Why the interactions of fast ions with foils are distinct from charge exchange

The intensity patterns of some Rydberg line series in the X-ray spectra of foil-excited fast ion beams bear a resemblance to patterns seen with SN remnants or after charge exchange (CX). Closer scrutiny of the ion-foil interaction process reveals why this is no more than a chance resemblance, because the underlying processes are very different. However, electron beam ion traps are suitable for CX observations.