Metamorphosis of helical magnetorotational instability in the presence axial electric current

TL;DR

This paper investigates how axial electric currents influence the stability of liquid metal flows in magnetic fields, revealing new electromagnetic instabilities that extend the classical magnetorotational instability.

Contribution

It introduces a novel electromagnetic instability mechanism in helical magnetorotational flows with axial currents, expanding understanding of flow stability in liquid metals.

Findings

Electric current extends HMRI range indefinitely.

Identifies internal pinch-type electromagnetic instability.

Discovers a new steady circulation driven by current-field interaction.

Abstract

This paper presents numerical linear stability analysis of a cylindrical Taylor-Couette flow of liquid metal carrying axial electric current in a generally helical external magnetic field. Axially symmetric disturbances are considered in the inductionless approximation corresponding to zero magnetic Prandtl number. Axial symmetry allows us to reveal an entirely new electromagnetic instability. First, we show that the electric current passing through the liquid can extend the range of helical magnetorotational instability (HMRI) indefinitely by transforming it into a purely electromagnetic instability. Two different electromagnetic instability mechanisms are identified. The first is an internal pinch-type instability, which is due to the interaction of the electric current with its own magnetic field. Axisymmetric mode of this instability requires a free-space component of the azimuthal magnetic field. When the azimuthal component of the magnetic field is purely rotational and the axial component is nonzero, a new kind of electromagnetic instability emerges. The latter driven by the interaction of electric current with a weak collinear magnetic field in a quiescent fluid gives rise to a steady meridional circulation coupled with azimuthal rotation.