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11 juin 2025 - Energétique, aérodynamique et intensification des jets d’arcs de coupage plasma / Soutenance de thèse de Frederic CAMY-PEYRET
Doctorant : Frederic CAMY-PEYRET 

Date et lieu : le 11 juin à 14h00 ; amphi N°3 - Centrale Méditerranée

Résumé : Le procédé de découpe plasma des matériaux conducteurs d’électricité, en particulier des métaux, est l’un des trois grands procédés de découpe thermique des tôles, avec le laser et l’oxycoupage. Ce procédé s’est progressivement répandu dans l’industrie depuis les années 70, et est aujourd’hui incontournable dans les activités de construction et de fabrication métallique. Les améliorations de cette technologie ont historiquement été apportées par les équipementiers industriels en utilisant une approche très empirique et technologique, probablement en raison de la faible intensité en R&D fondamentale du secteur, et surtout de la complexité multi-physique de la phénoménologie des jets plasmas utilisés en découpe des métaux. Ceux-ci peuvent en effet être décrits par un écoulement trans-sonique sous-détendu dans une tuyère sonique de taille millimétrique, au col de laquelle une colonne plasma fortement ionisée est chauffée au-delà de 20000 K par le passage d’un courant électrique.
La complexité de l’objet d’étude réside dans les nombreux effets physiques, souvent eux-mêmes difficiles à appréhender isolément, qui peuvent a priori contribuer à déterminer la structure du jet plasma : propriétés thermodynamiques et coefficients de transport du plasma, géométrie de la tuyère et son aérodynamique, pression, intensité du vortex, turbulence, transferts radiatifs à haute température, couplage électrostatique et électromagnétique entre le passage du courant et le plasma, phénomènes aux électrodes, …
Depuis les années 2000, et en collaboration avec le monde académique, certains acteurs industriels, ont engagé des approches plus fondamentales et scientifiques permettant de mieux comprendre et décrire ces objets technologiques, démarche à laquelle l’auteur a contribué depuis 25 ans.
Après un résumé de la carrière et des travaux de recherche menés ou supervisés par l’auteur dans les domaines connexes de la mécanique des fluides, de la combustion, des matériaux, et des procédés, la problématique abordée dans ce manuscrit de thèse portera donc sur l’étude de l’énergétique et de la phénoménologie des jets de plasmas d’arc transféré utilisés pour la découpe. Ce choix est d’abord celui du fil rouge scientifique et technologique le plus dense de mon expérience de chercheur industriel, et aussi un sujet dont la nature multi-physique a beaucoup bénéficié de l’expérience acquise dans d’autres spécialités.
Nous introduirons les technologies et les procédés, nous couvrirons les méthodes expérimentales et de simulation numérique utilisées, nous étudierons les échelles énergétiques à l’œuvre en découpe plasma en comparaison au procédé laser concurrent, et détaillerons les mécanismes d’apport et de distribution de l’énergie à la tôle. Nous présenterons les avancées dans la compréhension de la constriction de la colonne plasma à travers la description du fonctionnement de la tuyère jusqu’à son col sonique, avant de poursuivre vers l’aval en analysant la structure du jet plasma sous-détendu entre la tuyère et la tôle. Enfin, nous conclurons par l’étude de l’influence de la cohérence spatiale de la densité de puissance du plasma sur la précision de coupe et les mécanismes d’amélioration de celle-ci.

Mots clés : procédé de coupage plasma, plasma d’arc électrique, efficacité énergétique, découpe des aciers, jet plasma supersonique sous-détendu, adaptation aérodynamique, thermodynamique des plasmas thermiques, transferts thermiques à haut flux, intensification de la densité de puissance.

Jury
Stéphane PELLERIN  / Professeur, Université d'Orléans / Rapporteur
Luc VERVISCH  / Professeur, INSA de Rouen / Rapporteur
Françoise BATAILLE  / Professeur, Université de Perpignan Via Domitia / Examinatrice
Philippe ROBIN-JOUAN  / Fellow Expert, GE Vernova / Examinateur
Sergey GAVRILYUK  / Professeur, Aix-Marseille Université / Président du jury
Eric SERRE  / DR CNRS, M2P2 / Examinateur
Pierre BOIVIN  / CR CNRS, M2P2 / Directeur de thèse
Pierre FRETON  / Professeur, Université de Toulouse / Co-directeur de thèse
Bernard LABEGORRE  / Senior Expert, Air Liquide / Membre invité
13 décembre 2024 - A 3D electromagnetic model in SOLEDGE3X: Application to turbulent simulations of tokamak edge plasma / Raffael Düll PhD Defense
Doctorant : Raffael Düll

