Influence of electromagnetic boundary conditions onto the onset of dynamo action in laboratory experiments

The influence of electromagnetic boundary conditions on the onset of dynamo action of the Riga and Karlsruhe experiments with the addition of an external wall was studied. A wall of different thickness, conductivity and permeability was considered. The first experiment was monocellular with a dynamo mechanism based on a laminar kinematic approach and the second was multicellular with scale separation between the flow and magnetic field. In Karlsruhe experiments, the addition of an external wall with a conductivity larger than the fluid conductivity or the permeability larger than vacuum led to a reduction of the dynamo instability threshold.