A hydrocarbon reaction model for low temperature hydrogen plasmas and an application to the Joint European Torus

A model of collisional processes of hydrocarbons in hydrogen plasmas has be en developed to aid in computer modeling efforts relevant to plasma-surface interactions. It includes 16 molecules (CH up to CH4, C2H to C2H6, and C3H to C3H6) and four reaction types (electron impact ionization/dissociative ionization, electron impact dissociation, proton impact charge exchange, an d dissociative recombination). Experimental reaction rates or cross section s have been compiled, and estimates have been made for cases where these ar e not available. The proton impact charge exchange reaction rates are calcu lated from a theoretical model using molecular polarizabilities. Dissociati ve recombination rates are described by the equation A/T-B where parameter A is fit using polarizabilities and B is estimated from known reaction rate s. The electron impact ionization and dissociation cross sections are fit t o known graphs using four parameters: threshold energy, maximum value of th e 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 (1985)] MARK II carbon inner divertor using the WBC M onte Carlo impurity transport code. The updated version of WBC, which inclu des the full set of hydrocarbon reactions, helps to explain an observed asy mmetry in carbon deposition near the divertor. (C) 2000 American Institute of Physics. [S1070-664X(00)00505-X].