short-range Order and Electron ultra-fast DYnamics in Phase transition from solid to laser-generated Warm Dense Matter – OEDYP

The study of the so-called Warm Dense Matter (WDM) is an emerging and challenging field that is at the crossroads of condensed matter and plasma physics. Here the density goes from one percent of the solid density up to 10 times its value; the temperature varies from 0.1 to 100 eV. In this regime, matter is mostly degenerate, strongly coupled and non-ideal. That gives rise to a physical complexity that ought to be a fertile ground for exciting scientific discoveries, as illustrated by the important number of recent international workshops and schools related to WDM. Warm Dense Matter science covers a wide range of physical phenomena from astrophysics and geophysics (stellar and planetary interiors), inertial confinement fusion (early stage initial conditions) up to industrial applications such as laser ablation, laser machining and laser damaging. Great uncertainties remain regarding the physics of these states of matter. From the theoretical point of view, there are many models using various assumptions about electronic and ionic structure. Those quantities are difficult to obtain in the WDM regime where self-consistent description for electronic and ionic structure is needed. Ab initio simulations are ideally suited for describing this regime precisely because no adjustable parameters or empirical inter-ionic potentials are needed. Moreover, ab initio simulations provide a self-consistent description of ionic and electronic structure. This technique has been successfully used to compute equation of states and electronic transport properties of various systems from solid to Warm Dense Matter. X-ray absorption is a new field of applications for this approach and experiments are needed to test its accuracy and the validity of various approximations used. As temperature increases up to 10 eV, the method is pushed to its intrinsic limits. The simplification in the numerical work brought by an approach based on dense plasma theory such as the modified hypernetted chain ' average atom (MHNC-AA) model also requires to be validated. Therefore, the key issue is to create those conditions in the laboratory to provide stringent tests for both the ab initio pseudo-potential calculations of X-ray absorption spectra and the atomic physics approach valid at higher temperatures. This project aims at producing homogeneous Warm Dense Plasmas (WDP) and studying the ultra-fast dynamics of their structural (short-range order expected) and electronic properties during the phase transition from solid to WDM. We propose to generate WDM by isochoric heating with an ultra-short laser pulse (femtosecond scale) and with a laser-produced proton burst (picosecond scale). Two complementary laser installations will be used: high repetition rate and moderate energy at CELIA for study of WDM in the 0.1 ' 1 eV range (laser heating); single-shot high energy laser at LULI (30 J) to extend this study up to the 1 ' 10 eV range (proton heating). Mainly, two time-resolved diagnostics will be used. The first one is the near-edge X-ray absorption spectroscopy to probe the short-range order of ions, and more generally the frontier between bound and free electronic states. The second one consists in the measurement of the phase shift and reflectivity of an optical beam to probe the free electron dynamic. This project takes advantage of complementary experimental, numerical and theoretical skills. It is the result of a discussion within the frame of the working group MHEDOC (Matière à Haute Densité d'Energie et Ondes de Choc, resp. M. Koenig) gathering the main players in the general field of High Energy Density science in the French community. The development of laser-based ultra-fast near-edge X-ray absorption spectroscopy to probe correlated matter has been also identified as a major topic in the GDR AppliX (Applications des nouvelles sources X, resp. A. Klisnick).