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Simulations and diagnostics of high-energy-density plasmas and warm dense matter rely on models of material response properties, both static and dynamic (frequency-dependent). Here, we systematically investigate variations in dynamic electron–ion collision frequencies ν(ω) in warm dense matter using data from a self-consistent-field average-atom model. We show that including the full quantum density of states, strong collisions, and inelastic collisions lead to significant changes in ν(ω) ⁠. These changes result in red shifts and broadening of the plasmon peak in the dynamic structure factor, an effect observable in x-ray Thomson scattering spectra, and modify stopping powers around the Bragg peak. These changes improve the agreement of computationally efficient average-atom models with first-principles time-dependent density functional theory in warm dense aluminum, carbon, and deuterium.

Data for the pre-print "First-principles derivation and properties of density-functional average-atom models", https://arxiv.org/abs/2103.09928. Each data folder is named according to the corresponding figure in the paper. For any questions, please contact the authors.

In simulations of the warm dense matter regime, it is typical to use a combined finite-temperature Kohn-Sham density-functional theory (KS-DFT) and molecular dynamics approach. However, in KS-DFT, (i) scaling worsens with increasing temperature, and (ii) temperature dependence is usually neglected in the exchange-correlation (XC) functional. We present a derivation from first-principles which reduces the full many-body Hamiltonian to an average-atom model in the dilute gas limit, which significantly reduces the computational cost of the KS-DFT calculation. We also show preliminary results including a comparison of temperature-dependent and zero-temperature XC functionals.

We have studied the propagation of fast electrons through laser irradiated Ti foils by monitoring the emission of hard X-rays and K-�� radiation from bare foils and foils backed by a thick epoxy layer. Key observations include strong refluxing of electrons and divergence of the electron beam in the foil with evidence of magnetic field collimation. Our diagnostics have allowed us to estimate the fast electron temperature and fraction of laser energy converted to fast electrons. We have observed clear differences between the fast electron temperatures observed with bare and epoxy backed targets which may be due to the effects of refluxing.

Spatially resolved K-shell spectroscopy is used here to investigate the interaction of an ultrashort laser pulse (lambda = 800 nm, tau = 40 fs) with a Ti foil under intense irradiation (I lambda(2) = 2 x 10(18) W mu m(2) cm(-2)) and the following fast electron generation and transport into the target. The effect of laser pulse polarization (p, s, and circular) on the K alpha yield and line shape is probed. The radial structure of intensity and width of the lines, obtained by a discretized Abel deconvolution algorithm, suggests an annular distribution of both the hot electron propagation into the target and the target temperature. An accurate modeling of K alpha line shapes was performed, revealing temperature gradients, going from a few eV up to 15-20 eV, depending on the pulse polarization. Results are discussed in terms of mechanisms of hot electron generation and of their transport through the preplasma in front of the target.

We report experiments demonstrating enhanced coupling efficiencies employing high contrast laser irradiation of nano-fabricated conical targets. Peak temperatures near 200 eV are observed with modest laser energy (10 J), revealing similarities in hot electron localization and material heating to reduced mass targets. Collisional particle-in-cell simulations attribute this enhancement to self-generated resistive magnetic fields forming within the cone wall, which effectively confine the energetic electrons to heat a reduced volume at the cone tip.

International audience; no abstract

Positron emission tomography (PET) using 2-[18F]fluoro-2-deoxy-D-glucose (FDG) is a useful diagnostic tool for the detection of tumours. Using dynamic FDG PET, net metabolic clearance of FDG, K, can be calculated by Gjedde-Patlak analysis of the time course of the radioactivity concentrations in tissue and arterial blood. We examined whether time-activity curves (TACs) based on arterial blood sampling could be replaced by TACs obtained from the descending aorta in dynamic PET scans of patients with liver tumours. The study was performed in two parts, using data from dynamic liver scans with arterial blood sampling in human subjects: First, data from four patients with no liver tumours and five patients with liver tumours were used as a training group. Volumes of interest were defined in the descending aorta (aorta VOIs) by four different methods. K values were calculated based on the corresponding TACs and compared with those based on TACs of the arterial blood sample radioactivity concentrations. The aorta VOI which gave K values that were in best agreement with the K values based on the arterial blood sample measurements was called the AORTA-VOI. Use of the AORTA-VOI was subsequently tested in a test group of 19 tumour patients by comparing the K values from the AORTA-VOI with the K values based on the arterial blood sample measurements. The AORTA-VOI consisted of the sum of small regions of interest (ROIs) drawn in the centre of the aorta (approximately six pixels of 2.4x2.4 mm per transaxial slice of 3.1 mm thickness) in as many transaxial slices as possible (30-40 slices). There were no statistically significant differences between the two sets of K values. The ratio of K values in tumour tissue to K values in reference tissue was 2.1-9.7:1 (mean, 5.4:1) based on the AORTA TACs, and 2.1-8.4:1 (mean, 4.6:1) based on blood sample TACs (P>0.3). We conclude that arterial blood sampling can be replaced by the present AORTA-VOI in the calculation of the net metabolic clearance of FDG in dynamic PET studies of liver tumours in human subjects. Udgivelsesdato: 2000-Apr

Measurements of snow creep pressures from the centre section of a 3.4 m high and 15 m long avalanche-defence supporting structure located on a mountain in western Norway are presented. The site has a deep snow cover and an average slope angle of 25 degrees. The measurement configuration corresponds to plane-strain conditions and the data, along with measured snowpack parameters, allow comparison with simple theoretical predictions. The analysis shows that the average pressure on the structure may be calculated fairly accurately using linear, viscous modelling for the snow deformation. The maximum pressures proved to be higher than that provided by a linear model and this is considered characteristic of nonlinear material. The implications of these results for estimates of design loads are discussed.

Measurements of snow creep pressure on an avalanche-defence structure in western Norway are presented. Two different types of measurement methods are described and evaluated. Pressure data from four winters are correlated with measured snowpack properties. The results show that maximum and average pressures are strongly dependent on the product of density and snow depth as well as snowpack stiffness. The highest pressures were observed in spring prior to melting of the snowpack.