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1. Disentangling the effects of non-adiabatic interactions upon ion self-diffusion within warm dense hydrogen

   https://doi.org/10.1098/rsta.2023.0034  

   Angermeier, William A. ; Scheiner, Brett S. ; Shaffer, Nathaniel R. ; White, Thomas G. ( August 2023 , Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   Warm dense matter is a material state in the region of parameter space connecting condensed matter to classical plasma physics. In this intermediate regime, we investigate the significance of non-adiabatic electron-ion interactions upon ion dynamics. To disentangle non-adiabatic from adiabatic electron-ion interactions, we compare the ion self-diffusion coefficient from the non-adiabatic electron force field computational model with an adiabatic, classical molecular dynamics simulation. A classical pair potential developed through a force-matching algorithm ensures the only difference between the models is due to the electronic inertia. We implement this new method to characterize non-adiabatic effects on the self-diffusion of warm dense hydrogen over a wide range of temperatures and densities. Ultimately we show that the impact of non-adiabatic effects is negligible for equilibrium ion dynamics in warm dense hydrogen. This article is part of the theme issue ‘Dynamic and transient processes in warm dense matter’. 

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2. Proton stopping measurements at low velocity in warm dense carbon

   https://doi.org/10.1038/s41467-022-30472-8  

   Malko, S. ; Cayzac, W. ; Ospina-Bohórquez, V. ; Bhutwala, K. ; Bailly-Grandvaux, M. ; McGuffey, C. ; Fedosejevs, R. ; Vaisseau, X. ; Tauschwitz, An. ; Apiñaniz, J. I. ; et al ( May 2022 , Nature Communications)

   Ion stopping in warm dense matter is a process of fundamental importance for the understanding of the properties of dense plasmas, the realization and the interpretation of experiments involving ion-beam-heated warm dense matter samples, and for inertial confinement fusion research. The theoretical description of the ion stopping power in warm dense matter is difficult notably due to electron coupling and degeneracy, and measurements are still largely missing. In particular, the low-velocity stopping range, that features the largest modelling uncertainties, remains virtually unexplored. Here, we report proton energy-loss measurements in warm dense plasma at unprecedented low projectile velocities. Our energy-loss data, combined with a precise target characterization based on plasma-emission measurements using two independent spectroscopy diagnostics, demonstrate a significant deviation of the stopping power from classical models in this regime. In particular, we show that our results are in closest agreement with recent first-principles simulations based on time-dependent density functional theory.

    

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3. Toward an integrated platform for characterizing laser-driven, isochorically heated plasmas with 1  µm spatial resolution

   https://doi.org/10.1364/AO.446182  

   Allen, C_H ; Oliver, M. ; Divol, L. ; Landen, O_L ; Ping, Y. ; Schölmerich, M. ; Wallace, R. ; Earley, R. ; Theobald, W. ; White, T_G ; et al ( March 2022 , Applied Optics)

   Warm dense matter is a region of phase space that is of high interest to multiple scientific communities ranging from astrophysics to inertial confinement fusion. Further understanding of the conditions and properties of this complex state of matter necessitates experimental benchmarking of the current theoretical models. We discuss the development of an x-ray radiography platform designed to measure warm dense matter transport properties at large laser facilities such as the OMEGA Laser Facility. Our platform, Fresnel diffractive radiography, allows for high spatial resolution imaging of isochorically heated targets, resulting in notable diffractive effects at sharp density gradients that are influenced by transport properties such as thermal conductivity. We discuss initial results, highlighting the capabilities of the platform in measuring diffractive features with micrometer-level spatial resolution.

    

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4. Spatiotemporal dynamics of fast electron heating in solid-density matter via XFEL

   https://doi.org/10.1038/s41467-024-51084-4  

   Sawada, H. ; Yabuuchi, T. ; Higashi, N. ; Iwasaki, T. ; Kawasaki, K. ; Maeda, Y. ; Izumi, T. ; Nakagawa, Y. ; Shigemori, K. ; Sakawa, Y. ; et al ( September 2024 , Nature Communications)

   High-intensity, short-pulse lasers are crucial for generating energetic electrons that produce high-energy-density (HED) states in matter, offering potential applications in igniting dense fusion fuels for fast ignition laser fusion. High-density targets heated by these electrons exhibit spatially non-uniform and highly transient conditions, which have been challenging to characterize due to limitations in diagnostics that provide simultaneous high spatial and temporal resolution. Here, we employ an X-ray Free Electron Laser (XFEL) to achieve spatiotemporally resolved measurements at sub-micron and femtosecond scales on a solid-density copper foil heated by laser-driven fast electrons. Our X-ray transmission imaging reveals the formation of a solid-density hot plasma localized to the laser spot size, surrounded by Fermi degenerate, warm dense matter within a picosecond, and the energy relaxation occurring within the hot plasma over tens of picoseconds. These results validate 2D particle-in-cell simulations incorporating atomic processes and provide insights into the energy transfer mechanisms beyond current simulation capabilities. This work significantly advances our understanding of rapid fast electron heating and energy relaxation in solid-density matter, serving as a key stepping stone towards efficient high-density plasma heating and furthering the fields of HED science and inertial fusion energy research using intense, short-pulse lasers.

    

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5. Upland and Riparian Surface Soil Processes in an Urban Creek with Native and Non-Native Vegetation after Fire

   https://doi.org/10.3390/fire5020032  

   Kinoshita, Alicia M. ; Becerra, Rey ; Miletić, Marta ; Mladenov, Natalie ( April 2022 , Fire)

   Wildfires can pose environmental challenges in urban watersheds by altering the physical and chemical properties of soil. Further, invasive plant species in urban riparian systems may exacerbate changes in geomorphological and soil processes after fires. This research focuses on the 2018 Del Cerro fire, which burned upland and riparian areas surrounding Alvarado Creek, a tributary to the San Diego River in California. The study site has dense and highly flammable non-native vegetation cover (primarily Arundo donax) localized in the stream banks and has primarily native vegetation on the hillslopes. We estimated the post-fire organic matter and particle distributions for six time points during water years 2019 and 2020 at two soil depths, 0–15 cm and 15–30 cm, in upland and riparian areas. We observed some of the largest decreases in organic matter and particle-size distribution after the first post-fire rainfall event and a general return to initial conditions over time. Seasonal soil patterns were related to rainfall and variability in vegetation distribution. The riparian soils had higher variability in organic matter content and particle-size distributions, which was attributed to the presence of Arundo donax. The particle-size distributions were different between upland and riparian soils, where the riparian soils were more poorly graded. Overall, the greatest change occurred in the medium sands, while the fine sands appeared to be impacted the longest, which is a result of decreased vegetation that stabilized the soils. This research provides a better understanding of upland and riparian soil processes in an urban and Mediterranean system that was disturbed by non-native vegetation and fire. 

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