Experimental station for laser-based picosecond time-resolved x-ray absorption near-edge spectroscopy

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Nonequilibrium warm dense matter investigated with laser–plasma-based XANES down to the femtosecond

International audienceThe use of laser–plasma-based x-ray sources is discussed, with a view to carrying out time-resolved x-ray absorption spectroscopy measurements, down to the femtosecond timescale. A review of recent experiments performed by our team is presented. They concern the study of the nonequilibrium transition of metals from solid to the warm dense regime, which imposes specific constraints (the sample being destroyed after each shot). Particular attention is paid to the description of experimental devices and methodologies. Two main types of x-ray sources are compared, respectively, based on the emission of a hot plasma, and on the betatron radiation from relativistic electrons accelerated by laser

Comparisons of x-ray sources generated from subpicosecond laser-plasma interaction on clusters and on solid targets

International audienc

Understanding XANES spectra of two-temperature warm dense copper using ab initio simulations

Using ab initio molecular-dynamics simulations combined with linear-response theory, we studied the x-ray absorption near-edge spectra (XANES) of a two-temperature dense copper plasma. As the temperature increases, XANES spectra exhibit a pre-edge structure balanced by a reduction of the absorption just behind the edge. By performing systematic simulations for various thermodynamic conditions, we establish a formulation to deduce the electronic temperature T e directly from the spectral integral of the pre-edge that can be used for various thermodynamic conditions encountered in a femtosecond heating experiment where thermal nonequilibrium and expanded states have to be considered. Time resolved x-ray absorption near-edge spectroscopy (TR-XANES) was recently extended to the warm dense matter (WDM) regime [1]. This highly transient physics has required specific development to get an appropriate time-resolution (picosecond or less) on XANES measurements. TR-XANES gives a complete picture by probing simultaneously the valence electrons and the atomic local arrangement modifications in WDM situations well beyond the melting. However, the physical interpretation of XANES spectra is not straightforward and a strong connection between theory and experiment is needed to extract information. This issue has been addressed using ab initio molecular dynamics (AIMD) simulations, providing a consistent description of both the electronic and ionic structures, together with x-ray absorption spectra calculation [2,3]. In the past decade, both theoretical and experimental XANES spectroscopy was used to get a deep understanding of the electronic structure modification of metals in the WDM. On aluminum, XANES modification during the solid-liquid-vapor phase transition was observed with picosecond time resolution [4]. With the support of AIMD simulations, this has shown the connection between electronic and ionic modifications. Comparison between measurements and calculations for molybdenum have demonstrated that XANES spectra can be simply interpreted in terms of electron DOS modification when solid molybdenum turns to WDM [5]. Besides the understanding of the phenomena involved, it is possible to have access to the time scales of phase transitions [4,6,7]. XANES spectra were also used to shed light on the DOS modification induced by laser shock compression in metals such as aluminum [8,9] and iron [10,11]. When WDM is transiently produced by femtosecond laser heating, matter faces nonequilibrium situations. It is a great * [email protected] scientific challenge to independently resolve the electron and ion dynamics. In principle, TR-XANES can address this physics, but it is necessary to disentangle the corresponding features in XANES spectra, which depend on the considered element. In some light metals such as Be [12] and Al [13], the electronic temperature can be retrieved directly from the slope of the absorption K edge. In most metals with a localized d band, the situation is more complex and this methodology cannot be applied. Recent experiments have been dedicated to femtosecond laser-heated warm dense copper on a synchrotron beamline [3,14]. Using 2-ps time-resolved x-ray spectroscopy, Cho et al. investigated the modification of XANES spectra near the L 2,3 edge. By direct comparison between measured spectra and spectra computed using AIMD, they retrieved the time evolution of the electron temperature and compare it to the behavior obtained using a two-temperature model. The purpose of the present paper is to go further in the calculations and analysis of warm dense copper using AIMD simulations. The interpretation of XANES spectra especially for transition metal is nontrivial and requires a careful analysis. The relation between the electronic temperature T e and pre-edge peak is reproduced. A quasilinear function is extracted to deduce absolute values of T e (up to 3 eV) from the spectral integration of the pre-edge. The impacts of ion temperature T i and density are carefully and independently studied. This shows the validity of this function in the various thermodynamic conditions encountered in a femtosecond heating experiment, including strong thermal nonequilibrium. This provides a practical T e diagnostic to analyze any XANES experiment without the need of additional AIMD calculations. It has been used in a recently published paper that revisits the electron-ion thermal equilibration dynamics on a tabletop setup, emphasizing the critical role of target expansion [15]. Other features are identified above the L edge, in close relation with the crystalline structure

