27 mars 2024
- Study of the dynamics and passive control of heavy ions produced by plasma-wall interaction: toward the elaboration of a predictive model in the suite of codes SOLedge2D-EIRENE from experiments on the tokamak WEST / PhD Defense Luca Cappelli
Doctorant : Luca CAPPELLI
Date : le 27 Mars 2024 à 15h00 ; CEA, IRFM, F-13108 Saint-Paul-Lez-Durance, bâtiment 506, salle René Gravier
Abstract: Fusion, in the context of tokamak devices, is suggested as an alternative for sustainable and high performance energy production. To date, one of the issues preventing its industrial development, concerns plasma-wall interactions. Power discharged from the plasma to the tokamak vessel has two main consequences: gradual erosion of mechanical components and contamination of the plasma by eroded species. Tungsten (W) has emerged as a promising material to enhance the lifespan of mechanical components in tokamaks. However, managing W concentrations in the plasma, especially in comparison to lighter species like carbon, poses a challenge. Ongoing fusion research aims for reliable modeling of erosion and transport of impurities. Modeling is fundamental to support experiments, design and prepare optimized scenarios for future reactors as ITER. In particular fluid models are a promising tool for this end, but they need proper boundary conditions. This Ph.D. thesis delves into the modeling of Werosion and redeposition through Monte Carlo techniques and analytical models in the limit of a collisionless plasma sheath and a smooth eroded surface. The research involves simulating W erosion and transport in the plasma region adjacent to a solid wall, encompassing the combined Debye sheath and Chodura sheath (referred to as the plasma sheath) and a segment of the SOL with dimensions on the order of the Larmor radius of the eroded species. The plasma sheath plays a pivotal role in the erosion and redeposition of W. It is characterized by the presence of a dominant electric field and strong electron density gradients, making kinetic models the only way to solve transport within it. A portion of this research is dedicated to building a model for the sheath, which serves as a plasma background where eroded particle transport is solved using Monte Carlo techniques. The Monte Carlo model computes redeposition probability maps, offering an alternative method to particle tracking for the calculation of redeposition. The study also explores the impact of different impinging energy distributions of ions in both erosion and redeposition. Results indicate that approximating the energy distribution of impinging ions on the eroded material with the average energy underestimates erosion and redeposition only if the average energy is close to the sputtering energy threshold. Otherwise, even using the average energy is accurate. A large database regarding W redeposition was rapidly created using the redeposition probability maps. Data was then used to train a Neural Network (NN), capable of estimating redeposition as a function of local plasma parameters. Additionally, an analytical model is provided to explain the main mechanisms of redeposition and how to roughly estimate redeposition through integration. This approach is less accurate but it is more flexible because it does not need the use of probability maps, nor it is valid for a set of fixed conditions, as the NN. The analytical model allows to rapidly estimate other important parameters such as temperature and average charge state of sputtered and not-redeposited W. Experimental testing validates the sheath model assumptions regarding the electric potential drop, showing good agreement between modeling and experimental measures. Furthermore, the analytical model is applied to the WEST database to gain insights into W sources at the lower divertor. This Ph.D. thesis contributes in plasma-material research providing fundamental insights into the physics of local W erosion and redeposition in the collisionless limit. Beyond its theoretical contributions, this work has practical implications since it provides different modeling tools to estimate local W erosion. Such tools can be integrated into multispecies plasma solvers, paving the way for innovative time-dependent simulations. This in turn could be valuable for the design and optimization of future tokamak reactors incorporating W.
Keywords: Tungsten, Erosion, Redeposition, Plasma-wall interactions, kinetic models, Monte Carlo
Jury :
David TSKHAKAYA - Czech Academy of Sciences / Rapporteur
Andreas KIRSCHNER - Institute of Energy and Climate Research / Rapporteur
Richard PITTS - ITER organization / Examinateur
George TYNAN - University of California San Diego / Examinateur
Clarisse BOURDELLE - IRFM, CEA / Examinatrice
Tiberiu MINEA - University Paris-Saclay / Président du jury
Eric SERRE - M2P2, CNRS / Directeur de thèse
Nicolas FEDORCZAK - IRFM, CEA / Co-encadrant de thèse
Yannick MARANDET - PIIM, CNRS / Membre invité
