I. Murakami

J.G. Wang

R.K. Janev

T. Kato

National Institute for Fusion Science (NIFS-Repository)

§5. Cross Section Database for Electron-Impact Ionization of Hydrocarbon Molecules Janev, R.K. (Macedonian Acad. of Sci., Skopje), Wang, J.G. (Univ. of Georgia, USA), Murakami, I., Kato, T. Carbon-based materials are currently used as plasma facing materials in the divertors of all medium- and large-size fusion plasma devices. The interaction of divertor plasma with carbon materials produces hydrocarbon molecular fluxes that enter the plasma and degrade its purity 1). The transport and distribution of carbon impurities in divertor region is to a large extent determined by the collision processes of hydrocarbon molecules (and their ions) with plasma electrons and protons. Theoretical modeling of this transport requires accurate cross section information for all processes involved in the transport kinetics. Among the most important collision processes of hydrocarbon molecules CxHy present in divertor plasmas is their electron impact ionization. For typical divertor hydrocarbon molecules x = 1-3 and 1::s; y ::s; 2x+2, and the number of ionization channels for a single molecule may be very large. Besides the direct ionization channel e + CxHy ~ e + CxH/ + e there may be many other dissociative ionization channels involving various fragmentation modes of the intermediate collision complex, (1) e + CxHy ~ e + CH/ + 1: Cni-Ln + e (2) where the sum runs over all neutral fragmentation channels. The number of dissociative channels for the C3Hs molecule has been experimentally determined to be 22. We have undertaken an effort to compile, critically assess and generate a complete cross section database for the ionization processes (1) and (2) for all CxHx molecules with x = 1-3 and 1 ~ y ~ 2x + 2. Only the most important dissociative channels, Eq. (2), were included in our analysis. The experimental cross section data information for the processes (1) and (2) was found very fragmentary, limited mainly to the weakly reactive hydrocarbons and to the ion-production cross sections in reaction (2) (i.e. with no discrimination of neutral fragmentation channels, represented by the sum over n and min Eq. (2)). A limited number of theoretical calculation were available only for the total ionization cross section (for all the channels of (1) and (2)). The missing total and partial (channel) cross sections were generated by formulating and applying certain semi-empirical cross section scaling relationships, based on the "additivity rules" for molecular bonds and associated continuum collision strengths. The generated ionization cross section database for considered CxHy molecules, which covers the energy range from threshold to the MeV region, will soon be published as a NIPS-DATA Report 2). References 1) Hofer, W.O., Roth, J. Eds., "Physical Processes of the Interaction of Fusion Plasmas with Solids". (Academic Press, San Diego, 1996). 2) Janev, R.K., Wang, J.G., Murakami, I., Kato, T. NIPS-DATA (2001, to be published). 377 

Cross Section Database for Electron-Impact Ionization of Hydrocarbon Molecules

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