Carbon erosion mechanisms in tokamak divertor materials: insight from molecular dynamics simulations

While there is much experimental and theoretical interest in the surface ph ysics of amorphous hydrogenated carbon under hydrogen bombardment, the atom ic-scale interactions taking place under low-energy (E similar to 10 eV) pa rticle bombardment still remain obscure. We now describe an erosion mechani sm for low-energy ions, which leads to carbon erosion yields much higher th an expected from physical sputtering alone, and also find that the erosion yield from this mechanism is highly sensitive to the detailed surface struc ture. We show that this sensitivity can explain why the erosion of tokamak divertor carbon surfaces has been shown to decrease sharply at extremely hi gh hydrogen fluxes (similar to 10(19) ions/cm(2) s). The high-flux hydrogen bombardment produces a very high hydrogen content at the sample surface, w hich cm lowers the carbon sputtering yield by an order of magnitude by decr easing the carbon bond collision cross-section. (C) 2001 Elsevier Science B .V. All rights reserved.