J.G. Wang

R.K. Janev

T. Kato

National Institute for Fusion Science (NIFS-Repository)

§4. Cross Section Database for Proton Charge Transfer Reactions with Hydrocarbon Molecules Janev, R.K. (Macedonian Acad. Sci., Skopje), Kato, T., Wang, J.G. (Univ. of Georgia, USA) In most presently operating medium- and large-size fusion devices the plasma facing components in the divert or (divert or plates and side walls) contain carbon-based materials (graphite, carbon-carbon composites, etc.). The interaction of divertor hydro genic plasma with these materials generates significant amounts of hydrocarbon molecules (mainly by chemical erosion) that enter the plasma. The composition of these fluxes depends on the energy of hydro genic particles bombarding the carbon surfaces and on the surface temperature 1). At bombarding energies in the range 30 - 500 e V, the lighter hydrocarbons, such as CH3, CH4 and C2Hz, are preferentially formed, while with decreasing the impact energy towards 1 -10 eV, heavier hydrocarbon molecules, such as CzH4, CzH6, C3H4, C3H6 and C3Hs, become Increasingly dominant in the hydrocarbon fluxes from the walls. Once in the plasma, these molecules are subjected to various collision processes with plasma electrons and protons. Most of these processes lead to dissociation of reacting molecule and result in generation of all types of hydrocarbon molecules CxHy, independent of the composition of original erosion influxes. (Obviously, x and y should be smaller than Xmax and Ymax of the most complex CxHy molecule coming from the walls.) The proton impact charge transfer reactions with hydrocarbon molecules H+ rH H + rHy+ + '--'X y ~ '--'X (1) At the collision energies pertinent to divertor plasma temperatures (0.5- 20 eV), the charge transfer reactions have significant cross sections, particularly when the number of 376 hydrogen atoms in the molecule is large. This is mainly due to the large values of polarizabilities of hydrocarbon molecules, the exothermic character of reaction (1) for all CxHy molecules and the existence of internal degrees of freedom (vibrational and rotational) of CxHy+ product ions, which can absorb the reaction exothermicity. This last circumstance facilitates the fulfillment of energy resonance condition for the reaction, resulting in a large value of its cross section. In an effort to establish a complete database for the cross sections of reactions (1) with x = 1 - 3 and y ~ 2x + 2, we have compiled and critically assessed the sparse experimental and theoretical cross section data from the literature, and on their basis derived semi-empirical cross section scaling laws to generate the unavailable cross sections for most of the reactions of type (1 ). For the molecules CxHy withy~ 2x-2, it was found that reaction (1) in the energy region below ---20 keV has a resonant character, with cross f . d 10-15 2 section of the order o magn1tu e em (and larger in the eV region). For y < 2x-2, the resonant condition for the reaction (1) is not fulfilled and the corresponding cross sections are relatively small. However, the large polarizabilities of CxHy molecules nevertheless ensure large cross sections(~ 10-16 cm2) in the sub-e V energy region even for this case. The generated proton charge transfer cross section database for all CxHy (x = 1-3, 1~ y ~ 2x+2) has been published in Ref.2. References 1) Haasz, A.A. et al., Atom. Plasma-Mater. Int. Data Fusion, 7, (Part A) 5 (1998). 2) Janev, R.K. Kato, T., Wang, J.G., NIPS-DATA, 64 (2001). 

Cross Section Database for Proton Charge Transfer Reactions with Hydrocarbon Molecules

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