Multi-scale modeling of hydrogen isotope diffusion in graphite

The importance of plasma-wall interaction processes for the edge plasma is well known: creation of impurities by different sputtering mechanisms or recycling properties of the walls are examples of processes determining the divertor characteristics and the edge plasma profiles. To be able to have a better understanding of the plasma-wall interaction process itself, a multi-scale procedure is followed: molecular dynamics calculations resolve the microscopic length scale and deliver quite precise input data for kinetic Monte Carlo calculations (jump frequencies, migration energies, jump step-sizes) used for meso-scale up to the macroscopic system length. To cover the whole length scale involved - from microscopic up to macroscopic - several subsequent levels of kinetic Monte Carlo are needed, each providing the necessary input data for the next level. With this procedure the corresponding time scales spanning from picoseconds atomic interaction times to wall equilibration times of at least milliseconds will be spanned. Inclusion of a realistic structure model is also important, like for porous graphite where the void structure and orientation of the microcrystallites have to be included. First results of such a multi-scale calculation are presented studying the diffusion of hydrogen isotopes in porous graphite and are compared with experimental results from the literature. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA. Wenheim.