Behavior of matter under isothermal dynamic compression: displacement of chemical reactivity; synthesis of new metastable materials; phase transition mechanisms.
The Diamond Anvil Cell equipped with piezoelectric actuators, or d-CED, is an innovative device that can generate dynamic compressions and decompressions over a wide range of pressure variation rates. The d-CED thus enables finely controlled dynamic stresses to be applied, with (de)compression rates that can vary over several orders of magnitude along isothermal paths. This paves the way for the creation of reference databases for the validation of microscopic mechanisms. Furthermore, the compression or decompression rates can be equated to ultra-fast heating or cooling rates of the sample, offering the possibility of exploring, in a highly controlled manner, certain phenomena still debated in the literature, such as the maximum stability of a solid beyond its melting point.
The objective of this thesis is to exploit the new possibilities offered by d-CED to demonstrate new phenomena or gain a detailed understanding of certain effects discussed in the literature, by performing ultra-fast temperature variations. A first application will consist of studying the nucleation kinetics of rare gases (Ar, Ne, Kr) as a function of the compression rate, and comparing them with recent measurements made at the XFEL in cryogenic jets. A second objective will be to study chemical changes, with an initial study focusing on the modification of the reactivity of nitromethane, a reference explosive. Another area of study will concern the synthesis of new molecular compounds from mixtures of dense molecular fluids (N2, H2, O2).
Applications using laser-accelerated relativistic electrons with PETAL
This PhD project focuses on the physics of plasmas generated by ultra-high-power and high-intensity lasers. The work will be carried out at the LMJ facility, using the PETAL laser which operates at intensities exceeding 10¹8 W·cm?² and enables the production of high-energy particles.
The main objective of the thesis is to investigate the generation and acceleration of relativistic electron beams in a gas jet. The potential applications of these beams will be assessed for electron–positron pair production and for electron-beam-based radiography.
The research will combine experimental and numerical approaches. The PhD candidate will take part in experimental campaigns scheduled for 2026–2027, including the implementation of diagnostics and data analysis. In parallel, Particle-In-Cell and Monte Carlo simulations will be performed to support the interpretation of the experimental results.
In a second phase, the thesis will contribute to the qualification of upgrades to the PETAL laser, focusing in particular on secondary sources of electrons, protons, and hard X-ray radiation generated by laser–matter interactions, within the framework of the PETAL-UPGRADE project.
Measurement of the speed of sound in H2 and He, key components of gas giant interiors
The goal of this thesis is to study hydrogen-helium mixtures in the fluid phase under high pressure and high temperature using Raman and Brillouin spectroscopy. The experiments will be conducted in a diamond anvil cell with laser heating, allowing exploration of a wide range of pressure and temperature conditions representative of the interiors of gas giant planets (1-300 GPa, 300-4000 K). Raman spectroscopy will be used to probe possible chemical changes occurring under extreme conditions, while Brillouin spectroscopy will provide access to the adiabatic sound velocity and the equations of state of these fluid mixtures. These data will be particularly useful for improving the modeling of Jupiter and Saturn’s interiors.
Study of radiative decay of the nucleus using a technic like Oslo-method
Radiative neutron capture is a nuclear reaction forming a compound nucleus which decays by emitting gamma-rays at excitation energy around the neutron binding energy. This well-known reaction which we known how to accurately measure its cross section at low incident neutron energies for most stable and few unstable nuclei close the stability valley, remains difficult to measure for exotic nuclei like fission fragments. Nuclear reaction models based essentially on stable nuclei, also struggle to provide reliable predictions of cross sections for these exotic nuclei. However, in the recent years, progress made related to the models and the measurements for the radiative capture show that significant improvements in including microscopic ingredients studies. These micoscopic ingredients: gamma strength function and nuclear level density, remain accesible to the experiment. These ingredients which respectively manage the way of how the gamma cascade occurs and the nuclear structure at high excitation energy can also be measured and calculated to be compared and suggest ways to improve the predictability of models. This kind of improvements have a direct impact for instance on the cross sections for these exotic nuclei which are produced in the stellar nucleosynthesis. The subject of thie thesis is to measure these quantities for a nucleus involved in the nucleosythesis using a new setup called SFyNCS.
