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

While there is much experimental and theoretical interest in the surface physics of amorphous hydrogenated carbon under hydrogen bombardment, the atomic-scale interactions taking place under low-energy (E∼10 eV) particle bombardment still remain obscure. We now describe an erosion mechanism for low-energy ions, which leads to carbon erosion yields much higher than expected from physical sputtering alone, and also find that the erosion yield from this mechanism is highly sensitive to the detailed surface structure. We show that this sensitivity can explain why the erosion of tokamak divertor carbon surfaces has been shown to decrease sharply at extremely high hydrogen fluxes (∼1019 ions/cm2 s). The high-flux hydrogen bombardment produces a very high hydrogen content at the sample surface, which lowers the carbon sputtering yield by an order of magnitude by decreasing the carbon bond collision cross-section.