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

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 disks. 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.

Standard and Helical Magnetorotational Instability

“…In [45][46][47] it was shown that the HMRI traveling wave appears only in the predicted finite window of the magnetic field intensity, with a frequency of the traveling wave that was in rather good accordance with numerical simulations. Some disturbing effects of this early version (PROMISE 1), connected with the recirculating radial jet at midheight of the cylinder, were overcome in the follow-up PROMISE 2 experiment by splitting the axial end caps to suppress the Ekman pumping [48,49]. By comparing experimental and numerical (based on [50]) results for a wide variety of parameter dependencies, it was possible to identify the observed instability as an absolute one, distinguishing it clearly from a noise triggered convective instability as speculated on in [51].…”

Local instabilities in magnetized rotational flows: a short-wavelength approach

“…The significance of the LLL, together with a variety of further predicted parameter dependencies, was experimentally confirmed in the PROMISE facility, a Taylor-Couette cell working with a low Pm liquid metal (Stefani et al , 2007(Stefani et al , 2009. Present exper-imental work at the same device (Seilmayer et al 2014) aims at the characterization of the azimuthal MRI (AMRI), a non-axisymmetric "relative" of the axisymmetric HMRI, which dominates at large ratios of B φ to B z ).…”

Current status and future prospects for laboratory study of angular momentum transport relevant to astrophysical disks

“…The HMRI is an inductionless instability since it survives at the vanishing magnetic Reynolds number limit, in contrast to the standard MRI. It exhibits a form of traveling waves (Liu et al, 2007) which have been subsequently reproduced in the PROMISE experiments (Stefani et al, 2006;Stefani et al, 2009). Once again, the importance of axial boundary conditions was demonstrated: in the original device with significant Ekman circulation the traveling waves were disrupted by returning radial flow slightly above the mid height, while the traveling waves propagate smoothly throughout the flow when Ekman circulation is reduced by modified end caps, see Fig.2.…”