  The paper reports on the observation of a new type of instabilities in the
atomic cloud of a magneto-optical trap (MOT). They appear as large amplitude
self-oscillations, either periodic or erratic, depending on the parameters. An
original model taking into account the shadow effect and an homogeneous
distribution of the atoms in the cloud is developped. It allows us to show that
these instabilities originate on the one hand from an anomalous Hopf
bifurcation, and on the other hand from the extreme sensitivity of the system
to noise.
