Avis de Soutenance
Monsieur Stefano DI GENOVA
Soutiendra publiquement ses travaux de thèse intitulés
«Heavy ions migration in tokamak boundary plasmas :
development of a numerical model to interpret WEST
experiments»
Soutenance prévue le vendredi 1er decembre 2023 à 14h
Lieu : CEA – IRFM, salle René-Gravier, Bat 506
Et par Skype
Composition du jury :
|
Karl Krieger |
Rapporteur |
Max-Planck-Institut für Plasmaphysik |
|
David Donovan |
Rapporteur |
University of Tennessee Knoxville |
|
Khaled Hassouni |
President du jury |
Université Sorbonne Paris Nord |
|
Emmanuelle Tsitrone |
Examinatrice |
IRFM, CEA |
|
Sebastijan Brezinsek |
Examinateur |
Forschungszentrum Jülich |
|
Paolo Innocente |
Examinateur |
Consorizo RFX, CNR |
|
Eric Serre |
Directeur de thèse |
M2P2, CNRS |
|
Guido Ciraolo |
Co-directeur de thèse |
IRFM, CEA |
|
Nicolas Fedorczak |
Invité (encadrant) |
IRFM, CEA |
|
Alberto Gallo |
Invité |
IRFM, CEA |
Abstract
Tungsten (W) is considered to be the most suitable material for the Plasma-Facing Components (PFCs) of future tokamak fusion reactors. Nonetheless,the deployment of this material in tokamak experiments has been shown to be detrimental to plasma discharges: W is eroded from the wall and contaminates the plasma, causing large power losses through radiation. Plasma operations in the W Environment Steady-state Tokamak (WEST) are heavily influenced by W contamination. In WEST discharges, the power loss due to W contamination is, on average, around 50% of the total power injected into the plasma. Moreover, the radiated power fraction (fRad) is insensitive to plasma conditions. The causes behind this experimental trend are not fully understood.
Furthermore, in experiments, it is not possible to detect which eroded PFCs impact the plasma W content the most. In fact, even if the gross W erosion flux is measured through visible spectroscopy, the W influx is screened by the plasma. The resulting net W flux cannot be measured in experiments. For these reasons, the experimental analysis of W contamination in WEST must be supported by modelling activities. The modeling of W migration in WEST helps estimate the W screening at the different PFCs and analyse the contamination trends in the tokamak. During this Ph.D. thesis, two well-established numerical tools (SOLEDGE and ERO2.0) are used to model the boundary plasma and the W migration in WEST plasma discharges.
Parametric analyses allow for investigating the role of the concentration and charge of light impurities in W erosion and screening, as well as the impact of turbulent diffusion on W transport. The W dynamics is analysed, focusing on how the collisional forces can affect the W momentum. Based on simulations, a close analysis of the erosion of each WEST PFC and its impact on the plasma W content is performed. Results show that the WEST lower divertor is the most eroded PFC, but it is also the most screened one. On the other hand, less eroded components could impact the plasma W content more than the lower divertor. The tokamak upper divertor, the external surface of the baffle, and the antenna protections might be unscreened enough to influence the plasma W content even at low erosion rates. The research activity focuses on the antennas: 3D simulations of the boundary plasma are carried out using a complex wall geometry, the 3D wall is equipped with toroidally localized objects representing WEST antennas. The antenna protections are weakly screened, and the impact of their erosion on the plasma W content is predominant over the other PFCs one. The 3D model is used to analyze W migration over the WEST operational domain. The WEST database is sampled to obtain a scan of simulation input parameters that mimic the WEST plasma conditions over an experimental campaign. The simulation results are compared to WEST diagnostics data (reflectometry, Langmuir probes, and visible spectroscopy) to verify that the simulated plasma conditions are compatible with the WEST database. The W migration trend is analysed: the W density (nW) increases proportionally with the power entering the scrape-off layer (PSOL) and strongly drops when the radial distance between the separatrix and the antennas (Radial Outer Gap, ROG) increases. The radiated power (PRad) is estimated in simulations with a simple 0D model. At a given ROG, PRad is proportional to the total power PTOT, with fRad which is not sensitive to plasma conditions. These trends are qualitatively and, at times, quasi-quantitatively comparable to what is observed in WEST experiments. In simulations, the fRad insensitivity to plasma conditions is related to the low screened W influx caused by the erosion of the antenna protections. This research activity shows how simplified numerical simulations of the boundary plasma and W migration can give a realistic picture of the W migration trends in tokamak experiments. The results also underline the importance of the main chamber PFCs located close to the confined plasma. Finally, this work points out how the net W influx coming from low screened PFCs might be weakly affected by plasma conditions and how it might become challenging to control during plasma discharges.
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