Identification of a non-axisymmetric mode in laboratory experiments searching for standard magnetorotational instability

TL;DR

This paper reports the first observation of a non-axisymmetric magnetohydrodynamic instability in a laboratory Taylor-Couette setup, providing experimental clues towards confirming the astrophysically significant standard magnetorotational instability.

Contribution

It presents experimental evidence of a non-axisymmetric instability potentially related to SMRI, supported by numerical simulations, advancing understanding of MHD instabilities in laboratory conditions.

Findings

Observed non-axisymmetric MHD instability in liquid-metal flow.

Instability growth matches numerical simulations and occurs under specific shear and magnetic field conditions.

Results suggest conditions for SMRI are achievable below predicted thresholds.

Abstract

The standard magnetorotational instability (SMRI) is a promising mechanism for turbulence and rapid accretion in astrophysical disks. It is a magnetohydrodynamic (MHD) instability that destabilizes otherwise hydrodynamically stable disk flow. Due to its microscopic nature at astronomical distances and stringent requirements in laboratory experiments, SMRI has remained unconfirmed since its proposal, despite its astrophysical importance. Here we report a nonaxisymmetric MHD instability in a modified Taylor-Couette experiment. To search for SMRI, a uniform magnetic field is imposed along the rotation axis of a swirling liquid-metal flow. The instability initially grows exponentially, becoming prominent only for sufficient flow shear and moderate magnetic field. These conditions for instability are qualitatively consistent with SMRI, but at magnetic Reynolds numbers below the predictions of linear analyses with periodic axial boundaries. Three-dimensional numerical simulations, however, reproduce the observed instability, indicating that it grows linearly from the primary axisymmetric flow modified by the applied magnetic field.