Avis de soutenance de thèse de Monsieur Yann Munschy

Avis de Soutenance

Monsieur Yann MUNSCHY

Soutiendra publiquement ses travaux de thèse intitulés :

«Kinetic and Gyrokinetic physics of plasma-wall interaction in tokamaks»

Soutenance prévue le vendredi 29 novembre 2024 à 10h

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

Et par Skype

Composition du jury :

Abstract

The interaction between a tokamak plasma and its material boundary affects edge plasma turbulence and thus influences the quality of confinement. Extending gyrokinetic simulations of transport and turbulence up to the very edge of the plasma is a necessary requirement to achieve reliable predictions (i) of the overall plasma confinement and (ii) of possible routes towards improved confinement regimes. However, it is still ongoing work. A critical point is to correctly describe within the gyrokinetic framework the physics of plasma-wall interaction. The present work aims at improving the edge description in the gyrokinetic framework by proposing a model for plasma-wall interaction that is compatible with the gyrokinetic theory.

Prior to proposing such a model, the interaction between charged particles and a solid wall is studied in the two-dimensional—1D in space, 1D in velocity space—framework of the Voice code. The description of the wall is based on immersed boundary conditions, also called penalization technique. Voice shows that penalization allows for the recovery of key aspects of plasma-wall interaction physics: a positively charged region—reminiscent of a Debye sheath—develops at the plasma boundary, which ensures quasineutrality within the plasma. The physics at play within the sheath results in distribution functions departing from Maxwellians. Studying the system using the hierarchy of fluid equations shows that (i) the Bohm criterion is inoperant in the kinetic framework, and (ii) the speed of sound used to express this criterion critically depends on the closure of the fluid hierarchy. This last point could bear consequences for fluid codes that use the Bohm criterion as a boundary conditions on plasma-facing units. So as to prepare for the future extension of the Gysela code to include plasma-neutral interaction, we describe a so-called pressure-diffusion fluid model for neutrals. We couple this neutral fluid model to the kinetic plasma description of Voice, and illustrate the effect of the neutral dynamics on the plasma.

Based on this preliminary study and by reviewing already-existing gyrokinetic plasma-wall interaction models, we propose a sheath model that is compatible with the global framework of the gyrokinetic code Gysela. The model ensures quasineutrality in  the plasma withouth solving the characteristic time and spatial scales of the sheath, by matching the electron and ion gyrocenter fluxes on spatial average over the entire surface of the plasma material boundary. The model uses penalization so that the
material boundary is immersed within the simulation domain. Performing global simulations shows that the absorption of ions within the material boundary along with the reflection of slow electrons on the boundary surface works as expected. We however observe a mismatch between the electron and ion fluxes at the material boundary. It leads to the buildup of a negative charge in front of the boundary, effectively breaking quasineutrality on timescales too short to study turbulence. We describe possible ways for closing this mismatch and discuss its possible origin.

Keywords

      Plasma-wall interaction, gyrokinetic simulations, kinetic simulations, Plasma-neutral interaction

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