFEL) to study the creation and characterization of nonthermal electron driven warm dense matter with a high-power short-pulse laser. The XFEL’s hard x-ray pulses penetrate through dense plasma to provide the conditions of its interior, while the ultrashort pulses enable capturing snapshots of time-evolving plasma states. Ultrafast time-resolved measurements of short-pulse laser-driven WDM could both help develop theoretical and computational models connecting condensed matter physics and plasma physics, and reveal the role of nonthermal electrons in unexplored WDM regimes relevant to astrophysical and fusion plasmas. This project will also include training of graduate and undergraduate students at state-of-the-art XFEL facilities in Japan and Germany.The goals of this project are to infer conditions of nonthermal electron driven warm dense matter with XFEL-based diagnostic techniques, and to obtain time-resolved information on heating, equilibration and cooling phases of matter. The interaction of a femtosecond relativistic intensity laser with a solid target can produce a beam of nonthermal electrons with the energies ranging between 10's of keV to several MeV. The transport of the electrons isochorically heats and transforms the solid into warm dense matter, while the density remains constant. Using ~10 fs XFEL pulses, instantaneous conditions of the heated target, specifically electron temperatures and ionization states, can be diagnosed with X-ray Thomson Scattering and x-ray transmission imaging with a ~100 fs temporal resolution. Subsequently, time histories of the plasma conditions can be constructed by varying a time delay between the XFEL and the optical laser. Experimental results will be compared with two-dimensional particle-in-cell plasma simulations and density functional theory-based quantum electron simulations. This project is jointly funded by Division of Physics and the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.