Impurity release and transport from a toroidally symmetric limiters in DIII-D

Dual toroidal limiters will be inserted into the outer mid-plane of the DIII-D tokamak to symmeterize the far scrape-off layer. The limiters will consist of graphite tiles and may eventually be replaced by silicon carbide (SiC) tiles. Due to the large uncertainty of plasma conditions in the far scrape-off layer, impurity release and tranport from the limiters remains an outstanding question. Impurity release is modeled with a homogenous mixed material model with particle-surface interactions according to the RustBCA sputtering model. Subsequent impurity transport and redeposition is modeled with the Global Impurity Transport (GITR) 3D Monte Carlo code. The fractional release and deposition of eroded particles are measured as functions of anticipated driving forces in the far scrape-off layer: temperature, density, flow velocity, and perpendicular transport. It is found that due to the low temperature and plasma densities, impurities undergo substantial long-range transport as opposed to the prompt redeposition observed in divertor regions. The long-range transport of impurities, from both the limiters and the current DIII-D configuration, leads to highly localized deposition in locations far from the outer mid-plane. SiC tiles result in less C erosion than the pure C case, leading to the formation of Si-rich interaction regions in the surface layers that may improve the oxygen gettering properties of the main wall.