Lorentz force effects in the Bullard–von Kármán dynamo: saturation, energy balance and subcriticality

We report an experimental study of a turbulent dynamo in a liquid metal flow. The semi-synthetic dynamo is achieved thanks to an induction process generated by the turbulent shearing motion of liquid gallium and a feedback loop with external amplification, using coils. The external amplification allows the excitation of the dynamo instability at magnetic Reynolds numbers of order-one. This semi-synthetic dynamo is studied here in a regime where saturation is achieved when Lorentz forces modify significantly the bulk flow structure. We describe the supercritical bifurcation, intermittent and saturated regimes, the scalings of the dynamo magnetic field and we detail the power budget. We also report self-killing dynamos for which the dynamo magnetic field cannot be sustained, when the flow is dominated by the action of Lorentz forces, and subcritical regimes in which the flow only sustains a dynamo when it is already dominated by the action of Lorentz forces.