Date : Vendredi 13 Décembre 2024 à 15:00 ; CEA Cadarache, bât. 506, 13108 Saint-Paul-Lez-Durance

Abstract: In the tokamak edge, steep gradients and magnetic curvature generate large-scale turbulent structures that transport plasma particles from the hot core, where fusion occurs at around 10 keV, to the much colder Scrape-Off-Layer (SOL), where magnetic field lines intersect the physical wall. Turbulence reduces plasma confinement and defines the region where strong heat fluxes impact the divertor. The drift-reduced fluid code SOLEDGE3X, developed by CEA/IRFM in collaboration with Aix-Marseille University, has proven effective in simulating electrostatic resistive drift-wave turbulence in realistic tokamak geometries. However, both experimental and numerical results have shown that electromagnetic effects significantly impact drift-wave dynamics, and thus, edge plasma turbulence. This thesis introduces a new electromagnetic model in SOLEDGE3X for the vorticity equation, incorporating magnetic induction, electromagnetic flutter, and electron inertia. Magnetic induction accounts for the time variation of the parallel magnetic vector potential Apara in the definition of the parallel electric field, and Apara is related to the parallel current density Jpara via Ampère's law. Fluctuations in the magnetic field, termed flutter, are added at first order and are assumed to be small compared to the equilibrium field. Electron inertia, represented by a finite electron mass in Ohm's law, is necessary to constrain shear Alfvén wave speeds to physical values. The new fields Apara and Jpara are integrated into the flux-surface-aligned FVM framework on a poloidally and toroidally staggered grid. Flutter affects the parallel transport equations and gradients in Ohm's law, and its implementation required special care to account for the new radial component of the parallel direction. To handle timesteps larger than Alfvénic, electron thermal, or electron-ion collision times, the corresponding inductive, inertial, and resistive effects are solved implicitly in a coupled 3D system for the potentials Phi and Apara. The model was verified with manufactured solutions and validated on a linear slab case, which demonstrated the expected transition from Alfvén to thermal electron waves as the perpendicular wavenumber increased. Flutter contributes minimally to cross-field transport but affects the non-adiabatic potential response to density fluctuations in Ohm's law. Simulations in slab, circular (limited), and X-point (diverted) geometries consistently show that electron inertia and magnetic induction destabilize drift-wave turbulence, while flutter stabilizes it in both the linear and nonlinear phases. On open field lines, magnetic induction reduces the sensitivity of turbulent structures to sheath effects, promoting further turbulence spreading in the SOL. Numerically, electron inertia significantly improves the condition number of the vorticity system, especially in hot plasmas with low resistivity, providing a factor-four speedup even in electrostatic scenarios. However, adding flutter degrades code performance, as it requires solving implicit 3D systems for viscosity and heat diffusion problems that were previously treated as uncoupled 2D systems on each flux surface. As an extension to this work, perturbations to the magnetic equilibrium were externally imposed in a transport mode simulation to study heat deposition in a non-axisymmetric magnetic configuration with ripple on WEST. 

Jury:
Directeur de these    M. Eric SERRE CNRS M2P2
Rapporteur            M. Benjamin DUDSON Lawrence Livermore National Laboratory
Rapporteur            M. Boniface NKONGA Université Côté d'Azur
Examinateur            M. Paolo RICCI EPFL
Président            M. Eric NARDON CEA Cadarache
Examinateur            Mme Daniela GRASSO Politecnico de Torino
Co-encadrant de these M. Hugo BUFFERAND CEA Cadarache
11 décembre 2024 - Advanced numerical modelling of transport in tokamak plasma and confrontation to experiments / Ivan Kudashev PhD Defense
Doctorant : Ivan Kudashev

Date : jeudi 12/12 à 14h00, amphi N°3 ; Centrale Méditerranée ; 38 rue Joliot-Curie, 13013 Marseille

