The VKS Experiment: A Turbulent Dynamo

The VKS experiment studies experimentally the dynamo effect in the flow generated inside a cylinder filled with liquid sodium by the rotation of coaxial impellers (the von Karman geometry). We first report observations related to the self-generation of a stationary dynamo when the flow forcing is symmetric, i.e. when the impellers rotate in opposite directions at equal angular velocities. The bifurcation is found to be supercritical, with a neutral mode whose geometry is predominantly axisymmetric.
We discuss observations of dynamo generations for asymmetric forcings, i.e. when global rotation is imparted to the flow. Time-dependant magnetic fields are generated, with a dynamics which shares similarities with low dimensional dynamical systems.