In this paper a theoretical model is presented, which is based on a pass to
pass analysis of the localized interaction between a short laser pulse wit
h a wider electron distribution. It can be applied to a large class of phys
ical phenomena and, in particular, to the case of a storage-ring free-elect
ron laser (FEL). Numerical results are confirmed by experimental measuremen
ts done on the ACO and Super-ACO FELs.