Iron and iron alloys on extreme conditions probed with X-ray Diagnostics on FEL and intense laser facilities – IronFEL

Warm Dense Matter (WDM), the regime lying between condensed matter and plasma physics, is of great interest in many contexts including planetary formation, planetary interiors, and inertial confinement fusion research. These states of matter are characterized by solid densities and temperatures up to hundreds of eV (several million degrees K). The prediction of behaviour in such regimes, where neither condensed matter nor plasma models are applicable, is particularly difficult. In this context, creating and probing warm dense matter in the laboratory has recently received much attention for the in situ measurement of structural and physical properties of matter at such extreme conditions.
In planetology, the characterisation of the key physical properties of WDM is of the utmost importance. Matter under extreme high pressure (up to several tens of Mbar) and high temperature (up to a few eV) constitutes the core of our planet yet has hardly been studied, and the available models of its physical properties are in urgent need of validation.
Iron alloys are major components of the cores of Earth and Earth-like planets. The nature and abundance of the core impurities are actively debated, and are intimately related to critical open questions for geosciences: the Earth’s bulk composition, core-mantle differentiation and its implications for the mantle composition and mineralogy, the nature of the core mantle boundary, and the generation of the Earth's magnetic field. Understanding and modelling warm dense iron and iron alloys is a direct step towards understanding the Earth’s magnetic field, and important in the study of Earth-like exoplanets. More precisely, the phase transitions and melting temperature of iron alloys at the solid-liquid Outer-core Boundary are fundamental to understanding such issues. Despite its importance, the melting line is the subject of long controversy. Until now, advances in this field have been limited by difficulty of recreating such states of matter in the laboratory for study.
Determination of the iron melting curve at Mbar pressures is essential for the description of dynamics of the Earth’s core. A significant step towards this goal has been made by compressing matter up to 10 Mbar pressure with innovative ultra high energy laser techniques. Isochoric heating with fs laser pulses allows the investigation of high temperature phase transitions at solid density. Because energy is deposited via electrons, a transient non-equilibrated system is initially created. The pathways for thermal relaxation and atomic reorganisation are as yet unknown due to a lack of any direct experimental validation.
The recent development of Free Electron Lasers (FEL) has succeeded in coupling exciting aspects of synchrotron radiation with intense laser properties, giving ultrafast, high intensity laser beams in the X-ray and XUV energy range. Such properties are of great interest for WDM studies, as X-rays allow homogeneous and volumetric isochoric heating. Also, in the last five years the FEL community has demonstrated how such X-ray pulses are effective as a diagnostic probe, giving new insights into physics and numerous high ranking publications. In addition, the FEL facilities are now equipped with complementary optical lasers opening many possibilities for dynamic studies with pump – probe techniques.
This project will focus on warm and dense iron and iron alloys. We propose to study the high-density regime using laser compression, and the solid density high temperature regime with isochoric heating with fs FEL pulses. This second aspect will develop the possibility of performing sub-100 fs time resolved pump – probe experiments to access the ultrafast transient WDM states, and thus validate current models. An essential part of our project will consist of developing X-ray diagnostics such as X-ray absorption spectroscopy and X-ray scattering coupled to ultrafast pump – probe schemas.