Eur. Phys. J. B 77, 459-468 (2010)
https://doi.org/10.1140/epjb/e2010-00272-5

Dynamo regimes and transitions in the VKS experiment

¹ Laboratoire de Physique Statistique, CNRS & École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France
² Laboratoire de Physique de l'École Normale Supérieure de Lyon, CNRS & Université de Lyon, 46 allée d'Italie, 69364 Lyon Cedex 7, France
³ Service de Physique de l'État Condensé, CNRS & CEA Saclay, 91191 Gif-sur-Yvette, France
⁴ Laboratoire des Écoulements Géophysiques et Industriels, CNRS & Université Joseph Fourier, BP53, 38041 Grenoble Cedex 9, France

Corresponding author: ^a podier@ens-lyon.fr

Received: 17 April 2010
Published online: 22 September 2010

Abstract

The Von Kármán Sodium experiment yields a variety of dynamo regimes, when asymmetry is imparted to the flow by rotating impellers at different speed F₁ and F₂. We show that as the intensity of forcing, measured as F₁+F₂, 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 = (F₁–F₂)/(F₁+F₂), 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.