Experimental realization of dynamo action: present status and prospects

Giesecke Andre a.giesecke@hzdr.de Helmholtz-Zentrum Dresden-Rossendorf Germany
Stefani Frank, Helmholtz-Zentrum Dresden -Rossendorf
Gundrum Thomas, Helmholtz-Zentrum Dresden -Rossendorf
Gerbeth Gunter, Helmholtz-Zentrum Dresden -Rossendorf
Nore Caroline, Laboratoire d’Informatique pour la Mecanique et les Sciences de lIngenieur, Orsay, France
Leorat Jacques, Observatoire de Paris-Meudon


Abstract
Cosmic magnetic fields are an ubiquitous phenomenon arising in and around many astrophysical objects like planets, stars, or galaxies. There is a common consensus that these fields are generated by (mostly) turbulent flows of conducting liquids or plasmas. Whereas dynamo action in the astrophysical context seems to be quite simple, essentially because of the large scales of the involved flows, the experimental realization of dynamo action on typical laboratory scales is a demanding task. So far only three facilities have been able to demonstrate fluid flow driven self-generation of magnetic fields. The first dynamos in Riga and in Karlsruhe were characterized by non-axisymmetric geometries of the eigenfield which can be well explained utilizing a laminar flow structure or a mean-field model, respectively. More dynamical effects like bursts, oscillations or sudden field reversals have been observed in the von-Karman-Sodium (VKS) dynamo experiment conducted at Cadarache in France. In that experiment a flow of liquid sodium is driven by two opposing impellers located close to the lids of a cylindrical container. However, dynamo action is obtained only when at least one of the flow driving impellers is made of soft iron with a relative permeability around ~65. In contrast to the previous experiments the observed magnetic field geometry is dominated by an axisymmetric mode which is in contradiction with the expectations from simulations as well as with the restrictions from Cowling's anti-dynamo theorem. Our kinematic simulations of an axisymmetric model of the Cadarache dynamo show a close linkage between the exclusive occurrence of dynamo action in the presence of soft iron impellers and the axisymmetry of the magnetic field. We observe two distinct classes of axisymmetric eigenmodes, a purely toroidal mode that is amplified by paramagnetic pumping at the fluid-impeller interface, and a mixed mode consisting of a poloidal and a toroidal contribution that is rather insensitive to the impeller permeability. In the limit of large permeability, the purely toroidal mode is close to the onset of dynamo action with a slightly negative growth-rate that is rather independent of the flow field. However, since in our axisymmetric configuration the purely toroidal mode is decoupled from any poloidal field component no dynamo action can be expected from this mode alone. Thus, a satisfying explanation of the observed axisymmetric dynamo mode requires mean field effects like the alpha-effect. Since the flow is considerably turbulent such effects are undoubtedly operative, however, so far their properties (e.g. spatial distribution or amplitude) are only speculative. Further progress in the experimental examination of dynamo action is expected from the planned liquid sodium facility DRESDYN (DREsden Sodium facility for DYNamo and thermohydraulic studies). Within this framework, a homogeneous dynamo, driven exclusively by precession, will represent the most ambitious compound. We present recent results of preparatory water experiements and design studies, and delineate the scientific prospects for the final set-up.