Molecular Dynamics Simulation of Hydrocarbon Reflection and Dissociation Coefficients from Fusion-Relevant Carbon Surfaces

Physica Scripta, T111, 145-151 (2004).

Alman, D. A., Ruzic, D. N.

Reflection coefficients for carbon and hydrocarbon atoms/molecules on carbon-based surfaces are critically needed for plasma-surface interaction analysis in fusion devices, as carbon will continue to be used in next step devices like ITER. These have been calculated at different energies and angles with a molecular dynamics code using the Brenner hydrocarbon potential. Hydrogen saturated graphite was prepared by bombarding a graphite lattice with hydrogen, until a saturation at ~0.42 H:C. Carbon at 45° has a reflection coefficient (R) of 0.64 ± 0.01 at thermal energy, decreasing to 0.19 ± 0.01 at 10 eV. Carbon dimers (R_thermal = 0.51, R_{>1 eV} ~ 0.10) tend to stick more readily than carbon trimers (R_thermal = 0.63, R_{10 eV} = 0.16). Hydrocarbons reflect as molecules at thermal energies and break up at higher energies. The total reflection via these fragments decreases with energy, the number of unpaired electrons, and changing hybridization from sp³ to sp² to sp. The results compare reasonably well with binary collision modeling for higher energies and experimental sticking data at thermal energies. A second surface, representing a “soft” redeposited carbon layer formed by the deposition of hydrocarbons onto a graphite surface, is also analyzed. In general, reflection is lower from the “soft” surface by 0.1–0.2. This reflection data can and has been incorporated in erosion/redeposition codes to allow improved modeling of chemically eroded carbon transport in fusion devices.