Computer simulation of erosion and deposition patterns on graphite target plates exposed to divertor plasmas

A plasma materials interaction code, EDDY, is modified to calculate the erosion and deposition patterns on divertor plates of fusion devices. An area in the plate is divided into segments, where dynamic erosion and deposition processes on the plate are simulated, and the ionisation and dissociation processes of released impurities in a divertor plasma are followed. Using the modified code, the erosion and deposition patterns on inner and outer vertical targets (graphite) in a designed fusion device, ITER, in a semi-detached plasma condition and on a divertor target plate in JT-60U are calculated. An erosion zone and a deposition zone are obtained in a private flux region and a scrape-off layer (SOL) region, respectively, on the outer vertical target. Near the strike point in the SOL region on the inner vertical target where the plasma temperature is low (<5 eV), the erosion dominates due to low ionisation and dissociation rates of chemically sputtered hydrocarbons if the same chemical sputtering yield (0.04) as for the outer vertical target is assumed. Physical sputtering is much less important for the patterns, in particular, near the strike points and in the private flux region. The two-dimensional plots of redeposited hydrocarbons and carbons for both targets reveal the dependence of the local redeposition process on the temperature and density of the plasma and the energy of the released particles. (author)