Theoretical Modelling Of Interaction Between Hydrogen And Carbon-based Wall Materials

Carbon-based materials due to their excellent thermomechanical properties and low atomic number have been chosen as a primary candidate for plasma facing materials in tokomaks. However, carbon-based materials have high erosion yield resulting from the impact of energetic hydrogen and severe tritium retention via co-deposition, which limit their applications in next-step fusion devices. Therefore, further studies on carbon-based materials are needed to overcome these shortcomings. In this paper, the following works have been done:Firstly, a model describing both energy and temperature dependence of carbon chemical erosion is presented. In the model, the chemical erosion is separated into the contributions from three mechanisms:thermal chemical erosion, energetic chemical sputtering, and ion-enhanced chemical erosion. By comparing the contributions from the three different erosion mechanisms, we found that when the target temperature is lower than about 400 K, the chemical sputtering dominates; As the target temperature increases, the contributions from the thermal chemical erosion and the ion-enhanced chemical erosion increase; Noticeably, when the ion impact energy is lower than 25 eV, the contribution of the thermal chemical erosion is comparable to that of the ion-enhanced chemical erosion, but as the ion impact energy increases further, the ion-enhanced chemical erosion becomes utterly dominant. The newly developed model can reproduce the experimental data well. Further, the new model is more general:when the temperature is at or near room temperature, the new model reduces to a model similar to Hopf's model; when the incident energy of the hydrogen ions is down to the energy range of thermal hydrogen atoms, this model reduces to Roth's well-known thermal chemical erosion formulation.Secondly, a dynamic Monte Carlo model is constructed to study the hydrogen retention, re-emission, and thermal desorption from porous graphite. For hydrogen retention, the simulation results show that most of the hydrogen atoms are retained at low temperatures; as the target temperature increases, the retained fraction starts to decrease rapidly; larger porosity and higher incident energy lead to larger hydrogen inventory. For hydrogen re-emission, the simulation results show that the released flux is made up mainly of molecular hydrogen at low temperatures and atomic hydrogen at high temperatures; the released flux of hydrogen molecules increases with the porosity and incident energy. For thermal desorption, the simulation results show that the number of desorption peaks depends on the fluence of pre-implanted H+, the concentration of trap sites, porosity and mean crystallite volume of graphite. Low implantation fluence and high concentration of trap sites easily lead to the occurrence of single desorption peak at around 1000 K while high implantation fluence and low concentration of trap sites favor the occurrence of double desorption peaks, respectively, at around 820 and 1000 K. It is also found that small porosity of graphite and large crystallite volume facilitate the occurrence of single desorption peak, and that large porosity and small crystallite size benefit the occurrence of double desorption peaks. In addition, experimentally observed third desorption peak at 400 K is reproduced by the simulation with assuming a small concentration of solute hydrogen atoms in graphite. It suggests that the solute hydrogen atoms play a critical role for the appearance of the third desorption peak.Thirdly, a molecular dynamic code is developed with REBO interaction potential for analyzing the collision processes between H/CHx and co-deposition layers. During the collision processes, the differences between hard film and soft film are studied in detail. The study results show that the adsorption yield hydrogen atom in soft film is higher than that in hard film, especially when the incident energy of hydrogen atom is below 15 eV. At fixed incident energy, the atomic range in soft film is deeper than in hard film. Higher adsorption yield and deeper range in soft film are the reason why the soft film easily traps a large amount of hydrogen isotopes. Moreover, the molecule orientations of CHx impinging on the substrate surface have an important effect on reflection yield below 5 eV. Reflection coefficients for CHx increase with increasing number of hydrogen atom but decrease with increasing the incident energy.Lastly, several unresolved issues are summarized as the topics of future research. A multi-scale scheme including first principles calculation, classical molecular dynamics, quantum molecular dynamics, and dynamic Monte Carlo is designed to solve these proposed issues.