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

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

Measurements of the K-shell opacity of a solid-density magnesium plasma heated by an X-ray free electron laser

We present measurements of the spectrally-resolved X-rays emitted from solid-density magnesium targets of varying sub-μm thicknesses isochorically heated by an X-ray laser. The data exhibit a largely thickness-independent source function, allowing the extraction of a measure of the opacity to K-shell X-rays within well-defined regimes of electron density and temperature, extremely close to local thermodynamic equilibrium (LTE) conditions. The deduced opacities at the peak of the K-α transitions of the ions are consistent with those predicted by detailed atomic-kinetics calculations