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

TL;DR

This study uses numerical simulations to analyze how high permeability disks influence magnetic field generation in the Cadarache dynamo experiment, revealing a skin effect mechanism and a dominant axisymmetric toroidal mode.

Contribution

It introduces a detailed numerical investigation of the impact of soft iron disks on dynamo thresholds and mode selection, highlighting a skin effect and a new dominant mode.

Findings

The critical magnetic Reynolds number decreases with increasing permeability.

A purely toroidal axisymmetric mode becomes dominant at high permeability.

High permeability disks significantly influence mode selection, unlike idealized boundary condition models.

Abstract

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 {\mu} that represents soft iron material within the conducting fluid flow is investigated. The critical magnetic Reynolds number Rm^c for dynamo action of the first non-axisymmetric mode roughly scales like Rm^c({\mu})-Rm^c({\mu}->infinity) ~ {\mu}^(-1/2) i.e. the threshold decreases as {\mu} 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 {\mu}. 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 crucial 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 x n = 0).