Mean electromotrice force for a ring of helical vortices

We study the dynamo mechanism for a flow made of a ring of stationary helical vortices in an electrically conducting media.
The choice of this flow is related to the one obtained in thermal convection in a rotating shell which is also expected in the Earths outer-core.
This choice is also related to a sodium experiment, carried out in Grenoble, based on a spherical Taylor-Couette model.
Applying the mean field approach and relying on the second order correlation approximation we derive the mean electromotive force (e.m.f.) produced by such a flow.
 We find that such a ring of helical vortices may produce, from an azimuthal mean magnetic field,
an azimuthal mean e.m.f. leading to the generation of a poloidal magnetic field.