Vizualisation of 3D NURBS meshes
For several years, CEA DAM has been developing Magix3D, a mesh generation software, and Themys, a visualization software, both based on the VTK (Visualization ToolKit) rendering library. The latter provides numerous algorithms for visualizing numerical simulation results in an HPC context.
The objective of the proposed PhD thesis will be to enrich the VTK library with mechanisms for visualizing and manipulating curved meshes represented by NURBS elements of arbitrary degree. Two key elements will need to be taken into account: the ability to measure the fidelity of the visualized data in relation to the initial data, and the visualization of large data sets representative of the studies conducted at CEA DAM. To this end, volume rendering algorithms will need to be studied and extended to handle NURBS elements.
Study of corrosion mechanisms of ceramics in molten chlorides salts
CEA Valduc operates processes involving molten salts. These salts, based on chloride compounds, can exhibit corrosive properties, particularly in the presence of impurities that lead to oxygen contamination. This results in the degradation of materials used in these processes. The study proposed here aims to understand these degradation mechanisms in order to identify the materials that best meet the needs of CEA Valduc. Beyond the specific requirements of CEA Valduc, this study also fits more broadly within ongoing research efforts to understand and mitigate corrosion in high-temperature molten salts environments, a major technological challenge for advanced modular reactors (AMRs).
The proposed work aims to study and compare various refractory materials in contact with chloride salts. Oxide materials (MgO, Y2O3, Ta2O5) and carbides (TaC) will be investigated in contact with CaCl2, NaCl, and KCl salts. The solubility of these materials in different molten salt media will be measured. The ultimate goal is to evaluate the behavior of these materials under aggressive conditions and to understand the mechanisms of their degradation.
Several studies have highlighted the predominant role of the material microstructure in relation to chemical durability. Initial characterization of the materials will be carried out using the facilities of Institute Jean Lamour (SEM/TEM, XRD). A thermodynamic study using the FactSage software will also be performed to predict material behavior and possible chemical reactions. The core of the thesis will consist of corrosion tests. Solubility constants of these different materials in chloride salts will be measured, followed by an investigation of phenomena occurring at the salt/material interface on sintered samples. Literature underscores the crucial influence of oxygen content on the corrosive nature of molten salts. Precise control and in situ measurement of oxygen levels is therefore critical for this work. To this end, the PhD candidate will have access to CEA’s facilities that enable work under inert atmosphere and analytical electrochemical measurements. Post-corrosion elemental analyses (ICP-AES/MS, UV-Vis spectroscopy) of the salts will be combined with microstructural characterizations of the samples to propose corrosion mechanisms for each material.
All experiments will take place at the CEA Valduc site, with occasional travel required to the IJL facilities in Nancy.
Study of the laser-driven ion acceleration in the relativistically induced transparency regime using ultra-thin foils and cryogenic targets
Laser-driven ion acceleration (LDIA) presents a compact and cost-effective alternative to traditional particle accelerators. Recent developments have enabled proton energies up to 160 MeV using ultra-thin foil targets irradiated by ultra-intense laser pulses, exploiting relativistic transparency regimes. This regime occurs when the laser pulse penetrates a near-critical plasma, generated by tailoring target thickness to the laser parameters, enabling multi-stage acceleration and enhancing proton energies without the need for contrast-enhancing techniques like plasma mirrors. This PhD project aims to further optimize proton acceleration in the transparency regime, with the goal of achieving 200 MeV energies using high-repetition-rate laser systems.
The first phase involves 3D Particle-In-Cell (PIC) simulations with Smilei, focusing on the sensitivity of laser-target interaction to temporal laser profiles for robust acceleration. The second phase investigates cryogenic hydrogen ribbon targets, developed by CEA, as an alternative to solid foils. These targets are near-critical in density, tunable in thickness, and compatible with high-repetition-rate operation, while providing mono-species proton beams. Experimental work will be conducted in collaboration with LULI and CEA, with preparations for experiments at the Apollon facility.
Quantum simulation of atomic nulei
Atomic nuclei constitute strongly correlated quantum many-body systems governed by the strong interaction of QCD. The nuclear shell model, which diagonalizes the Hamiltonian in a basis whose dimension grows exponentially with the number of nucleons, represents a well-established approach for describing their structure. However, this combinatorial explosion confines classical high-performance computing to a restricted fraction of the nuclear chart.
Quantum computers offer a promising alternative through their natural ability to manipulate exponentially large Hilbert spaces. Although we remain in the NISQ era with its noisy qubits, they could revolutionize shell model applications.
This thesis aims to develop a comprehensive approach for quantum simulation of complex nuclear systems. A crucial first milestone involves creating a software interface that integrates nuclear structure data (nucleonic orbitals, nuclear interactions) with quantum computing platforms, thereby facilitating future applications in nuclear physics.
The project explores two classes of algorithms: variational and non-variational approaches. For the former, the expressivity of quantum ansätze will be systematically analyzed, particularly in the context of symmetry breaking and restoration. Variational Quantum Eigensolvers (VQE), especially promising for Hamiltonian-based systems, will be implemented with emphasis on the ADAPT-VQE technique tailored to the nuclear many-body problem.