Ultrafast Thermal Melting in Nonequilibrium Warm Dense Copper

International audienc

Electron-ion thermal equilibration dynamics in femtosecond heated warm dense copper

International audienc

Polycapillary lenses for Soft-X-ray transmission: Model, comparison with experiments and potential application for tomographic measurements in tokamaks

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Source X-UV pour la spectroscopie d'absorption en régime femtoseconde

Les processus dynamiques se produisant lors de transitions de phase ultra-rapide peuvent être déduits à partir de mesures de diffraction ou d'absorption de rayonnement X. Les lasers femtosecondes ont récemment été utilisés pour étudier la dynamiques de la matière au moyen d'une pompe optique et d'une sonde X : du rayonnement X K alpha produit par interaction laser plasma. Nous présentons nos plus récents résultats concernant le développement d'un sytème de spectroscopie d'absorption du rayonnement X (XAS) basée sur une source laser-plasma large bande dans la gamme 1-5 nm permettant d'atteindre une résolution temporelle femtoseconde. Le système est conçu pour sonder les dynamiques électroniques ayant lieu durant la transion de phase semiconducteur-métal du dyoxide de vanadium (VO2) lorsque celle-ci est initiée par une impulsion laser femtoseconde. Dans la présente expérience, un spectre large bande proche du seuil L du vanadium (511 eV) et du seuil K de l'oxygène (525 eV) du VO2 a été généré et mesuré avec un haut rapport signal sur bruit (100), une grande résolution spectrale ($\Delta {\rm E/E} = 4.2\times 10^{-3})$, et une résolution temporelle de 1,2 ps

Radiative cooling of an Al plasma in an AlTi mixture heated by an ultraintense laser pulse

International audienceThe rapid heating and cooling dynamics of thin solid foils driven by an ultraintense (∼ 10 18 Wcm −2) picosecond laser pulse has been experimentally studied through time-integrated and time-resolved x-ray emission spectroscopy as well as 2D x-ray imaging. Targets consisted of plastic foils with buried Al or Al 42 Ti 58 layers, with Al as a tracer to infer the plasma conditions. Our measurements indicate that the Al K-shell emission occurs over a shorter duration and from a narrower region in AlTi mixtures compared to pure Al samples. The experimental data are consistent with a simple model describing the fast heating and expansion of the foil target, and pinpoint the importance of radiative cooling in high-Z samples

Two-channel high-resolution quasi-monochromatic X-ray imager for Al and Ti plasma

International audienceHigh-resolution, high-sensitivity X-ray imaging is a real challenge in high-energy density plasma experiments. We present an improved design of the Fresnel ultra high-resolution imager instrument. Using an Ultra-High-Intensity (UHI) laser to generate hot and dense plasma in a small volume of an Al-Ti mixed target provides simultaneous imaging of both Al and Ti X-ray emission. Specifically, the Al Heβ (or Lyβ) and the Ti Heα lines are imaged with a resolution of (2.7 ± 0.3) μm and (5.5 ± 0.3) μm, respectively. It features two transmission Fresnel phase zone plates fabricated on the same substrate, each associated with a multilayer mirror for spectral selection. Their spatial resolution has been measured on the PTB synchrotron radiation facility laboratory at BESSY II and on the EQUINOX laser facility. Results obtained on an UHI experiment highlight the difference of emission zone sizes between Al and Ti lines and the versatility of this instrument