Gross and Net Erosion of Silicon from SiC-coated PFCs in the DIII-D Divertor.

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 a thickness of 80 nm and 250 microns, respectively, were exposed near an attached outer strike point of L-mode plasmas using the Divertor Material Evaluation System (DiMES). Plasma density and electron temperature near the center of the coatings were $n_{e\thinspace }=$ 4x10$^{\mathrm{19\thinspace }}$m$^{\mathrm{-3}}$ and $T_{e\thinspace }=$ 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, possibly because of the higher surface binding energies in the latter. An average net Si erosion rate of 3x10$^{\mathrm{16\thinspace }}$cm$^{\mathrm{-2}}$s$^{\mathrm{-1}}$ was measured with Rutherford backscattering on the amorphous coatings with toroidal extent of 1 mm, in good agreement with ERO-OEDGE modeling. Using this rate and corrections from the modeling, an effective SXB coefficient for the Si II 636 nm line of 52 and a Si sputtering yield of 0.017 Si/D were calculated. Compared to previous DiMES measurements of net C erosion from pure carbon coatings at 3 times lower $n_{e}$ and comparable $T_{e}$, the effective reduction of the C erosion rate from SiC is about an order of magnitude. Recently experiments with SiC were repeated at lower $n_{e}$; data analysis is in progress.