Large scale dynamos with helicity loss through boundaries

Dynamo action is investigated in simulations of locally isotropic and homogeneous turbulence in a slab between open boundaries. It is found that a "pseudo-vacuum" boundary condition (where the field is vertical) leads to strong helicity fluxes which significantly reduce the amplitude of the resulting large-scale field. On the other hand, if there is a conducting halo outside the dynamo-active region, the large scale field amplitude can reach larger values, but the time scale after which this field is reached increases linearly with the magnetic Reynolds number. In both cases, most of the helicity flux is found to occur on large scales. From the variety of models considered we conclude that open boundaries tend to lower the saturation field strength compared to the case with periodic boundaries. The rate at which this lower saturation field strength is attained is roughly independent of the strength of the turbulence and of the boundary conditions. For dynamos with less helicity, however, significant field strengths could be reached in a shorter time.