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

Abstract

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 freefree 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 x-ray to ultraviolet wavelengths. We extend our calculations across the melt to the warm-dense matter regime, and find good agreement with advanced plasma models based on inverse bremsstrahlung at temperatures above 10 eV.

Time-Resolved XUV Opacity Measurements of Warm Dense Aluminum

“…The first is electronic, due to thermal broadening of the plasmon peak [13], increases in the many-body screening length and a reduction in the electron degeneracy as the electrons heat [14]. The second is due to the change in the ion structure factor as the ions heat and the system melts [10,13]. The electron contribution is readily observed on femtosecond timescales in our data.…”

“…3(c), with 1σ (68%) and 2σ (95%) confidence intervals. We also show the theoretical predictions from time-dependent DFT calculations based on the work of Hollebon et al for both an equilibrated system [10] and for a system with ions at 300 K, and from Iglesias [14]. Having understood how a single XUV pulse heats the sample, we now turn our attention to the pump-probe measurements.…”

“…At solid density, particle correlations, degeneracy and many-body effects all play an important role in determining how materials interact with light [5,6], invalidating the classical Coulomb-logarithm-based inverse bremsstrahlung picture widely applied in plasma physics modeling [7][8][9]. At the same time, the need to treat finite temperatures makes detailed approaches from condensed matter theory challenging to implement in practice, and increasingly unfeasible for temperatures exceeding of order 10 eV [10].…”

“…Because of its convenient electronic structure and the near-free behavior of its valence electrons, aluminum irradiated at extreme ultraviolet (XUV) wavelengths is an ideal test bed to study free-free light-matter interactions at high electron densities. In this context, recent attempts have been made aimed at understanding historical discrepancies in the predicted and measured absorption coefficients in ground state Al at XUV wavelengths [10], with further investigations pushing well into the warm dense matter regime [11,12]. Current theoretical predictions generally agree that the free-free absorption cross section should initially increase as the temperature of the system is raised to the Fermi temperature [10,[13][14][15], before starting to fall off at higher temperatures according to the T −3=2 temperature dependence of inverse bremsstrahlung (IB) theory [16].…”

Measurements of free-free absorption in warm dense aluminium

“…The measured diffraction profile must be matched to the output from the diffraction modelling code, this requires optimising seven different parameters simultaneously, while allowing for the fact that these parameters are often not independent from each other. The parameters that the code can vary are the presample beam profile, sample width, sample position, XUV transmission and phase shift, source size 8 and sample edge smoothness 9 . It was determined that Bayesian optimisation would be a suitable method for doing so-this is a commonly used method for finding minima of highly non-linear functions that are time-consuming to evaluate.…”