[en] Starting from the experimental data of the Micromafios ion source, a new model has been built to explain the observed C.S.D. The effect of plasma on neutral density is analysed. The relation between C.S.D. in the plasma and in the extracted beam is recalled. Electrons and ions collision rates are evaluated, including temperature effects. A physical description in the confinement mechanism is given, from which diffusion times are evaluated. The model is compared with experimental data, showing good agreement. The effect of parameters variations is investigated, and C.S.D. are computed for the ECREVIS source presently under construction. Finally, a very simple method is presented to investigate magnetic bottle design, by reconstruction of field lines. (orig.)