Simulation of hydrocarbon redeposition in the gaps between divertor tiles

Hydrocarbon redeposition in the gap between divertor tiles is calculated by a Monte Carlo simulation. Using the energy- and species-dependent reflection coefficient on the tile, hydrocarbons repeatedly reflected from the side surfaces redeposit deep in the gap. The calculated results reproduce both the short and long decay lengths of tritium profiles observed on the gap sides of the TFTR bumper limiter [T. Tanabe et al., Fus. Sci. Technol. A 48 (2005) 577]. Redeposition profiles are influenced by the gap width and whether the plasma penetrates the gap or not. At high plasma temperature, the hydrogen concentration of the depositing species in the gap is low due to dissociation of hydrocarbons during long range transport in the plasma. At low plasma temperature, it is high due to deposition of low-energy neutral hydrocarbons, which have a low dissociation probability if the plasma does not penetrate the gap.