The Magnetorotational Instability (MRI): Observation in a Mass/Spring System and the Effects of Conductive Boundaries on a Free Stewartson-Shercliff Layer as a Step Towards MRI in a Liquid Metal*

The magnetorotational instability (MRI) has been proposed as a powerful mechanism for rapid angular momentum transport in many accretion disks, but has not been confirmed by observation or experiment. The PPPL apparatus was designed to identify the MRI mechanism in a magnetized liquid metal in a Taylor-Couette flow. A water-filled device was used to directly observe the MRI mechanism by measuring the angular momentum growth of a mass tethered to a spring, confirming the validity of the picture of MRI that is often offered as an explanation of the mechanism¹. The liquid-metal-filled apparatus operates with conductive endcaps to reinforce the MRI-unstable mean flow, increasing the saturation amplitude of the MRI. SFEMaNS code results are presented that demonstrate this improvement and suggest how to distinguish MRI from residual Ekman flows². The stronger field/galinstan coupling has been experimentally observed to modify the Shercliff layer instability³. Previous measurements, using insulating endcaps, showed that the instability occurs when the Elsasser number exceeds unity⁴. New measurements, using conducting endcaps, are presented showing that stronger coupling reduces the threshold field for the instability. However, the line-tying of the field to the endcaps and the galinstan supports the shear layer, resulting in a highly-sheared mean flow with significant fluctuations. Guided by simulations, the shear profile of the experiment can be changed to mitigate the centrifugal instability during the search for the MRI signature.  Recent progress on identifying the MRI mechanism in liquid metal experiments will also be presented.

¹ D. M. H. Hung et al., arXiv:1801.03569

² X. Wei et al., PRE 94, 063107 (2016)

³ K. Caspary et al., PRE 97, 063110 (2018)

⁴ A. Roach et al., PRL 108, 154502 (2012)