Direct and dissociative single electron capture in slow collisions of He2+ with O2 and CO

For the next generation of magnetically confined fusion plasma experiments, an important part of the plasma heating will have to be provided by fast alpha particles resulting from D-T fusion reactions. In a so-called burning fusion plasma these He2+ ions transfer their initial 3.5 MeV kinetic energy in series of elastic collisions to the deuterium and tritium fuel ions. After having slowed down, they end up as 'He ash' which needs to be removed from the plasma to avoid inacceptable Bremsstrahlung losses and dilution of the D-T fusion fuel. Removal of the He ash occurs in so far not yet well understood collective plasma processes for which in the outermost plasma region also atomic collisions between He2+ and neutral plasma constituents might be of some relevance. In the plasma scrape-off layer (SOL) there are non-negligible concentrations of hydrogen and oxygen molecules which result from particle- and radiation-stimulated desorption from the first wall components. In collision of He2+ with molecular species, single electron transfer (SEC) is a highly probable process which may leave the product molecular ions in either bound or unbound (dissociated) states. For a satisfactory understanding of the reaction kinetics and energy balance in the edge plasma it is of interest to determine the absolute and relative importance of this non-dissociative and dissociative SEC processes. For this purpose we can apply as experimental method translational energy spectroscopy which determines the kinetic energy change of the (nearly) forward-scattered charge-exchange projectile ions. Since these ions assume most of the inelastic energy loss or -gain from the relevant SEC reaction channels, the latter can be identified in the respective translational energy spectrum (TES). By varying the projectile ions scattering angle one may also select the related impact parameter range and thereby enhance or suppress different SEC channels in relation to each other. For the present measurements, the He2+ projectile ions have been produced from 4He gas in a GHz ECR multicharged ion source. We utilized the SEC reaction He2+ + Ne → He+ (n=1) + Ne+ (2s2p62S) (δE = + 5.9 eV) for calibration of the TES kinetic energy gain-/loss scale, by adding Ne gas to the molecular target gas of interest into the collision cell of our translational energy spectrometer. If the fraction of Ne in the target gas mixture is well known, this method also provides a rather convenient way for determination of the absolute state-selective SEC cross section for the molecular target of interest, by way of comparison with the cross section for the above SEC from Ne. The latter cross section can be very conveniently measured, because at low impact energy the related SEC channel is the only one favoured by the reaction window and therefore correspond with good accuracy to the respective total SEC cross section. (author)