Interplay of Ion Impact Angle and Plasma Parameters in Predicting ELM-resolved Carbon Deposition and Tungsten Erosion in DIII-D

Using a near surface mixed material model, a shift of the average ion impact angle (θ) by several degrees towards the surface normal of the DIII-D Small Angle Slot V-shaped W (SAS-VW) divertor is predicted to increase the fractional carbon (C) coverage (σ_C) of the W divertor by tens of percent. Particularly, reflection coefficients (R) and sputtering yields (Y) of C are highly sensitive to θ at background plasma-thermalized ion impact energies (E_i). Without assessment of σ_C, our models vastly overpredict gross W erosion (Γ_W), especially during an Edge Localized Mode (ELM).

We predict higher σ_C for plasma discharges with higher C plasma impurity concentrations (%) and lower electron temperature (T_e) at the divertor. In an experiment, we injected varying rates of D₂ at the outboard midplane and assessed the impact on SAS-VW divertor σ_C and Γ_W. Binary collision code RustBCA computes R and Y of C and D ions on C and W materials. Variation in θ and E_i have a large effect on Y and R to influence σ_C and Γ_W.

With different ion sheath models, we provide novel estimations of C and D θ and consequent σ_C on a smooth W divertor surface with respect to varying key plasma parameters. WYKO profilometry obtained topographical data of the SAS-VW tiles to correct θ predictions for rough surfaces.