Speaker
Description
Fast-ion interactions with plasma-facing components remain an important open question for ITER, particularly after the recent transition to a full tungsten first wall. Tungsten sputtering is expected to be mostly caused by the thermal plasma, but lost fast ions such as fusion-born alpha particles may contribute to tungsten sputtering. The extent of fast ion contribution depends on both the spatial distribution of fast-ion losses and the corresponding sputtering yields, which vary with particle energy and angle of incidence. Previous studies indicate that sputtering yields decrease at higher energies, while for high-energy ions the yield increases with incidence angle. These considerations raise the central question of whether fast-ion losses can produce a significant tungsten source compared to thermal processes.
We combine ASCOT-based fast-ion wall interaction data with sputtering yield calculations derived from SDTrimSP for hydrogenic species and helium (alphas). High-resolution yield maps as functions of energy and incidence angle were constructed to enable numerical integration with fast-ion flux distributions. The methodology converts alpha particle fluxes to wall source rates and subsequently to tungsten sputtering rates by applying energy- and angle-dependent yields. These results are then compared against thermal sputtering contributions obtained from WallDyn modelling. Furthermore, we evaluate numerically the impact of CX lost alphas and NBI on tungsten sputtering under realistic ITER conditions.
The results indicate that the tungsten source due to fast alpha particles is significantly smaller than that from thermal processes, with differences on the order of 3–4 orders of magnitude. This large gap is attributed to both relatively low sputtering yields under relevant conditions and limited fast-ion fluxes to the wall. No clear mechanism was identified that could bridge this discrepancy under ITER-relevant conditions, and contributions from beam ions are expected to be even smaller.