Molecular dynamics studies of the chemical sputtering of carbon-based materials by hydrogen bombardment

A central issue in the performance of tokamak fusion devices is the choice of the plasma-facing materials. During the device operation the plasma-facing surfaces are subject to extreme heat and particle loads, as they are impinged on by escaped fusion plasma particles. Although carbon-based materials have excellent thermomechanical properties in view of the high heat flux, their use in the next-step fusion device ITER is restricted by high chemical sputtering yields by the impinging hydrogen even at energies as low as a few eV’s. The exact mechanism of the chemical sputtering has not been known. Using molecular dynamics simulations we have studied the erosion of amorphous hydrogenated carbon surfaces by low-energy ( 35 eV) hydrogenic atoms. The main finding of this thesis is a new sputtering mechanism, where an impinging hydrogen atom breaks the covalent bond between two carbon atoms by penetrating the region between them. This mechanism was first observed in simulations employing an empirical force model and was later verified with more accurate tight-binding calculations. The temperature and energy dependencies of the chemical sputtering were studied. A good overall agreement with experimental data was obtained. Our simulations also provided predictions of the carbon sputtering yields for lower hydrogen impact energies than those achieved in experiments. The experimentally observed isotope effect of the chemical sputtering was explained. Finally, the effect of Si doping on the chemical sputtering of carbon was studied. It was observed that the C sputtering yield decreases nonlinearly with increasing Si concentration. The inclusion of silicon was concluded to suppress the bond breaking in the C-Si network and enhance the rebonding of unbound hydrocarbon species.