A model of collisional processes of hydrocarbons in hydrogen plasmas has been developed to aid in computer modeling efforts relevant to plasma-surface interactions. It includes 16 molecules ͑CH up to CH 4 , C 2 H to C 2 H 6 , and C 3 H to C 3 H 6) and four reaction types ͑electron impact ionization/ dissociative ionization, electron impact dissociation, proton impact charge exchange, and dissociative recombination͒. Experimental reaction rates or cross sections have been compiled, and estimates have been made for cases where these are not available. The proton impact charge exchange reaction rates are calculated from a theoretical model using molecular polarizabilities. Dissociative recombination rates are described by the equation A/T B where parameter A is fit using polarizabilities and B is estimated from known reaction rates. The electron impact ionization and dissociation cross sections are fit to known graphs using four parameters: threshold energy, maximum value of the cross section, energy at the maximum, and a constant for the exponential decay as energy increases. The model has recently been used in an analysis of the Joint European Torus ͓P. H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 ͑1985͔͒ MARK II carbon inner divertor using the WBC Monte Carlo impurity transport code. The updated version of WBC, which includes the full set of hydrocarbon reactions, helps to explain an observed asymmetry in carbon deposition near the divertor.

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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 458 has a reflection coefficient ðRÞ of 0:64 AE 0:01 at thermal energy, decreasing to 0:19 AE 0:01 at 10 eV. Carbon dimers ðR thermal ¼ 0:51; R >1 eV $ 0:10Þ tend to stick more readily than carbon trimers ðR thermal ¼ 0:63; R 10 eV ¼ 0:16Þ: 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 sp 3 to sp 2 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.

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