Dynamo regimes and transitions in the VKS experiment

The Von Kármán Sodium experiment yields a variety of dynamo regimes, when asymmetry is imparted to the flow by rotating impellers at different speed F <Subscript>1</Subscript> and F <Subscript>2</Subscript>. We show that as the intensity of forcing, measured as F <Subscript>1</Subscript>+F <Subscript>2</Subscript>, is increased, the transition to a self-sustained magnetic field is always observed via a supercritical bifurcation to a stationary state. For some values of the asymmetry parameter θ=(F <Subscript>1</Subscript>–F <Subscript>2</Subscript>)/(F <Subscript>1</Subscript>+F <Subscript>2</Subscript>), time dependent dynamo regimes develop. They are observed either when the forcing is increased for a given value of asymmetry, or when the amount of asymmetry is varied at sufficiently high forcing. Two qualitatively different transitions between oscillatory and stationary regimes are reported, involving or not a strong divergence of the period of oscillations. These transitions can be interpreted using a low dimensional model based on the interactions of two dynamo modes. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010