Fundamental science of the synergy of multi-species interactions in a high plasma-heat-flux environment P. S. Krstic 1 , F. W. Meyer 1 , Y. K. Peng 2 , D. L. Hillis 2 , L. R. Baylor 2 , R. H. Goulding 2 , J. H. Harris 2 1 Physics Division, and 2 Fusion Energy Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 Introduction. In the recent Greenwald Report [1], areas of urgent knowledge gaps toward developing a magnetic fusion demonstration reactor were identified. These include plasma-facing components (PFC’s), plasma-wall interactions (PWI’s), and fusion materials. All three areas were identified as requiring major extrapolations from the present knowledge base, with no solutions in hand or with solutions foreseen but not yet achieved. Plasma-surface interaction (PSI) science is strongly linked to all these areas. The choice and deployment of plasma-facing materials should be optimized to ensure the performance and sustainment of the burning plasma, and therefore depends on PSI science knowledge to design and build viable heat-bearing components. An important issue for the fuel cycle is the retention of tritium fuel in the PFC materials; this is a current subject of study in plasma-surface interactions, but one for which orders of magnitude reduction in retention are required from present experiments to a DEMO. The third area emphasizes the strong interaction and coupling between the plasma and wall materials, and highlights the present lack of predictive capabilities. PSI, in particular, chemical and physical sputtering, cause erosion of the limiter/divertor plates and vacuum-vessel walls, whether these are made of C, Be, W, or some combination (i.e., mixed materials). Such impurities can degrade fusion performance by diluting the fusion fuel and cooling of the core excessively via line radiation from partially ionized impurity atoms. Hydrocarbon re-deposition onto plasma-facing components can lead to long-term accumulation of large in-vessel tritium inventories via co-deposition. The choices of wall material has profound effects on confinement of fusion-grade plasmas, assuming adequate power and particle handling of the plasma facing components and other internal components. The present knowledge of plasma interactions with these surfaces is still mainly empirical, and “wall conditioning” remains an art. Although carbon-based materials have superior thermal–mechanical properties, they are expected to trap high levels of tritium by co-deposition with eroded carbon and thereby severely constrain safe plasma operations. Thus, a mix of several different plasma-facing materials is now proposed in ITER to optimize the requirements of areas with different power and particle flux characteristics. The slow rate of progress in the area of tritium removal, together with progress of divertor tokamaks with high-Z (e.g. tungsten) walls, suggests investigations of all-metal surfaces, which are held, in case of DEMO, at elevated temperatures. The stability of the modified surfaces under high transient heat fluxes during Edge Localized Modes (ELMs) and disruptions is another important question and expands the range of plasma parameters that needs to be studied both experimentally and theoretically. This white paper addresses the research to understand and predict individual phenomena both under well-controlled conditions as well as under more realistic large-area, high plasma heat flux conditions where synergistic multi-species interactions play significant and possibly dominant roles. Scientific issues of synergistic interactions. The likely use of high-Z refractory metals for plasma facing materials in the DEMO reactor at sustained elevated temperatures and in continuous operation, as well as the need for efficient power conversion, raises important issues related to the tritium retention, permeation, recombination, surface erosion, chemical reactions, damage annealing, codeposition, nanostructures formation, blistering (in e.g. tungsten), and their dependences on plasma and wall temperatures, as well as plasma and neutron fluences . High wall temperatures (>600C) and high fluence conditions, in particular, have not been adequately studied to date. The key issues for surface studies on ITER-relevant carbon based materials are erosion, reflection, impurity transport in the interacting plasma, redeposition, T uptake and removal. Specific needs [2], among others, include determination and characterization of the composition of eroded species such as