Absolute experimental and model cross sections for slow ions of very high charge colliding with He

We present absolute experimental cross sections for removing one and two electrons from He by slow projectiles with charge-states ranging between 11 and 31. The one- (two-) electron removal cross section is found to increase faster (slower) than linear in the charge state q. These effects cannot be accounted for by the extended classical over barrier model. We therefore propose a modification of this model which takes a transfer excitation process (capture by the projectile and excitation of the target) into account. This leads to excellent agreement with the experimental results. True double-electron capture is found to be much less important than transfer ionization in removing both electrons from He in its ground state. We argue that the energy-loss per capture event involving highly charged metal impurity ions and helium in a magnetically confined fusion plasma essentially equals the ionization potential Iq-1 of the highly charged product ion. From a comparative study of experiments using Xe as a target, we infer that the energy loss per capture event might be larger for removal of both electrons from excited He (or other more easily ionized atoms). This effect would be due to the larger tendencies for the intermediate doubly excited states to stabilize radiatively. (orig.)