[SY-B3] Hydrogen super-saturated layers in plasma loaded tungsten: a global model combining Density Functional Theory data, Thermodynamic and Kinetic models
In this work, we combined Density Functional Theory data, a Thermodynamic and a kinetic model to build a global model enabling to determine the total concentration of hydrogen implanted in the sub-surface of tungsten exposed to a hydrogen flux. This is achieved given a flux of hydrogen, a temperature of implantation, and the energy of the incoming hydrogen ions. This model is built step by step; an equilibrium with a molecular hydrogen gas phase is first considered and the resulting solubility is compared with experimental results. A kinetic model is subsequently used to determine a chemical potential for hydrogen in the sub-surface of tungsten. Combining both these models, two regimes are then established in which hydrogen is trapped at interstitial sites or in vacancies. The existence of these two regimes are driven by the temperature of implantation; above a temperature of implantation is the interstitial regime, below is the vacancy regime in which super-saturated layers are formed. A simple analytical and easy to use expression is work-out for the transition temperature, which allows to plot a diagram of existence of the super-saturated layer depending on the implantation temperature, the incident energy and the flux of the hydrogen ions.