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

这个项目将探索温暖的致密物质的基本性质，这是一种温度相对较低但由电子和离子组成的密度非常高的等离子体。 特别是，所谓的“非热”电子，一个小的，但重要的人口的高能电子，不与其余的等离子体处于热平衡的属性，将进行实验和计算研究。 温密物质（WDM）的温度和密度范围太冷、太密，无法应用传统的等离子体物理理论，而其温度又太高，无法用凝聚态物理技术进行处理，因此对WDM的理论和数值模拟具有挑战性。在实验中，WDM的高密度和低温也阻止了传统等离子体诊断的使用。 为了克服这些挑战，该项目将使用X射线自由电子激光器（XFEL）提供的超短硬X射线脉冲来研究用高功率短脉冲激光器产生和表征非热电子驱动的温暖致密物质。XFEL的硬X射线脉冲穿透稠密的等离子体，以提供其内部的条件，而超短脉冲能够捕获随时间变化的等离子体状态的快照。 短脉冲激光驱动WDM的超快时间分辨测量可以帮助开发连接凝聚态物理和等离子体物理的理论和计算模型，并揭示非热电子在与天体物理和聚变等离子体相关的未探索WDM制度中的作用。 该项目还将包括在日本和德国的最先进的XFEL设施中培训研究生和本科生，该项目的目标是利用基于XFEL的诊断技术推断非热电子驱动的温暖致密物质的条件，并获得关于物质加热、平衡和冷却阶段的时间分辨信息。 飞秒相对论强度激光与固体靶相互作用可以产生能量在几十keV到几MeV之间的非热电子束。电子的传输等容地加热并将固体转化为温暖的致密物质，而密度保持不变。使用~10 fs的XFEL脉冲，被加热的目标的瞬时条件，特别是电子温度和电离状态，可以诊断与X射线汤姆逊散射和X射线透射成像与~100 fs的时间分辨率。随后，可以通过改变XFEL和光学激光器之间的时间延迟来构建等离子体条件的时间历程。实验结果将与二维粒子在细胞等离子体模拟和基于密度泛函理论的量子电子模拟进行比较。 该项目由物理部和激励竞争性研究的既定计划（EPSCoR）共同资助。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.