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Influence of high permeability disks in an axisymmetric model of the Cadarache dynamo experiment

Giesecke, A.; Nore, C.; Stefani, F.; Gerbeth, G.; Leorat, J.; Herreman, W.; Luddens, F.; Guermond, J. L.

Numerical simulations of the kinematic induction equation are performed on a model configuration of the Cadarache von-K\'arm\'an-Sodium dynamo experiment. The effect of a localized axisymmetric distribution of relative permeability mur that represents soft iron material within the conducting fluid flow is investigated. The critical magnetic Reynolds number Rmc for dynamo action of the first non-axisymmetric mode roughly scales like Rmc μ-Rmc ∝ μr -1/2 i.e. the threshold decreases as μr increases. This scaling law suggests a skin effect mechanism in the soft iron disks. More important with regard to the Cadarache dynamo experiment, we observe a purely toroidal axisymmetric mode localized in the high permeability disks which becomes dominant for large μr.
In this limit, the toroidal mode is close to the onset of dynamo action with a (negative) growth-rate that is rather independent of the magnetic Reynolds number. We qualitatively explain this effect by paramagnetic pumping at the fluid/disk interface and propose a simplified model that quantitatively reproduces numerical results.
The decise role of the high permeability disks for the mode selection in the Cadarache dynamo experiment cannot be inferred from computations using idealized pseudo-vacuum boundary conditions (H ✕ n =0).

Keywords: dynamo; VKS; permeability; soft iron

Permalink: https://www.hzdr.de/publications/Publ-16362
Publ.-Id: 16362