Studies of hydrocarbon cracking by molecular beam injection in He plasmas in the TJ-II stellarator

Hydrocarbons (CH4 and C2H4) have been injected in TJ-II He plasmas by means of the supersonic helium beam set-up typically used for the edge characterization in hydrogen plasmas. The radial profiles of the emission of the CH(A-X) band and Hα were recorded along the hydrocarbon penetration. For all cases, the emission profiles of the recorder species can be fitted to a simple model with one single parameter, related to the velocity along the observation line of the CH and H radicals resulting from the initial cracking of the hydrocarbon. A H2 beam was also used for reference of velocity distribution. The penetration of H atoms, which emit Hα, is in the case of hydrocarbon cracking twice as large as in H2 dissociation, depending on hydrocarbon. A similar radial profile of the CH radicals was found for both hydrocarbons, thus suggesting the existence of a common precursor. The ratio of Hα/CH(A-X) photon emissions for the two hydrocarbons investigated is compatible with a reaction stoichiometry yielding 3 H atoms per CH.