VKS-dynamo – VKS

CONTEXT Self-generation of a magnetic field by the flow of an electrically conducting fluid has been proposed by Larmor in 1919 as a possible mechanism for the magnetic field of the sun. Such fluid dynamos have first been displayed at laboratory scale in 2000 in Karlsruhe and Riga by strongly constraining the geometry of the flow lines within solid boundaries. Our group has obtained the first dynamo generated by a large scale turbulent flow of liquid sodium without strong geometrical constraints in 2006 (the VKS experiment). The experiment also displayed, the first occurrence in the laboratory of magnetic field reversals that share some similarities with the Earth magnetic field. Several other types of dynamics have also been observed depending on the flow forcing. The VKS-dynamo project is aimed at quantitatively studying these different dynamos. Upgraded magnetic field measurements, new velocity measurement techniques together with global flow characterisation will be developed. They will enable us to fully characterize the observed dynamo modes and their non-linear interactions. They will also allow us to study the precise influence of the boundary conditions (electrical, magnetic) on the dynamo threshold and saturation. OPEN QUESTIONS They are related to the influence of the flow turbulence, since liquid metal flows capable of dynamo action are strongly turbulent (at laboratory scale). • A first important question is to understand the respective roles of the mean and fluctuating parts of the velocity field. In particular, as in the case of phase transitions, it is of interest to study the scaling laws of the mean field and of the moments of its fluctuations above the dynamo threshold. Is there a breakdown of Landau theory of instabilities in the vicinity of the dynamo threshold due to the presence of strong turbulent fluctuations? It is one of the main motivations for achieving self-generation without using high magnetic permeability boundaries (which make the bifurcation imperfect). • Another factor concerns the influence of system-wide rotation. There is a widespread belief that rotation may assist generation by lowering the dynamo threshold. There is however no clear-cut demonstration or experimental evidence of this behaviour. It can be studied in VKS by running with various flow forcing schemes. • The back-reaction of the magnetic field on the flow leads to the saturation of the growth of the magnetic field. It is often believed that the velocity fluctuations are first affected at small scales when their kinetic energy density is of the order of the magnetic energy density of the growing field. However, this has not been studied experimentally. • A last aspect concerns the dynamics of the large scale magnetic field above the dynamo threshold, and in particular, the possible mechanisms for field reversals. Possible theoretical models invoke the escape mechanism from a metastable state in the presence of noise, relaxation oscillators or heteroclinic orbits in low dimensional dynamical systems. These issues should be clarified from the analysis of experimental data. WORKPLAN The first part of our project consists of a detailed study of the dynamo regimes found so far. A large effort will be devoted to instrumentation and additional measurements will be performed in order to get a better knowledge of the correlations between the velocity and the magnetic fields, the power consumption of the different types of dynamos, etc, so as to address the issues above. The second part of our project concerns the generation of a dynamo without using ferromagnetic boundaries. Until now, the VKS experiment has displayed self-generation only when soft-iron impellers have been used. Several modifications of the set-up will be performed in order to get a better understanding of the effect of boundaries with a high magnetic permeability on the dynamo effect. IMPACT From a fundamental point of view, the detailed measurements that will be performed on the observed VKS dynamo regimes will lead to a precise understanding of their growth, saturation and dynamics above threshold. In particular, we expect a significant increase of our understanding of mechanisms for field reversals both at laboratory scale and for natural dynamos. If successful, the observation of self-generation in a turbulent flow without using boundaries with high magnetic permeability will be an important step. On a more experimentally-oriented note, the instrumentation that will be developed in the framework of the present project will be of primary interest for the study of liquid metal flows as envisioned, for instance, for fourth generation nuclear reactors.