Dynamo regimes and transitions in the VKS experiment - Berhanu, M. and Verhille, G. and Boisson, J. and Gallet, B. and Gissinger, C. and Fauve, S. and Mordant, N. and Petrelis, F. and Bourgoin, M. and Odier, P. and Pinton, J. -F. and Plihon, N. and Aumaitre, S. and Chiffaudel, A. and Daviaud, F. and Dubrulle, B. and Pirat, C.
EUROPEAN PHYSICAL JOURNAL B 77, 459-468 (2010)
Abstract : The Von Karman Sodium experiment yields a variety of dynamo regimes,
when asymmetry is imparted to the flow by rotating impellers at
different speed F (1) and F (2). We show that as the intensity of
forcing, measured as F (1)+F (2), 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 theta = (F (1)-F (2))/(F (1)+F (2)), 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.