Optimum reduction of the dynamo threshold by a ferromagnetic layer located in the flow

TL;DR

This paper demonstrates that inserting a ferromagnetic layer with an optimal magnetic permeability in a fluid dynamo can lower the critical magnetic Reynolds number, enhancing dynamo onset efficiency, and explains recent experimental observations.

Contribution

It introduces the concept of an optimal ferromagnetic layer permeability to minimize the dynamo threshold, providing a new mechanism for dynamo control.

Findings

Existence of an optimal magnetic permeability for the layer.

Critical magnetic Reynolds number is minimized at this optimal permeability.

Similar effects occur with high electrical conductivity of the layer.

Abstract

We consider a fluid dynamo model generated by the flow on both sides of a moving layer. The magnetic permeability of the layer is larger than that of the flow. We show that there exists an optimum value of magnetic permeability for which the critical magnetic Reynolds number for dynamo onset is smaller than for a nonmagnetic material and also smaller than for a layer of infinite magnetic permeability. We present a mechanism that provides an explanation for recent experimental results. A similar effect occurs when the electrical conductivity of the layer is large.