Characterization of the nonaxisymmetric MHD mode in the Princeton magnetorotational instability experiment using simultaneous velocity and magnetic field measurements

We report comprehensive experimental and numerical characterization of the recently identified magnetohydrodynamic (MHD) instability in a modified Taylor-Couette experiment using Galinstan as the working fluid [Wang et al., Nat. Commun. 13, 4679 (2022)]. In the experiment, the inner cylinder, outer cylinder and upper (lower) endcaps corotate independently at an fixed angular velocity ratio Ω₁:Ω₂:Ω₃=1:0.1325:0.55, creating a hydrodynamically stable (quasi-Keplerian) flow in the bulk of the container. Coils surrounding the outer cylinder provide a uniform magnetic field Bi along the rotation axis. We used Hall probes and ultrasound velocimetry to simultaneously measure the radial magnetic field B_r and azimuthal velocity u_φ at different azimuths in a single run. The B_r measurements were made on the inner cylinders surface at different heights, and u_φ measurements were made from the outer cylinder at the midplane. The nonaxisymmetric instability is observed in B_r and u_φ. It has an exponential growth at its onset and a dominant azimuthal wavenumber m=1. The instability is global with a radially uniform rotation frequency (angular phase velocity), and an axial phase velocity as well. Our numeral simulations reproduce the experimental findings, and further reveal that the nonaxisymmetric instability contributes to outward radial angular momentum transport, as with the magnetorotational instability. Possible mechanisms for the instability are also discussed.