Helical magnetorotational instability in a Taylor-Couette flow with strongly reduced Ekman pumping

TL;DR

This study demonstrates the observation of helical magnetorotational instability (HMRI) in a Taylor-Couette flow with reduced Ekman pumping, showing a more global and sharply defined instability consistent with numerical models.

Contribution

The paper introduces an improved experimental setup with split end-caps that significantly reduces Ekman pumping, enabling clearer observation of HMRI across the entire cell height.

Findings

HMRI observed as a global instability rather than noise-driven.

HMRI appears sharper and aligns better with numerical predictions.

Reduced Ekman pumping allows for more accurate study of HMRI.

Abstract

The magnetorotational instability (MRI) is thought to play a key role in the formation of stars and black holes by sustaining the turbulence in hydrodynamically stable Keplerian accretion discs. In previous experiments the MRI was observed in a liquid metal Taylor-Couette flow at moderate Reynolds numbers by applying a helical magnetic field. The observation of this helical MRI (HMRI) was interfered with a significant Ekman pumping driven by solid end-caps that confined the instability only to a part of the Taylor-Couette cell. This paper describes the observation of the HMRI in an improved Taylor-Couette setup with the Ekman pumping significantly reduced by using split end-caps. The HMRI, which now spreads over the whole height of the cell, appears much sharper and in better agreement with numerical predictions. By analyzing various parameter dependencies we conclude that the observed HMRI represents a self-sustained global instability rather than a noise-sustained convective one.