Estimates of carbon sputtering from fast-ion losses on DIII-D QH-mode plasmas*

Modeling of carbon sputtering by fast ion losses in quiescent H-mode mode plasmas indicates minimal impact toward high Z_eff conditions, counter to previous expectations. This indicates the divertor sources the majority of the carbon leading to Z_eff>3 in these scenarios. The 3D spatial distribution of fast-ion flux to the DIII-D wall is computed with Monte-Carlo simulations with the full-orbit code SPIRAL, retaining local distributions of energy and impact angle and using a realistic 3D wall, including features as bumper limiters, beam ducts and diagnostic ports. The RustBCA code is used to compute the distribution of the C physical sputtering source, finding contributions at bumper limiters and inner wall from prompt losses and at outer target from partially thermalized ions. Under conservative assumptions on impurity assimilation probability and confinement time, a contribution to Z_eff of the order of 0.1 is inferred. The results suggest a dominant role for the divertor C source, exacerbated by the high electron temperature temperature and C dilution at the divertor targets in this scenario.