Chaotic and cooperative regimes for the micromaser.

We describe the micromaser in the framework of an exact semiclassical model for regular injection, allowing the case of more than one atom at a time in the cavity to be considered. For high pumping and high atom fluxes we find a new kind of phase transition corresponding to a strongly nonlinear chaotic behavior. In addition to the customary period-doubling route to chaos, unexpected scenarios are exhibited, which are a consequence of the multistable competition between different attractors for time evolution