Monte Carlo simulation of hydrocarbon redeposition on a graphite surface in hydrogen plasma

To investigate the hydrocarbon redeposition process on the graphite tile surface, we developed a modeling code that considers a complete set of collisional reactions in the plasma, energy- and species-dependent reflection coefficients, and fragmentation at the tile surface. The model calculations revealed local redeposition characteristics. The redeposition rate, which is the number of redeposited particles on the tile surface per launched CH4 molecule, decreases with decreasing plasma temperature due to a steep decrease in the rate coefficients for electron impact ionization at temperatures of less than 10 eV. At elevated plasma temperatures, large numbers of singly or multicharged carbon ions are redeposited because of the high sticking coefficient of the multicharged ions, which is strongly accelerated by the sheath field. The hydrogen concentration of the deposited species increases because of a decrease in the number of dissociative reactions in the plasma with decreasing plasma density. Using energy- and species-dependent sticking coefficients, low-energy ion species are reflected by the tile surface where dissociation occurs and are subsequently dissociated due to transport in the plasma; accordingly, the hydrogen concentration of deposited species is low. (author)