Abstract : To control heat deposition on the Plasma-Facing Components (PFCs) of current tokamaks and future reactors, a major effort is underway to develop fluid codes that can model turbulent transport in the plasma edge. Understanding the underlying physical processes is one of the key challenges in magnetic fusion research, especially with the upcoming launch of the International Thermonuclear Experimental Reactor (ITER). Despite significant advances in plasma fluid simulations, several challenges in the modelling remain. These include limitations related to fixed magnetic equilibrium, simplified boundary conditions to model plasma wall interactions, time-consuming neutral transport simulations, crude perpendicular turbulent transport models, and limited coupling between the plasma core and the Scrape-Off Layer (SOL). These limitations hinder full-discharge simulations, restricting analysis to a few snapshots of relatively stable plasma phases, which still carry significant uncertainties. This thesis contributes to the ongoing development of the SolEdge-Hybridized Discontinuous Galerkin (HDG) code, a magnetic equilibrium-free fluid plasma solver. The research focuses on improving the physical completeness of the code and enhancing its ability for experimental validation. A detailed overview of the SolEdge-HDG code is provided, highlighting the initial models and assumptions. The implementation of the HDG method is discussed, which allows the use of high-order meshes that are not aligned with the magnetic field, enabling precise descriptions of tokamak wall geometries. Key developments in the SolEdge-HDG suite include the creation of synthetic diagnostics (bolometer and visible range cameras), improving the code's ability to compare simulations with experimental data. These comparisons revealed shortcomings in the initial physical models, which have been addressed in this thesis. Improvements include a more consistent neutral fluid model, which is crucial for understanding tokamak fueling, as well as the introduction of new heat sources and a self-consistent heuristic perpendicular turbulent transport fluid model. The enhanced SolEdge-HDG code successfully captures key plasma regimes, such as sheath-limited, high-recycling, and detached states. A detailed study is conducted on the plasma’s response to variations in gas puffing, demonstrating its impact on tungsten sputtering. The extension of the bolometer system in WEST for more accurate measurements is also explored, as well as potential applications of visible camera diagnostics. The thesis demonstrates the first application of the self-consistent turbulent model to simulate a full cross-section during the ramp-up phase of a WEST discharge. The simulation results show qualitative agreement with experimental data. The interaction between evolving plasma equilibrium and the turbulent model is also discussed, with emphasis on its effect on divertor heat load predictions. Applications of the fully upgraded SolEdge-HDG model are further explored for steady-state plasmas with additional heating. Special attention is given to the turbulent model’s response to different heating methods and the process of ion-electron energy equilibration. Finally, the thesis illustrates the application of SolEdge-HDG and synthetic diagnostics to improve tokamak design. It examines the impact of reflections on bolometer signals and evaluates various approaches for tomographic inversions of plasma radiation. An application to the final design of the ITER bolometer system is also presented. This work demonstrates the expanded capabilities given by magnetic-equilibrium-free solver for tokamak design and operation. The integration of synthetic diagnostics not only allows to confront simulations to the experiments, but also sheds light to the model’s weaknesses. While some limitations remain, the code suite is already capable to solve key operational design challenges. 

Jury:
Directeur de these          M. Eric SERRE CNRS, M2P2
Co-encadrant de these  Mme Anna GLASSER CNRS, M2P2
Président                  Mme Pascale HENNEQUIN CNRS, LPP
Examinateur                  M. Alberto LOARTE ITER
Rapporteur                  Mme Eleonora VIEZZER University of Sevilla
Rapporteur                  M. Jeremy LORE ORNL
11 décembre 2024 - Mechanisms of interactions between organic and mineral matter (phosphates) during hydrothermal liquefaction of residual biomass: application to digestate from anaerobic digestion / Antonello Tangredi PhD Defense
Doctorant : Antonello TANGREDI

Date : Wednesday December 11, 2024 at 9.30am in the Cerege Amphitheatre at the Technopôle de l'Arbois-Méditerranée

Abstract : Phosphorus (P) is an essential nutrient for global food production, but intensive agriculture disrupts its natural cycle, increasing reliance on non-renewable sources. A sustainable alternative is recovering P from renewable organic waste streams such as sewage sludge and digestate. This PhD thesis investigates the hydrothermal treatment of sewage sludge to explore P conversion and speciation, aiming to valorize the mineral phase as fertilizer and the organic phase as bio-oil. Two types of sludge, differing in solids content and composition, were sampled from a wastewater treatment plant in southern France. The sludge was treated in a batch reactor at temperatures varying from 250 to 350 °C for 5 to 45 min. Products were centrifuged into a solid pellet and process water, followed by characterization of their physicochemical properties. Results show that temperature and treatment duration significantly impact by-product characteristics, including their P content and speciation. Higher temperatures and longer times improve bio-oil yield. Treatments at 250-300 °C promote organic P mineralization and increase soluble phosphate concentration in the process water, while treatments at 350 °C lead to greater P recovery yield in the solid pellet (> 90%). Higher calcium and iron contents of sludge improve orthophosphate precipitation, and calcium oxide addition enhances P recovery in the solid pellet as calcium phosphate. This research provides a framework for sustainable P recovery, suggesting hydrothermally treated pellets as potential slow[1]release fertilizers. Future work should include detailed bio-oil characterization to better assess the energy recovery potential.

Keywords : residual biomass, phosphorus recovery, hydrothermal liquefaction, sewage sludge, circular economy.