microstructure informed kinetic model : application to solid explosives
When an explosive composition is subjected to an intense stress, such as a shock, the wave generated interacts with the microstructure and in particular with the defects it contains. Due to the nature of the defects, the energy can be localised, as when porosity is compacted, which can lead to the appearance of hot spots. Beyond a certain critical size, these hot spots grow as a result of the chemical decomposition of the explosive, and in some cases this can lead to the creation of a detonation wave. The role of these hot spots is therefore decisive in the initiation of solid explosives. The majority of macroscopic models used to study the shock-detonation transition (SDT) are phenomenological models calibrated on experiments (e.g. multi-strand gauge experiments) and therefore do not take into account the microstructural peculiarities specific to each explosive. It then becomes necessary to recalibrate a model for each composition, which limits any predictive capacity.
Microtomographic studies of real microstructures of explosive compositions have revealed that these deviate significantly from an average description based on a spherical pore. Through image segmentation, these microtomographs can provide essential ingredients for mesoscopic-scale simulation codes: these microstructures can be used directly as input for calculations or as a basis for generating virtual but realistic microstructures, thereby extending the accessible database given the experimental difficulties in generating this type of image in large numbers.
The computing power available today means that we can now envisage explicit simulations of realistic microstructures of explosive compositions. These simulations, in two or even three dimensions, will form the basis for the construction of a macroscopic kinetics model for modelling the shock-detonation transition. The results expected from this work are cross-disciplinary and can be transposed to all composite energetic materials. The effect of thermal or mechanical damage on the behaviour of an explosive or a solid propellant (vulnerability issues) could also benefit from this project. This more detailed knowledge of the role of microstructure (grain shape, porosity, etc.) could also improve filler manufacturing processes (e.g. ‘Very Insensitive’-RDX).
Hydrodynamic simulations of porous materials for ductile damage
The mechanical behavior of metallic materials under highly dynamical loading (schock) and especially their damage behavior is a topic of interest for the CEA-DAM. For tantalum, damage is ductile : by nucleation, growth and coalescence of voids within the material. Usual ductile damage models have been developed using the simplifying assumption that voids are isolated in the materials. However, recent studies by direct simulations explicitly describing a void population in the material (and experimental observations after failure) have shown the importance of void interaction for predicting ductile damage. Yet, the microscopical mechanisms of this interaction remain little known.
The objective of the PhD is to study the growth and coalescence phases of ductile damage through direct numerical simulations of a porous material undergoing dynamic loading. Hydrodynamic simulations, in which voids are explicitly meshed within a continuous matrix, will be used to study relevant scales of length and time. Monitoring the void population throughout the simulation will provide valuable information on the influence of void interaction during ductile damage. Firstly, the bulk behavior will be compared to the one predicted by usual models of isolated voids, showing the macroscopic effect of void interaction. Secondly, the evolution of the size distribution in the void population will be monitored. The last objective will be to understand microscopic void-to-void interaction. In order to take advantage of the wealth of simulation results, approaches based on artificial intelligence (neural networks on the graph associated with the pore population) will be used to learn the link between a void's neighborhood and its growth.
The doctoral student will have the opportunity to develop their skills in shock physics and mechanics, numerical simulations (with access to CEA-DAM supercomputers), and data science.
Coarse-grained simulations and viscoelastic behavior of polymeric photoactuators: a bottom-up strategy
Mechanical actuators, like muscles, are materials that can change their own macroscopic shape to perform mechanical work when submitted to an external stimulus, such as light irradiation. The resulting photoactuators (PA) are based on a variety of photoactive materials including gels, crystals, liquid crystal elastomers (LCE) or polymer films forming polymeric PA (PPA). This project focuses on PPAs, usually made of elastomers in which photoactive molecules are inserted. To optimize the PPA properties, a precise understanding of the behavior of these materials at all scales is necessary. PPAs are viscoelastic by nature and therefore the continuous scale modeling of their behavior requires the knowledge of some specific mechanical properties, like the time-dependent relaxation moduli G(t) and K(t). At the supramolecular scale, these relaxation moduli can be obtained by Molecular Dynamics (MD) simulations using the Green-Kubo relation [3]. However, for these materials, the G(t) and K(t) timescales far exceed the accessible timescales of MD (on the order of thousands of seconds vs. microseconds). This PhD work has thus two main objectives to reduce this gap :(i) temperature-accelerated dynamics, (ii) anisotropic coarse-grained (CG) simulations.
