Speaker
Description
Carbon-based plasma-facing materials (PFMs) have been regarded as attractive candidates for fusion reactors owing to their high compatibility with plasma performance and favorable thermal properties. However, concerns over tritium retention and material activation have led to the adoption of metallic PFMs in recent devices. In contrast, experiments on the QUEST device identified a hydrogen transport barrier at the interface between carbon-containing redeposition layers and the metallic substrate. This finding suggests that active surface control of metallic PFMs could enable partial utilization of carbon’s advantageous properties while maintaining acceptable activation levels. Building on this insight, Hanada et al. proposed a new operational concept in which a small amount of carbon is intentionally introduced into the plasma (“carbon doping”) to form a carbon-containing redeposited layer over the entire plasma-facing surface, while a low-temperature region (<150 °C) serves as a “carbon pump” that preferentially collects carbon. This system aims to regulate the in-vessel carbon inventory and suppress hydrogen isotope retention below critical limits. To develop such a system, it is essential to understand the microscopic behavior of hydrogen within carbon-containing redeposited layers, including its bonding states, diffusion pathways, and molecular hydrogen formation. Recent first-principles molecular dynamics (FPMD) studies by Kusaba et al. have shown that H₂ formation in hydrogenated amorphous carbon (a-C:H) is strongly governed by the saturation of C–H bonds, with molecular hydrogen emerging primarily in voids. Motivated by these findings, the present study investigates the structural evolution and hydrogen retention characteristics of carbon-doped redeposited layers through atomistic simulations, systematically varying carbon density, hydrogen concentration, and temperature. Special attention is given to C–H bond saturation behavior, the kinetics of H₂ formation, the development of nanoscale voids on metallic substrates, and carbon re-condensation behavior on low-temperature surfaces. These analyses clarify the fundamental physical mechanisms by which the carbon-pump concept suppresses hydrogen isotope retention.
The outcomes of this study establish a scientifically grounded hydrogen-recycling model tailored to metallic-wall fusion systems and provide a quantitative basis for optimizing integrated operation scenarios that combine carbon doping with active carbon pumping. The results contribute to the design of next-generation plasma-facing components that leverage controlled carbon behavior while maintaining low activation and safe tritium handling.