Dynamo Efficiency Controlled by Hydro- dynamic Bistability

Hydrodynamic and magnetic behaviors in a modified experimental setup of the von Karman sodium flow - where one disk has been replaced by a propeller - are investigated.When the rotation frequencies of the disk and the propeller are different, we show that the fully turbulent hydrodynamic flow undergoes a global bifurcation between two configurations. The bistability of these flow configurations is associated with the dynamics of the central shear layer. The bistable flows are shown to have different dynamo efficiencies; thus for a given rotation rate of the soft-iron disk, two distinct magnetic behaviors are observed depending on the flow configuration. The hydrodynamic transition controls the magnetic field behavior, and bifurcations between high and low magnetic