Fluxes of molten metal can generate magnetic fields. This so-called dynamo effect creates cosmic magnetic fields, like those found on planets, moons, and even asteroids. In the years to come, a unique experiment in which a steel drum containing several tons of liquid sodium rotates around two axes is intended to demonstrate this effect. It will be carried out in the new DRESDYN facility at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), an independent German research laboratory. A study recently published in the scientific journal Physical examination letters confirms the chances of success of the experiment.
In the same way that a bicycle dynamo converts motion into electricity, conductive fluids in motion can generate magnetic fields. The so-called magnetic Reynolds number (the product of fluid flow velocity, expansion, and conductivity) primarily determines whether a magnetic field is actually generated.
In a spectacular experiment, scientists from Frank Stefani’s team at the HZDR Institute for Fluid Dynamics aim to achieve the critical value required for the appearance of the dynamo effect. For this purpose, a steel cylinder two meters in diameter containing eight tons of liquid sodium will rotate around one axis up to ten times per second and once per second around another, which is inclined with respect to the first. The technical term for this movement, which is often compared to a tilting top, is precession.
“Our experience in the new DRESDYN installation aims to demonstrate that precession, as a natural motor of flow, is sufficient to create a magnetic field”, explains André Giesecke, lead author of the study. In its simulations and in the accompanying water experiments – the mock-up was six times smaller than the large dynamo – scientists examined the structure of the flow caused by the precession.
“To our surprise, we observed a double-role symmetrical structure in a specific range of the precession rate, which should provide a dynamo effect at a magnetic Reynolds number of 430,” explains the physicist.
Unresolved: the role of precession in geodynamo
The center of the Earth is made up of a solid core surrounded by a layer of molten iron. “The molten metal induces an electric current, which in turn generates a magnetic field,” explains Giesecke. The common belief is that the convection driven by buoyancy, as well as the rotation of the Earth, is responsible for this geodynamo. However, the role played by precession in the formation of the Earth’s magnetic field is still unclear.
The Earth’s axis of rotation is tilted 23.5 degrees from its orbital plane. The axis of rotation changes position over a period of approximately 26,000 years. This precessive movement through space is considered to be one of the possible sources of energy for geodynamo. Millions of years ago, the Moon also had a strong magnetic field, as indicated by rock samples from the Apollo missions. According to experts, precession could be the main cause.
The experiments on liquid sodium at the HZDR are expected to start in 2020. Unlike previous experiments in the geodynamic laboratory, there will be no propeller inside the steel drum, as was used in the first successful experiment. dynamo event in Riga, Latvia in 1999, in which scientists from the HZDR were heavily involved. This experience and others in Karlsruhe, Germany and Cadarache, France provided groundbreaking research for a better understanding of geodynamo.
“In principle, we can define three different parameters for the experiments at DRESDYN: the rotation, the precession and the angle between the two axes,” explains Giesecke. On the one hand, he and his colleagues hope to obtain answers to the fundamental question of whether precession actually produces a magnetic field in a conductive fluid. On the other hand, they are interested in knowing which components of the flux are responsible for creating the magnetic field and the point at which saturation occurs.
Dual role in the Container
“In the simulations, we found that standing inertial waves occur over a wide range of parameters. Within a certain range, however, we have now noticed a characteristic dual-role structure that is proving to be extremely effective for the dynamo effect In principle, we are already aware of such a speed structure thanks to the French dynamo experiment, in which it was artificially produced by two propellers, while in our precession experiment it should emerge naturally. “
HZDR researchers used special ultrasound technology to measure the structure of the flow. “We were very surprised to see how closely the data from the experiment matched the results of the simulation. So we have an extremely robust prediction for the big DRESDYN experiment. For example, we know at what rotational speeds the dynamo effect occurs and what magnetic field structures we can expect, ”says Giesecke.
The scientific community involved in the dynamos is eagerly awaiting the results of the planned experiment, which will run to the limits of technical feasibility in many ways. “We are also awaiting detailed information on the general dynamics of liquid metal fluxes under the influence of magnetic fields. This will allow us to draw conclusions about fluxes in the industrial sector,” according to Giesecke.
Finally, the magnetic flux tomography developed at HZDR as part of its research on dynamo is of interest to many fields of steel foundry and crystallization. The work was funded in part by the Helmholtz Alliance “Liquid Metal Technologies” (LIMTECH).