Dielectronic satellite emission from a solid-density Mg plasma: Relationship to models of ionization potential depression

Producción CientíficaWe report on experiments where solid-density Mg plasmas are created by heating with the focused output of the Linac Coherent Light Source x-ray free-electron laser. We study the K-shell emission from the helium and lithium-like ions using Bragg crystal spectroscopy. Observation of the dielectronic satellites in lithium-like ions confirms that the M-shell electrons appear bound for these high charge states. An analysis of the intensity of these satellites indicates that when modeled with an atomic-kinetics code, the ionization potential depression model employed needs to produce depressions for these ions which lie between those predicted by the well known Stewart-Pyatt and Ecker-Kroll models. These results are largely consistent with recent density functional theory calculations.This work has received support from the Spanish Ministry of Science and Innovation under Research Grants No. PID2019-108764RB-I00 and No. PID2022-137632OB-I00

Time-resolved XUV Opacity Measurements of Warm-Dense Aluminium

The free-free opacity in plasmas is fundamental to our understanding of energy transport in stellar interiors and for inertial confinement fusion research. However, theoretical predictions in the challenging dense plasma regime are conflicting and there is a dearth of accurate experimental data to allow for direct model validation. Here we present time-resolved transmission measurements in solid-density Al heated by an XUV free-electron laser. We use a novel functional optimization approach to extract the temperature-dependent absorption coefficient directly from an oversampled pool of single-shot measurements, and find a pronounced enhancement of the opacity as the plasma is heated to temperatures of order the Fermi energy. Plasma heating and opacity-enhancement is observed on ultrafast time scales, within the duration of the femtosecond XUV pulse. We attribute further rises in the opacity on ps timescales to melt and the formation of warm-dense matter

Density functional theory studies of solid density plasmas

In warm dense matter (WDM) and dense plasma physics, Density Functional Theory (DFT) has become a standard approach over the past many years for simulating transport properties, equations of state, interpreting experimental measurements and many other applications. The main chapters, two to four, of this thesis cover original work by the author on three topics: excited state pseudopotentials, time-dependent DFT (TDDFT) and many-body theory. For an excited state pseudopotential, a specific excited ion core configuration is generated by externally imposing a set of occupation numbers in the same way as can be rigorously done for a non-interacting electron system. In chapter 2 results and a physical argument are presented seeking to justify this process when generating excited configurations of bound electron systems. Those electrons that might be considered as `free' within a plasma exhibit not only single-particle excitations, as one might label with a set of single-particle occupation numbers, but also significant collective behaviour i.e. plasmons. TDDFT linear-response theory is applied in chapter 3 as a rigorous means of modelling the general dynamic and wavelength-dependent response properties, and fluctuations, for quantum plasma systems. With help from the Langreth rules a fluctuation-dissipation relation for the electron dynamic structure factor is derived. Finally, the dynamic structure factor is computed for compressed Beryllium and CH plasma, with favourable comparison to experimental data and simulations by previous authors. In chapter four the free-free opacity of solid density Al plasma is considered. Both the tensor nature of the dielectric function, in the form of local field corrections, and an accurate description of bound-state properties, in the form of correct binding energies, are required to reproduce experimental room temperature measurements. Commonly used exchange-correlation functionals are insufficient for predicting the energy gap between bound states and the continuum in a linear response theory context. To this end, the author has implemented and demonstrated finite-temperature many-body quasi-particle calculations in the Abinit code. These many-body calculations are expensive however they are a potential future source of accurate theoretical predictions, covering a wide range of plasma conditions to which other, perhaps simpler models might be benchmarked.</p

Modelling of warm dense hydrogen via explicit real time electron dynamics: dynamic structure factors

We present two methods for computing the dynamic structure factor for warm dense hydrogen without invoking either the Born-Oppenheimer approximation or the Chihara decomposition, by employing a wave packet description which resolves the electron dynamics during ion evolution. Firstly, a semi-classical method is discussed, which is corrected based on known quantum constraints, and secondly, a direct computation of the density response function within the molecular dynamics. The wave packet models are compared to PIMC and DFT-MD for the static and lowfrequency behaviour. For the high-frequency behaviour the models recover the expected behaviour in the limits of small and large momentum transfers and show the characteristic flattening of the plasmon dispersion for intermediate momentum transfers due to interactions, in agreement with commonly used models for X-ray Thomson scattering. By modelling the electrons and ions on an equal footing, both the ion and free electron part of the spectrum can now be treated within a single framework where we simultaneously resolve the ion-acoustic and plasmon mode, with a self-consistent description of collisions and screening

Time-resolved XUV opacity measurements of warm-dense aluminium

The free-free opacity in plasmas is fundamental to our understanding of energy transport in stellar interiors and for inertial confinement fusion research. However, theoretical predictions in the challenging dense plasma regime are conflicting and there is a dearth of accurate experimental data to allow for direct model validation. Here we present time-resolved transmission measurements in solid-density Al heated by an XUV free-electron laser. We use a novel functional optimization approach to extract the temperature-dependent absorption coefficient directly from an oversampled pool of single-shot measurements, and find a pronounced enhancement of the opacity as the plasma is heated to temperatures of order of the Fermi energy. Plasma heating and opacity enhancement are observed on ultrafast timescales, within the duration of the femtosecond XUV pulse. We attribute further rises in the opacity on ps timescales to melt and the formation of warm dense matter

Ab initio simulations and measurements of the free-free opacity in aluminum

The free-free opacity in dense systems is a property that both tests our fundamental understanding of correlated many-body systems, and is needed to understand the radiative properties of high energy-density plasmas. Despite its importance, predictive calculations of the free-free opacity remain challenging even in the condensed matter phase for simple metals. Here we show how the free-free opacity can be modelled at finite-temperatures via time-dependent density functional theory, and illustrate the importance of including local field corrections, core polarization, and self-energy corrections. Our calculations for ground-state Al are shown to agree well with experimental opacity measurements performed on the Artemis laser facility across a wide range of extreme ultraviolet wavelengths. We extend our calculations across the melt to the warm-dense matter regime, finding good agreement with advanced plasma models based on inverse bremsstrahlung at temperatures above 10 eV

Ab initio simulations and measurements of the free-free opacity in aluminum

The free-free opacity in dense systems is a property that both tests our fundamental understanding of correlated many-body systems, and is needed to understand the radiative properties of high energy-density plasmas. Despite its importance, predictive calculations of the free-free opacity remain challenging even in the condensed matter phase for simple metals. Here we show how the free-free opacity can be modelled at finite-temperatures via time-dependent density functional theory, and illustrate the importance of including local field corrections, core polarization, and self-energy corrections. Our calculations for ground-state Al are shown to agree well with experimental opacity measurements performed on the Artemis laser facility across a wide range of extreme ultraviolet wavelengths. We extend our calculations across the melt to the warm-dense matter regime, finding good agreement with advanced plasma models based on inverse bremsstrahlung at temperatures above 10 eV