Avis de soutenance de thèse de Madame Virginia Quadri

Bonjour,

Nous vous prions de bien vouloir trouver ci-dessous l’annonce de la prochaine soutenance de thèse de Virginia Quadri : 

Avis de Soutenance

Mademoiselle Virginia QUADRI

Soutiendra publiquement ses travaux de thèse intitulés :

«Impact of neutrals and density regimes on turbulence in the edge plasma of tokamaks»

Soutenance prévue le jeudi 19 Décembre 2024 à 14h30

Lieu : CEA – IRFM, salle René Gravier, Bâtiment 506

Et par Skype

Composition du jury :

Abstract

Dissipative divertor regimes are the key stones of the strategy to manage heat exhaust issues in large-scale magnetic fusion reactors like ITER and DEMO. These regimes help mitigate plasma-wall interactions by exploiting plasma-neutrals interactions in the vicinity of the vessel wall. The width of the SOL, a key factor accessing these regimes, is largely determined by perpendicular transport mechanisms, where turbulent transport plays a significant role. For the last decades the modeling effort has relied on transport codes which provide a high fidelity description of plasma-neutrals interactions, at the expense of adopting an ad-hoc model for turbulent transport. Transverse transport in these codes is described by arbitrary diffusion coefficients that are usually tuned from experimental scaling laws, which have been derived for attached plasmas only. However, experimental findings indicate that transverse turbulent transport is impacted by the divertor exhaust regime. This thesis presents an investigation into the impact of plasma-neutral interactions and divertor regimes on edge plasma turbulent transport and equilibrium. The problem is addressed through numerical simulations with the SOLEDGE3X code. A fluid neutrals model has been implemented in the code to treat self-consistently plasma-neutrals interactions and turbulent transport. After describing the changes made to the code to handle neutrals physics, we first investigate the impact of recycling in circular limited configuration by comparing a simulation without neutrals, driven by a core particle flux, to a simulation driven by a gas puff and including neutrals recycling. The main outcome is that the inclusion of recycling in the model affects deeply both the plasma equilibrium and turbulence characteristics even in the considered low-recycling condition. New poloidal asymmetries emerge due to the localized recycling around the limiter. While neutrals barely impact the near SOL decay lengths, the far SOL widens by a factor of 2.4 in the presence of neutrals due to a flattening of density profiles. Mean-field transport coefficients reconstructed from turbulence data keep the same spatial distribution whether neutrals are included or not, but their amplitude increases by up to 100\%. These effects are showed to be due solely to the localization of the recycling source. Then, the impact of density regimes in a realistic TCV divertor geometry is studied. We demonstrate that our fluid neutrals model provides a sufficient approximation of density regimes. Two 3D turbulence simulations are analyzed, one at low density with an attached outer divertor, one at high density with a detached outer divertor. The transition from attached to detached regimes reveals substantial changes in both global and local properties of plasma and turbulence. As a consequence, one finds that detachment leads to an increase of both the near SOL and far SOL heat flux decay lengths by a factor of 1.6 and 2.7 respectively, in line with experimental trends. We finally reconstruct mean-field transport coefficients from our turbulence simulations. Detachment leads to a large increase of the particle diffusion coefficient by up to a factor of 5, but to the heat conductivity becoming negative in the outer divertor, raising fundamental questions about the validity of the ad-hoc anomalous transport description in mean-field codes. We propose several routes to solve this inconsistency, resulting in transport coefficients with a more physical behavior. Finally our work provides a first feedback concerning the methodology to run turbulence simulations with neutrals. Long physical time scales related to neutrals physics make it difficult to achieve a meaningful steady-state, even when applying dedicated strategies to shorten the convergence time. This critical point will need to be addressed in the future if turbulence simulations with self-consistent neutrals are to be used for heat exhaust studies.

Keywords

Edge plasma, Scrape-Off-Layer, Turbulence, Neutrals, Tokamaks, Simulations

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