Calculation of material erosion and ion-shadowed area at rough graphite and silicon carbide divertor surfaces

We present a computational investigation of material erosion and ion-shadowed areas as a function of the ion incident angle for rough graphite and silicon carbide divertor surfaces. Surface roughness affects erosion, material deposition, and, hence, plasma-facing component lifetime. Ion angle distributions (IADs) for D plasmas on the NSTX-U and DIII-D divertors were calculated by an equation-of-motion model that traces ion trajectories in the sheath. Then the effective sputtering yields and ion-shadowed area fractions are calculated by a Monte Carlo micro-patterning and roughness code that applies the calculated IADs to surface topographic data obtained from confocal microscopy of rough graphite and SiC surfaces from NSTX-U and DIII-D. The calculations find that the effective sputtering yields, the sputtering pattern, and the shadowed area are determined by the detailed surface topology. The mean surface inclination angle was found to be a useful parameter to estimate the local ion incident angle from the calculated IADs. We report global empirical formulas for the surface erosion and the shadowed area fraction from the main D ions for rough surface configurations of the divertor.