Incremental Generation of Polycubes Driven by Geometric Quality
The proposed work consists of designing and developing a new algorithm for generating block-structured hexahedral meshes using a “Polycube”-typed strategy. Usually, these methods deform a geometric domain G to be discretized into a polycube P, i.e. a polyhedron whose all the faces are orthogonal to one of the X, Y, or Z axes. This polyhedron can be easily discretized using a hexahedral mesh, which is then transformed via the inverse deformation to pave G.
Unlike traditional approaches, which are based on constructing the deformation function, we focus on the inverse function by treating the pair (geometry G, polycube P) as input. Our goal is to compute the inverse function F that transforms P into G, and, depending on some local properties of F, modify P to provide a “better” mesh of G from a geometric point of view.
Generation of Curved Hexahedral-Dominant Block Structures Using Median Objects
The automatic generation of block-structured hexahedral meshes is a challenging problem, solved in practice through the use of dedicated interactive software, such as Magix3D, which is developed and maintained at CEA DAM. Using such software, a specialist engineer may spend several days creating the expected mesh.
The objective of this thesis is to help such an engineer to quickly sketch initial block structures with a hexahedral-dominant structure by relying on the medial object of the 3D geometric domain to be discretized. When considering CAD-type geometric domains, represented by their boundary, the medial object provides volumetric information intrinsic to the domains, which will guide the engineer in creating individual hexahedral blocks.
The objective of this thesis is twofold: first, to propose an efficient method for generating 3D medial objects; and second, to develop an algorithm and an associated interactive tool for creating predominantly hexahedral block structures.
Modeling of Heating and Radiative Properties of a Solid Target Irradiated by an Ultra-Intense Laser
Ultra-high-intensity (UHI) lasers can deliver petawatt (10^15 W) beams over ultra-short durations (from femtoseconds to picoseconds). The interaction of such a laser with a solid target allows the study of matter under extreme conditions, i.e., at solid densities brought to temperatures close to 10^7 K. These dense and hot plasmas have radiative properties (X-ray emission/absorption) that provide information about their state (temperature, etc.) and that can be measured experimentally. However, the physical mechanisms governing the heating and radiative properties are not yet fully understood.
The thesis aims to simulate the laser-target interaction using the CALDER code, a Particle-In-Cell (PIC) type code developed at the CEA. The PhD student will study the physics of isochoric heating with this code and seek to understand the predominant physical effects in these plasmas. He will model recent experiments aimed at studying these plasmas and conducted on academic laser facilities. Prospective simulations will help prepare future experiments, such as those planned on the PETAL laser at the CEA. Atomic physics codes may be used to refine the simulated X-ray spectra. The work will also include adapting CALDER to better model this physics. The results will contribute to a better understanding of the interaction between ultra-high-intensity laser-plasma and the radiative properties of irradiated solid targets.
Prompt fission neutron spectrum precision measurement in the spontaneous fission of 252Cf
The 252Cf(sf) prompt fission neutron spectrum (PFNS) is a reference neutron data that is widely used as a well known neutron flux for cross section measurements and neutron detector characterization. The current evaluation of the spectrum dates from the work of Mannhart in 1988. With the improvement of detection systems, the uncertainty on the spectrum has an increasingly significant impact on the uncertainty of new nuclear data measurements using it as a reference. Improving the precision on the 252Cf(sf) PFNS would therefore have a wide impact on the nuclear data community and improve the uncertainties on all data that were measured with respect to this reference spectrum. The thesis aims to measure again the 252Cf(sf) PFNS with a focus on the region below 1 MeV and the region above 8 MeV, where the uncertainties are greatest, using recoil proton detectors. The chosen candidate will have to actively participate to the design and construction of the setup, leading the technological choices through simulations, will participate to the experiment and do the data analysis. The work will then be presented in international conferences and peer-reviewed articles.
Neutron elastic and inelastic scattering measurement on 9Be with VENDETA
9Be plays a central role in fusion technology and in material testing reactor (MTR) as a neutron source moderator. However, existing nuclear data for neutron scattering on 9Be exhibit significant uncertainties, particularly in the 1.5-15 MeV energy range. In order to provide high quality data for improving nuclear reaction models and evaluated nuclear data libraries, an experiment to measure the elastic and inelastic scattering of neutrons on a 9Be target was proposed and accepted at the Neutrons For Science (NFS) facility. Neutrons will be measured using the recently developed VErsatile Neutron DETector Array (VENDETA), formed of high-resolution time-of-flight detectors which combine excellent neutron/gamma discrimination and efficiency down to 100 keV. The experiment will employ the quasi-monoenergetic neutron beams available at NFS from the p+7Li reaction, enabling a systematic investigation of scattering observables as a function of incident neutron energy.
The chosen candidate will lead the analysis to extract angular differential cross sections for both elastic and inelastic channels as a function of the incoming neutron energy. The data will have a direct impact on applications in nuclear energy and shielding design, while their interpretation in terms of partial decay width to the elastic and inelastic channels will improve our understanding of 9Be and 10Be nuclear structure.
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".
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).
New tools for assessment of parameters of rocket launches in near-real-time from the ionosphere
The ionosphere is the upper part of the atmosphere between 100 and 1,000 km. It is partially ionized and is sensitive to multiple solar and geophysical phenomena (earthquakes, tsunamis) and anthropogenic events (such as explosions and rocket launches). Rocket launches can cause two main types of disturbances in the ionosphere, which are detected in measurements of total electron content (TEC) between the ground and a GNSS satellite (e.g., GPS or Galileo):
- Localized depletion of plasma density caused by rocket exhaust gases. These depletion events have a very distinctive spatial waveform, similar to a plume centered along the rocket's trajectory.
We have extensive experience in studying the ionospheric response to natural hazards and human-induced events. We are currently developing new techniques for near real-time assessment of ionospheric disturbance parameters related to natural hazards. Our methods enable us to automatically detect ionospheric disturbances related to earthquakes in TEC data, locate them, and, in future work, estimate the magnitude of the earthquake. However, unlike earthquakes, which are localized to a single point, rocket propagation is a more spatially complex phenomenon. In addition, some rockets may use different types of fuel that will have different chemical impacts on the atmosphere/ionosphere. Consequently, depending on the scenario, they will produce different types of disturbances in the ionosphere.
The main objective of the thesis is to study the ionospheric disturbances generated by different types of rocket launches. This will make it possible to evaluate the parameters of these events and develop methods for automatically analyzing the ionospheric disturbances generated by rocket launches.
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.