DOE OSTI2019
Plasma facing surfaces (PFS) have a profound influence on plasma performance and choice of material for PFSs is a burning issue for tokamak reactors. Using lithium wall conditioning in NSTX has resulted in a beneficial influence on plasma behavior including control of hydrogen recycling and improvements in energy confinement, longer confinement times, edge density control, and the elimination of Edge Localized Modes. A major challenge for the NSTX-U (increased power flux, increased campaign time) as well as for other magnetic fusion devices is to extend high-performance plasmas for very long durations, and to integrate this high performance with plasma facing components (PFCs) that can withstand very high heat and particle fluxes while maintaining structural integrity with minimal retention of fusion fuel. This project directly contributed to filling up the knowledge gaps and data for the dynamics of the plasma-facing interfaces in the NSTX-U at nano and nano-to-micro scales. This was achieved by both deciphering the poorly understood nanoscale phenomenology of plasma-surface interactions and providing the connection and comparison with the experiments which provide understanding at the mesoscopic scales. We modelled the dynamic response and evolution of a mixed material surfaces (D, Li, C, B, O) to bombardment by plasma particles, and investigated microstructure changes, erosion, surface chemistry, deuterium implantation and permeation. The main objectives of the project are i) a comparison of Li and B effects when deposited on carbon, and ii) the role of oxygen and other impurities (e.g. carbon, boron) in the lithium performance. In addition to predicting and understanding the phenomenology of the processes, the project achieved deliverables on the i) plasma induced erosion rates of PFCs (outer wall, divertor, etc.), including chemical and physical sputtering yields at various temperatures (300-700K), ii) deuterium uptake/recycling rates, and iii) accurate probability rates for the processes and reactions in i-ii which were used to understand the experimental results of the J.P. Allain and B. Koel NSTX-U projects. Lithium was a distinctive focus of the project. It easily charges in contact with other fusion material atoms, giving partially or fully its external electron cloud to other atoms in its vicinity and polarizing the mixed material. Our research strategy used experimentally validated multiscale theory applied to the large, multicomponent material surface systems. With the use of US supercomputing facilities we simulated evolution of atomic charges, chemistry, retention of deuterium, sputtering and morphology by introducing the corrections to the classical molecular dynamics, like are electronegativity equalization method (EEM) and quantum-classical molecular dynamics (QCMD). The goal was to understand and quantify the dynamics of the creation and evolution of the plasma-material interface under irradiation by atoms and molecules at boronized carbon, lithiated carbon, and boronized-lithiated carbon for both solid and self-healing liquid-metal divertor.