Contrib. Plasma Phys. 42 (2002) 2-4, 458-463 Obtaining Distributions of Plasma Impurities Using Atomistic Simulations E. Salonen a) , K. Nordlund a) , J. Keinonen a) , C. H. Wu b) a) Accelerator laboratory, P.O. Box 43, FIN-00014 University of Helsinki, Finland b) EFDA, MPI f¨ ur Plasmaphysik, D-85748 Garching bei M¨ unchen, Germany e-mail: msalonen@beam.helsinki.fi Abstract Carbon-based materials are widely used and studied divertor materials for tokamak fusion devices. During investigations of their plasma-facing properties, the carbon sputtering yields at all energies have received much attention. However, the chemical composition of the sputtered species is also very important as the species that leave the surface either redeposit on the original surface or drift elsewhere in the vacuum chamber, possibly entering the core plasma. Determining the fusion plasma dilution and the growth of hydrocarbon films in the fusion device vacuum chamber requires a good knowledge of the type of the eroded hydrocarbon species. We use molecular dynamics simulations to model the erosion of carbon surfaces under low energy (5 – 30 eV) H/D bombardment. The atomistic nature of the modeling allows us to unambiguously recognize the sputtered hydrocarbon species. Our results show that the predominant eroded species are the small hydrocarbons CH x /CD x and C 2 H x /C 2 D x , in agreement with experiments. Only a small fraction of heavier hydrocarbons are seen. 1 Introduction Carbon is a widely used divertor material in contemporary tokamak fusion devices due to its thermomechanical properties and low atomic number. However, notable physical and chemical sputtering from carbon divertors (usually graphite or carbon fiber composites) takes place under hydrogen bombardment at energies relevant to fusion reactor use, 1 - 100 eV [1, 2, 3]. The sputtered hydrocarbon species either redeposit back to the original surface, enter the core plasma, cooling and diluting it, or drift further away in the fusion device vacuum chamber. The latter case leads to growth of thin hydrocarbon films throughout the vacuum chamber walls [4]. As tritium is one of the fusion fuel components, the growth of the C:T films leads to the accumulation of a harmful tritium inventory that cannot be recycled. Clearly, determining growth rates of the hydrocarbon films, as well as evaluating the core plasma dilution requires knowledge of the chemical composition of the sputtered hydrocarbons. Measurements carried out during fusion device operation [5] give fractions of 54 %, 33 %, and 13 % for CH m D n ,C 2 H m D n and C 3 H m D n species, respectively. While some laboratory studies on carbon under low-energy hydrogen irradiation report only the total carbon and/or CH 4 yields, it is well known that other hydrocarbon species are also sputtered away [1, 2, 5]. Comprehensive studies of carbon erosion by H/D by Mech et al. [2, 6, 7] reached impinging hydrogen energies down to 10 eV. It was seen that at energies below 30 eV heavier hydrocarbons contribute to roughly 45 – 70 % of the total carbon sputtering yield at 300 K. However, in those studies the impinging projectile was a molecular H + 3 /D + 2 species, instead of a single atom or ion. The authors c  WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2002 0863-1042/02/2-404-0458 $ 17.50+.50/0