Observation of Axisymmetric Standard Magnetorotational Instability in the Laboratory

We report the first direct evidence for the axisymmetric standard magnetorotational instability (SMRI) from a combined experimental and numerical study of a magnetized liquid-metal shear flow in a TaylorCouette cell with independently rotating and electrically conducting end caps. When a uniform vertical magnetic field Bi is applied along the rotation axis, the measured radial magnetic field Br on the inner cylinder increases linearly with a small magnetic Reynolds number Rm due to the magnetization of the residue Ekman circulation. Onset of the axisymmetric SMRI is identified from the nonlinear increase of Br beyond a critical Rm in both experiments and nonlinear numerical simulations. The axisymmetric SMRI exists only at sufficiently large Rm and intermediate Bi, a feature consistent with theoretical predictions. Our simulations further show that the axisymmetric SMRI causes the velocity and magnetic fields to contribute an outward flux of axial angular momentum in the bulk region, just as it should in accretion disks.

Automated summary

This study presents the first direct evidence for the axisymmetric standard magnetorotational instability (SMRI) through a combined experimental and numerical approach. The study demonstrates that the radial magnetic field on the inner cylinder increases linearly with the magnetic Reynolds number when a vertical magnetic field is applied along the rotation axis. The onset of the axisymmetric SMRI is identified as a nonlinear increase of the radial magnetic field beyond a critical magnetic Reynolds number. This study also reveals that the axisymmetric SMRI exists only under specific conditions of the magnetic Reynolds number and the vertical magnetic field.