Generation of dust seeds by sputtering of carbon-based plasma facing materials under low-energy H/D/T ion bombardment

Low energy ion impact (with injection energy less than 50 eV or so) on the plasma facing wall materials of a magnetic confinement fusion device such as a tokamak can generate a significant amount of sputtered species that may grow into dust particles through agglomeration in the gas phase. In this work, we have studied sputtering properties of graphite and amorphous carbon substrates due to hydrogen (H), deuterium (D), and tritium (T) ion bombardment at low incident energies using classical molecular dynamics (MD) simulations. The classical interatomic potentials that we have used in this work are Brenner-type multi-body potential functions with weak Van der Waals interactions. MD simulations have been performed in such a way that we allow significant accumulation of incident species up to 1.25 x 1017 cm-2. Our simulation results indicate that a high level of H/D/T dose accumulation on the top surface is prerequisite for the formation of relatively large-sized sputtered hydrocarbon species. Significant isotopic dependence of sputtering yields has been also observed after the dose of incident D or T reaches about 1016 cm-2. It has been clearly shown that the sputtering yield can be lower at higher incident energies in the low energy range for D and T injections. These simulation results are consistent with some of earlier published experimental observations of carbon-based material sputtering. (author)