Jury:
Benedetta DE CAPRARIIS,       Reviewer, Associate professor, La Sapienza University of Rome
Véronique DELUCHAT,             Reviewer, Professor, Limoges University
Anthony DUFOUR,                   Examiner, Senior scientist, CNRS LRGP
Jean-Henry FERRASSE,           Examiner, Professor, Aix-Marseille University
Mathieu GAUTIER,                   Examiner, Professor, INSA Lyon
Elsa WEISS-HORTALA,            Examiner, Assistant professor, IMT Mines Albi
Olivier BOUTIN,                       Thesis supervisor, Professor, Aix-Marseille University
Cristian BARCA,                       Thesis co-supervisor, Associate professor, Aix-Marseille University
10 décembre 2024 - Numerical Investigation of the Hemodynamics of Aortic Valves and Their Surgical Treatments with a Focus on Fluid Structure Interaction Mechanisms / Tom Fringand PhD Defense
Doctorant : Tom FRINGAND

Date : 10 December 2024 at 1:30 pm in amphi N°3 - Centrale Méditerranée, 38 Rue Frédéric Joliot Curie, Marseille

Abstract: Cardiac pathologies are the leading cause of mortality worldwide. The heart is composed of four distinct chambers separated by valves that ensure unidirectional blood flow from one chamber to another. There are many categories of heart diseases that can affect various parts of the organ, but among them, aortic valve dysfunction has a significant weight. Aortic valve dysfunction is diagnosed as difficulties in opening and/or closing, which exhausts the patient’s heart and leads to poor hemodynamics. As a result, the aortic valve is the most commonly replaced part of the heart with prostheses that aim to replicate the characteristics of a healthy valve as closely as possible. These prostheses, mainly derived from porcine or bovine sources, have improved patients’ quality of life, however none of them are capable of fully restoring life expectancy to a level comparable to that of the general population. In this context, new valve replacement solutions are being explored. The Ozaki procedure appears to be a promising candidate, but its development remains limited for now. This technique avoids the use of external implants by using the patient’s own pericardial tissue to construct a new valve. This procedure is still relatively recent and offers many advantages, from the use of tissue already recognized by the body, to the design of the procedure itself. Nevertheless, several questions remain open about the Ozaki valve capacity to provide high-quality blood flow that lasts over time. This concern is understandable, given that the new aortic valve obtained after an Ozaki procedure has a significantly different shape compared to a healthy native valve or any of the prostheses available on the market. The objective of this thesis is to compare and quantify, from a biomechanical perspective, the differences in behavior and flow produced between an Ozaki-type valve and a healthy native valve. This comparison provides insights into the fundamental properties of this solution relatively to a healthy case in terms of durability and performance. To meet the expectations of surgeons, a bioprosthesis will also be included in the comparison to identify the advantages and disadvantages of the Ozaki valve compared to what is currently considered the reference in terms of replacement solutions. To carry out these three comparisons, a fully numerical and state-of-the-art approach has been developed, based on the Lattice BoltzmannMethod for blood flow simulation, with a finite element method to calculate the deformations experienced by the valve. These two methods are coupled using an immersed boundary formulation and an explicit time-stepping method with stabilization. The simulations were performed with a patient-specific objective, using an innovative process on geometries derived from clinical CT-scans with the use of Landmarks and Non-Uniformal Rational B-Splines (NURBS) interpolation. In terms of geometry, the Ozaki procedure nearly doubles (1.92 times) the coaptation surface of the leaflets compared to the native healthy aortic valve and increases the coaptation height by a factor of 3.7, impacting the behavior. The results of the fluid-structure interaction simulations reveal similar dynamics between the valves, but with the emergence of flutter in the Ozaki valve and higher flow velocities and wall shear stresses for the bioprosthesis. This thesis globally observes a biomechanical superiority of the Ozaki procedure compared to the bioprosthesis, suggesting the relevance of this new solution and its future development.

Key words: Numerical simulation, Fluid–structure interaction, Aortic valve, Ozaki procedure, Bioprosthesis, Hemodynamics.

Jury:
Lyes KADEM  -          Pr. Université de Concordia  -           Reviewer
Dominik OBRIST  -    Pr. Université de Berne  -                  Reviewer
Olivier BOUCHOT  -  PU-PH Université de Dijon  -             Examiner 
Morgane EVIN  -       Chargée de recherche au LBA, Université Gustave Eiffel  -   Examiner
Franck NICOUD  -     Pr. Université de Montpellier  -           President of the jury
Julien FAVIER -          Pr. Aix Marseille Université  -               Thesis director 
Loïc MACE  -             PU-PH. Aix Marseille Université  -        Thesis co-director
Archive des thèses soutenues au M2P2

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