Spatial Features of the Transition to Chaos in an Extended System

The study of the transition to chaos in convection experiments has provided much fundamental information concerning evidence of non linear scenarios. This is especially true for experiments dealing with a small number of convective rolls where the spatial organization remains unchanged during the transition. However, turbulence which presents a temporal chaotic behaviour also has spatial features. Attempts have been made, in extended convective patterns, to characterize the transition to time dependence of the flow. We have set-up an experiment in pressurized argon where we have shown that the occurrence of time dependence is directly related to repetitive nucleation of paris of dislocations in the structure. This mechanism is the first step to a more chaotic phase turbulence. We shall describe this mechanism and discuss the nucleation of localized pattern defects related to delocalized roll pattern instabilities.