The study of warm dense matter (WDM) is of interest to various fields ranging from laser processes, to astrophysics and fusion. The description of matter in this regime is highly challenging due to partial disorder, correlated ions, electron degeneracy and strong coupling between electron structure and ion bounding. It requires complex simulations at the atomic scale (DFT). Different laser techniques now make it possible to transiently reach these extreme conditions in a controlled manner in the laboratory. The recent development of time-resolved X-ray diagnostics is particularly relevant to access the microscopic scale of simulations. In particular, the X-ray Absorption Near-Edge Spectroscopy (XANES) brings invaluable information on both electron and atomic structures.
In this seminar, I will present experimental work on the development of laser-based time-resolved XANES, and on the necessary adaptations of this well-known condensed matter diagnostic to study WDM. It has been successfully used to study different regimes, from laser-shock compression (ns) to ultrafast laser heating (fs).
I will then focus on a series of recent experiments devoted to the investigation of non-equilibrium warm dense copper heated by a femtosecond laser pulse. Different laser-based devices have been used to resolve the dynamics of the electron-ion thermal equilibration (a few ps) , the ultrafast thermal melting (~ ps)  and more recently the non-ballistic electron energy transport (< 1 ps). The latter took advantage of the demonstration of femtosecond-XANES with an X-ray betatron source .
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 B. Mahieu et al., Nature Communications 9, 3276 (2018)
 B. Kettle et al., Physical Review Letters 123, 254801 (2019)