Multiscale modelling of twinning in tin
Twinning is a displacive deformation mechanism characterized by a continuous deformation of the material. Although widely studied for other industrial materials such as titanium alloys, this inelastic mechanism remains poorly understood and incompletely modeled for complex crystallographic structures. However, due to the reduced number of symmetries in these structures, dislocation slip is insufficient to accommodate deformation in certain loading directions, requiring the activation of twinning. This is the case for tin, which has a tetragonal structure. In particular, twinning contributes significantly to the mechanical response of tin at high strain rates and low temperatures. At intermediate temperatures and strain rates, a competition between dislocation plasticity and twinning plasticity can occur, making it crucial to describe the coupling between these two phenomena. Proposing a better description of this coupling will shed new light on the experimental data available at CEA DAM. The objective of the thesis is to develop a multiscale approach, from molecular dynamics to continuum mechanics, validated by experiments, to converge on a model that describes the behavior of tin over a wide range of temperatures and strain rates.
Metallurgy under extreme conditions
The microstructure-properties relationship is a core concept of metallurgy, and of materials engineering in general. For instance, the hardness of quenched steels emerges from their martensitic microstructure, induced by a phase change in iron. Here we are concerned about metallurgy under extreme conditions in which metallic samples undergo pressurizations in the 100 GPa (=1 million atmospheres) range, making it possible to synthesise new crystalline phases with potentially interesting properties (hardness, magnetism, etc.).
Studied systems will include tin, then indium and cobalt. The three of them exhibit a rich polymorphism under high pressure and temperature. We will seek to elucidate the role of defects such as twinning and plasticity on the mechanism and kinetics of these transitions. This will be done by comparing experimental observations with microstructure predictions obtained through mesoscopic simulation. High pressure/ high temperature will be mainly generated by laser-heated diamond anvil cells, and characterisation tools will include in situ X-ray imaging by diffraction and tomography, as well as electron microscopy. The X-ray sources used will be synchrotron sources and the European free-electron X-ray laser.
Development of theoretical Raman spectra with application on minerals from the surface of Mars
As we push the boundaries of space exploration with new missions to nearby planets, improving our investigation tools is crucial. Mars rovers have revealed a surface mineralogy unlike anything on Earth, shaped by the planet’s former hydrosphere followed by an extended dry and cold environment. For example, this favors the formation of perchlorates, or mixed silicate–salts glassy phases — minerals that are difficult to synthesize and stabilize on Earth but remain surprisingly stable on Mars. Recent Raman spectrometry data confirms their presence, highlighting an opportunity for deeper research. Understanding these minerals could offer new insights into Martian chemistry and planetary evolution.
Here we want to calculate the theoretical Raman spectra of perchlorates and other Martian minerals using the density functional perturbation theory (DFPT) as implemented in the ABINIT package. We want to obtain not only the position and the intensity of the peaks, but also the peak widths. They are necessary to correctly identify between similar spectra and to assess, by integration, the actual intensity of the peaks, which are directly comparable to experimental values on the field. These allow us to choose the representative peaks that can be used in identification and to analyze the displacement patterns associated with the vibrations. The results of our simulations will be compared and interpreted in the light of measurements performed by the current rovers on the surface of Mars.
For this, we need to implement several third- and fourth-order derivatives of the energy. This will be done as a series of DFPT terms, where the perturbations can be atomic displacements or electric fields. We will use a combination of the 2n+1 theorem and finite differences. The implementation will be done within the "Projector Augmented-Wave" approach (PAW) to DFT. The entire development effort will be integrated into the ABINIT package and made available to the entire community. ABINIT (www.abinit.org) is a highly visible international collaborative project for ab initio simulations based on DFT and DFPT. The computed spectra will be made available to the community via the WURM database.
The successful candidate will be co-advised between the IPGP (Paris) and the CEA (Bruyères-le-Chatel, S of Paris) groups. IPGP is a world-renowned geosciences research institute founded in 1921, associated with the CNRS, a component of the Université Paris Cité and employing more than 500 people. The group of Razvan Caracas is highly active in computational mineralogy, study of matter at extreme conditions, and planetology. The Quantum simulation of Matter group at CEA Bruyères-le-Chatel led by Marc Torrent is a main developer of the ABINIT package, highly active in density functional theory, projector augmented-wave, and high-performance computing.