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Higher Accuracy in Computer Simulation of the VKS Experiment

In 2007, the Von-Karman-Sodium (VKS) experiment in Cadarache showed that a macroscopic magnetic field can emerge spontaneously within a turbulent conductive liquid (by "dynamo" effect). New high-definition simulations of VKS highlight the influence of liquid resistivity and the nature of the turbines that agitate the liquid. These recent simulations contribute to a better understanding of the magnetic field of stars and certain planets.

Published on 31 May 2017

The VKS collaboration (between CEA, CNRS, ENS Paris and ENS Lyon) has endeavored to observe the dynamo effect in a laboratory: in the experiment, two turbine blades, located on each side of a cylinder filled with sodium, stir the conductive liquid into motion. Beyond a certain turbulence threshold, a macroscopic magnetic field emerges spontaneously. Depending on the turbulence regime obtained, magnetic field inversions can also occur, as is the case for the Earth on a geological scale, and for the Sun, approximately every 11 years. With this result, VKS was recognized as one of the most important experiments of the last ten years by the journal Physical Review Letters.

An international team involving scientists from IRAMIS and IRFU has carried out a new series of computer simulations with high spatial resolution for VKS. They show the influence of liquid resistivity and the nature of the turbine impellers on the generation and directional character (collimation) of the magnetic field. "We have found that for a perfectly ferromagnetic material, there is an effective increase in magnetic field collimation, supporting the emergence of the field, as shown in the experiment," said Jacobo Varela, a scientist now working at Oak Ridge National Laboratory, USA, after a postdoc at IRFU. A perfectly conductive material, on the other hand, reduces this collimation," he said. This explains why, in the experiment, the dynamo effect is more easily triggered with soft-iron impellers (ferromagnetic).

The researchers are further developing their model, which includes adding the shape of the impellers, to achieve a more accurate description of the geometry of the experiment and to provide feedback on optimizations of the experimental VKS system.

Effects of turbulence, resistivity and boundary conditions on helicoidal flow collimation: consequences for the Von-Kármán-Sodium dynamo experiment, Physics of Plasma.

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