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Modeling of hydrogen isotopes retention in plasma-facing components for fusion applications


Plasma-material interactions might strongly affect plasma performances and life-time of future magnetic fusion devices. For example, retention and recycling of hydrogen isotopes in plasma-facing components (PFC) may lead to dynamics plasma-material interactions and significant accumulation of tritium in material. Understanding the multifaceted physics of hydrogen retention in PFC is thus crucial, but remains challenging due to the wide spectrum of retention processes on PFC surface (erosion, co- deposition, etc.) and in PFC bulk (trap sites, bubbles, etc.) induced by long-time exposure of PFC to high flux of energy and particles. In this context, we revisit in this work some aspects of the reaction-diffusion models used to describe retention of hydrogen implanted in material in fusion relevant. We first focus on the analysis of thermal desorption spectroscopy (TDS) experiments, showing that the evolution of hydrogen concentration in material during TDS experiments is usually quasi-static. An analytic description of thermal desorption spectra (TDSP) is then obtained in quasi-static regime and is used to highlight dependencies of TDSP on hydrogen retention parameters. The interpretation of Arrhenius plots to characterize hydrogen retention processes is then revisited. Moreover, it is shown that retention processes can be characterized using the shape of desorption peaks in TDSP, and that long desorption tails in TDSP can be used to estimate activation energy of diffusion of hydrogen in PFC. Hydrogen retention induced by a large number of different types of traps is examined next. A reaction-diffusion model of TDSP with a large number of types of traps is presented for the first time. The application of this model is illustrated on several experimental TDSP available in literature, which are consistently reproduced using several types of traps with a unique broad spectrum of detrapping energies. The values of these detrapping energies are shown to be in agreement with values predicted by density functional theory simulations when several hydrogen atoms are trapped in one material vacancy. Effects of surface processes on hydrogen retention and recycling are investigated in the second part. First, long-term outgassing of hydrogen from PFC during off-plasma events is considered. The super- diffusive power-law decay in time of the hydrogen outgassing flux is modeled with a revisited single trap reaction-diffusion model, showing that hydrogen outgassing is either surface-limited or diffusion-limited. The outgassing regime is shown to be governed either by processes in the bulk or on the surface of material. The influence of hydrogen concentration profiles in material on the power-law exponents is analyzed as well. Finally, the different models proposed in the literature to describe power-law decays of hydrogen outgassing flux experimentally observed during off-plasma events are reconciled. Hydrogen recombination and desorption on tungsten surface is investigated next using molecular dynamics (MD) and accelerated molecular dynamics simulations. Adsorption states, diffusion, hydrogen recombination into molecules, and clustering of hydrogen on tungsten surfaces are analyzed. It is shown that tungsten hydrogen interatomic potential, available in literature and used in MD simulations, cannot reproduce main features of hydrogen molecular recombination on tungsten surface. Hydrogen clustering on tungsten surface is nevertheless observed during MD simulations. Effects of hydrogen clustering on hydrogen desorption are thus analyzed by introducing a kinetic model describing the competition between surface diffusion, clustering and recombination. Different desorption regimes are identified, which reproduce some aspects of desorption regimes experimentally observed when tungsten surface is saturated with hydrogen

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