Dynamo regimes and transitions in the VKS experiment

TL;DR

This paper investigates various dynamo regimes in the VKS experiment, showing how flow asymmetry and forcing influence transitions between stationary and oscillatory magnetic fields, explained by a low-dimensional dynamo mode interaction model.

Contribution

It reveals how flow asymmetry and forcing intensity induce different dynamo regimes and transitions, supported by a low-dimensional model explaining the observed phenomena.

Findings

Transition to self-sustained magnetic field is supercritical and stationary.

Time-dependent dynamo regimes occur with certain asymmetries and forcing levels.

Transitions between oscillatory and stationary regimes involve different bifurcation behaviors.

Abstract

The Von K{\'a}rm{\'a}n 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 $\\theta$ = (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.