  The saturated dynamics of a Single-Pass Free Electron Laser is considered
within a simplified mean-field approach. A method is proposed to increase the
size of the macro-particle, which is responsible for the oscillations of the
intensity of the wave. This approach is based on the reconstruction of
invariant tori of the dynamics of test particles. To this aim a dedicated
control term is derived, the latter acting as a small apt perturbation of the
system dynamics. Implications of these findings are discussed in relation to
the optimization of the laser source.
