On the axissymmetric dominance of the magnetic field in the VKS dynamo experiment.

In order to understand the results of recent dynamo experiments, the behavior of kinematic dynamos in cylindrical geometries is analyzed. Simulations are performed applying a hybrid finite volume/boundary element method that allows a stringent treatment of insulating boundary conditions.

A suitable prescribed velocity field, either analytic or -- more realistic -- from measurement data of water experiments, leads to dynamo action if a critical value for the magnetic Reynolds number is exceeded.

In case of an axisymmetric velocity field the simulations always result in a non-axisymmetric eigenfield which is dominated by the azimuthal m=1-mode. However, in contradiction to this expected result, the experimental realisation exhibits an axissymmetric field configuration.

Until today, no satisfactory explanations for the dominating m=0-mode are established. A recently presented approach is based on an alpha-effect caused by helical fluid motions between the impeller blades that drive the flow. However, it turned out, that the necessary
amplitude which is required for m=0 dominated solutions is well above realistic values that might be realized in the experiment.

Further potential explanations involve non-axissymmetric contributions either caused by a drifting large scale vortex structure as observed in water experiments or introduced through the azimuthal varying high permeability region from the ferrous impeller blades, which should ideally give rise to a strong axisymmetric azimuthal field component within the impeller region.