Net versus gross erosion of silicon carbide in DIII-D divertor

Abstract

Gross and net erosion rates of silicon from silicon carbide (SiC) coatings were measured in the divertor of DIII-D under well diagnosed reactor-relevant plasma conditions. Amorphous and crystalline SiC coatings on graphite with thickness of ∼80 nm and ∼250 μm, respectively, were exposed near an attached outer strike point of lower single null L-mode plasmas using the Divertor Material Evaluation System (DiMES). Plasma density and electron temperature near the center of the coatings were n e ∼ 4 × 1019 m-3 and T e ∼ 23 eV. Gross erosion of Si from all samples was measured spectroscopically using the Si II 636 nm line. It was found to be a factor of ∼4 higher for the amorphous coatings compared to the crystalline one. The thin amorphous coatings allowed measurements of net Si erosion with Rutherford backscattering. Net average Si erosion rate of ∼3 × 1016 cm-2s-1 was measured on the amorphous coatings with toroidal extent of 1 mm, where, according to ERO code modeling, the local redeposition of Si was about 30%. Using this rate, spectroscopic measurements, measured D+ ion fluxes, and corrections from ERO-OEDGE modeling, effective SXB coefficient for the Si II 636 nm line of ∼52 and Si sputtering yield of ∼0.017 Si/D were calculated. Deuterium retention on SiC coatings was measured by 2.5 MeV 3He nuclear reaction analysis at 0.5-2.5 × 1017 atoms cm-2, consistent with retention due to implantation into a surface undergoing net erosion.