Controlling chaos with parametric perturbations

We consider the effects of parametric perturbation on the onset of chaos in different dynamical systems. Favoring or suppression of chaos was observed depending on the phase or the frequency of the periodic perturbation. A lo wering of the threshold of chaos was observed in an electronic device simul ating a Josephson-Junction model and the suppression of chaos was obtained in a bistable mechanical device. We showed that in case of spatial instabil ity in a sample of liquid crystal, the action of the parametric perturbatio n is to modify the velocity and the onset of the defects. Considering that the emergence of defects precedes the threshold of spatio- temporal chaos, we infer that parametric perturbation can modify the thresh old of chaos in this spatial dynamical system.