A major challenge lies in accessing excited states, which are as crucial as the ground state in nuclear structure, while VQE primarily focuses on the latter. The thesis will therefore develop quantum algorithms dedicated to excited states, testing various methods: Hilbert space expansion (Quantum Krylov), response function techniques (quantum equations of motion), and phase estimation-based methods. The ultimate objective is to identify the most suitable approaches in terms of scalability and noise resilience for applications with realistic nuclear Hamiltonians.
Machine Learning-Accelerated Electron Density Calculations
Density Functional Theory (DFT) in the Kohn-Sham formalism is one of the most widespread methods for simulating microscopic properties in solid-state physics and chemistry. Its main advantage lies in its ability to strike a favorable balance between accuracy and computational cost. The continuous evolution of increasingly efficient numerical techniques has constantly broadened the scope of its applicability.
Among these techniques that can be associated with DFT, machine learning is being used more and more. Today, a very common application consists in producing potentials capable of predicting interactions between atoms using supervised learning models, relying on properties computed by DFT.
The objective of the project proposed as part of this thesis is to use machine learning techniques at a deeper level, notably to predict the electronic density in crystals or molecules. Compared to predicting properties such as forces between atoms, calculating the electronic density presents certain challenges: the electronic density is high-dimensional since it must be calculated throughout all space; its characteristics vary strongly from one material to another (metals, insulators, charge transfer, etc.). Ultimately, this can represent a significant computational cost. There are several options to reduce the dimensionality of the electronic density, such as computing projections or using localization functions.
The final goal of this project is to be able to predict, with the highest possible accuracy, the electronic density, in order to use it as a prediction or as a starting point for calculations of electron-specific properties (magnetism, band structure, for example).
In a first stage, the candidate will be able to implement methods recently proposed in the literature; in a second part of the thesis, it will then be necessary to propose new ideas. Finally, the implemented method will be used to accelerate the prediction of properties of large systems involving charge transfers, such as defect migration in crystals.
The nonresonant streaming instability in turbulent plasmas
The magnetic turbulence prevalent in many astrophysical systems, such as the solar wind and supernova remnants, plays a crucial role in accelerating high-energy particles, particularly within collisionless shock waves. By trapping particles near the shock front, this turbulence facilitates their energy gain through repeated crossings between the upstream and downstream regions – a process known as Fermi acceleration, believed to be the origin of cosmic rays.
It happens that the turbulence surrounding supernova remnants is likely generated by the cosmic rays themselves via plasma instabilities as they stream ahead of the shock. In the specific case of a shock wave propagating parallel to the ambient magnetic field, the dominant instability is thought to be the non-resonant streaming instability, or Bell's instability, which acts to amplify the preexisting turbulence.
The objective of this PhD is to build a comprehensive analytical model of this instability within a turbulent plasma, and to validate its predictions against advanced numerical simulations.
Electronic effects dans les cascades de collisions dans le GaN
In radiation environments like space and nuclear plants, microelectronic devices are subject to intense flux of particles degrading the devices by damaging the materials they are made of. Particles collide with atoms of the semi-conducting materials, ejecting them for their lattice site. Those displaced atoms also collide and set in motion a new generation of atoms, and so on, leading to a cascade of collisions which creates defects in the material. Moreover, primary or secondary particles (created following interaction with a neutron for example) also specifically interact with electrons of the target material, and lose kinetic energy in doing so by promoting electrons to higher energy bands. This aspect is called electronic stopping. Simulations of collision cascades must therefore describe both nuclei-nuclei collisions and electronic stopping effects.
The preferred method for collision cascades simulations at the atomic scale is Molecular Dynamics (MD). However, electronic effects are not included in this method as electrons are not taken into account explicitly. To circumvent this issue, additional modules have to be employed on top of MD to model electronic effects in a collision cascade. The state-of-the-art regarding electronic stopping simulation of a projectile in a target material is the real time - time dependent density functional theory (RT-TDDFT). The purpose of this thesis is to combine MD and RT-TDDFT to perform collision cascades simulations in GaN and study the influence of electronic effects. In addition to skills common to all thesis, the candidate will develop very specific skills in different atomic scale simulation methods, solid state physics, particle-matter interactions, linux environment and programming.
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.
Multiscale modeling of the magnetic response of heterogeneous material
The spectral dependence of the permeability of magnetic materials, whether in composite or dense materials, remains a complex issue due to the different scales of the phenomena involved. Approximate analytical models are often used to describe the frequency response of magnetic materials, particularly to improve their performance in areas such as power electronics. Recent results have shown that micro-magnetism codes can now predict the response of a system of coupled nanoparticles or a particle representing the volume of the materials in question. This thesis aims to use these tools to improve existing analytical models. An inclusion immersed in an effective field will be the paradigm from which the domain structure and the spectral response of the particle will be calculated using a micro-magnetism code. The materials studied include spherical particles or those with a high aspect ratio (magnetic oxides, ferromagnetic petals) at varying concentrations, ranging from dilute media to dense materials. This work will identify pathways to optimize the microstructure of materials for better performance in applications such as power electronics and microwave components. To this end, CEA provides a scientific computing environment with access to HPC resources, as well as facilities for sample preparation and static and dynamic magnetic characterization. At the end of this work, the candidate will have gained a solid understanding of the microstructure-property relationships described by a numerical approach applied to magnetic materials. More generally, this approach is expanding in the field of materials to improve their properties in various fields, under the designation "materials by design".