Description du sujet de thèse

This PhD thesis aims to deepen our understanding of impurity dynamics in key operational regimes of fusion plasmas. It will contribute to more predictive edge-core impurity modeling. Controlling impurity behavior is crucial for the success of ITER and future reactors.
Impurities, whether intrinsic (e.g., from plasma-facing components) or injected (e.g., for
radiative cooling), can radiate energy, dilute the core fuel, and impact confinement. This is especially critical in the edge pedestal region, where steep gradients and low collisionality create a complex interplay between collisional and turbulent transport mechanisms.
The national PEPR SupraFusion program (France 2030, CEA & CNRS) funds the
development of new high-temperature-superconducting (HTS) cables and magnets capable of reaching 20 – 30 T in steady-state operation. Such fields open the door to compact tokamaks, while maintaining high plasma pressure. This increase in magnetic field strength:
- reduces the ion gyroradius,
- modifies local collisionality, orbit widths, and neoclassical convection velocities,
- concentrates more power on the divertor, reinforcing the need for controlled radiative
regimes.
Among these regimes, the X-Point Radiator (XPR) [1] – obtained by injecting light
impurities (N, Ne, Ar, C) – dissipates up to 95% of the power just inside the Last Closed
Flux Surface (LCFS). On AUG, TCV, WEST [2], and JET, it improves plasma duration and mitigates ELMs [3].
However, no predictive model coupling turbulence, neoclassical effects, high magnetic field, and localized radiation has yet been validated. The GYSELA [4] code (CEA IRFM) – a global 5D gyrokinetic, multi-species code with full collisions – is ideal for analyzing this coupling.
Computing will be performed on supercomputers (e.g., TGCC/CEA and LEONARDO/CINECA)
The objective of this thesis is to investigate impurity transport in fusion plasmas with a focus on: i) Understanding turbulent and neoclassical contributions in the pedestal (a relatively narrow edge plasma region with significantly enhanced profile gradients, associated with an edge transport barrier, characteristic of the H-mode), under ITG (Ion Temperature Gradient) and TEM (Trapped Electron Modes) regimes, ii) Exploring the role of impurity temperature and its impact on impurity transport, iii) Introducing a proxy for radiative losses, as a first step toward realistic XPR (X Point Radiator).

The thesis will follow a progressive approach:
Year 1 – Turbulent-Neoclassical Transport in the Pedestal
• Use the Fortran version of GYSELA to compute impurity transport coefficients in
pedestal-like configurations.
• Focus on the ITG regime and implement an improved poloidal momentum source
(ongoing development) to better resolve the edge physics.
• Study the impact of decreasing collisionality (ITER-relevant regime) on neoclassical
impurity screening.
• Quantify the synergy and competition between turbulence and neoclassical
transport.
Year 2 – Inclusion of Kinetic Electrons and Temperature Effects
• Explore different plasma turbulence regimes, where the underlying instabilities are
driven either by ions or by electrons.
• Reassess impurity screening and transport in combined ITG/TEM scenarios.
• Investigate the influence of impurity temperature, and its consequences on fuel di-
lution and impurity accumulation.
Year 3 – Radiation Modeling and Complex Geometry
• Implement a "Point Radiator" model with concentric flux surfaces to estimate energy
losses due to impurity radiation. Add a proxy for radiative energy loss.
• Introduce 2D impurity source profiles (r, θ) for better spatial modeling of impurities.
• Evaluate the role of multiple impurity species with realistic charge states, subject to
computational feasibility.

Profile of the candidate :
- Graduated or will graduate soon, an Engineering degree or a Master's degree in plasma physics, fusion physics, or fluid dynamics.
- Knowledge of one or more programming languages.
- Knowledge of English (oral and written) is important. Knowledge of French would be an advantage
but is not necessary.
- Appreciative of teamwork, and collaborations between different laboratories.
- Taste for numerical simulations and analytical models.

Contexte de travail

The Institute Jean Lamour (IJL) is a joint research unit of CNRS and Université de Lorraine.
Focused on materials and processes science and engineering, it covers: materials, metallurgy, plasmas, surfaces, nanomaterials and electronics.
By 2025, IJL has 259 permanent staff (34 researchers, 133 teacher-researchers, 92 IT-BIATSS) and 374 non-permanent staff (136 doctoral students, 48 post-doctoral students / contractual researchers and more than 190 trainees), from some sixty different nationalities.
Partnerships exist with 150 companies and our research groups collaborate with more than 60 countries throughout the world.
Its exceptional instrumental platforms are spread over 4 sites ; the main one is located on Artem campus in Nancy.
The PhD student will work within the “Fusion Plasmas” research group based in Nancy, under the supervision of Etienne Gravier and Maxime Lesur, in strong collaboration with the Institute for Magnetic Fusion Research (IRFM) in CEA Cadarache (the correspondants in IRFM-CEA are Xavier Garbet and Yanick Sarazin). The PhD program will very likely include several stays, each a few weeks long, in CEA Cadarache.
This PhD thesis is part of the PEPR SupraFusion project.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.