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 Taylor-Couette cell with independently rotating and electrically conducting end caps. When a uniform vertical magnetic field $B_i$ is applied along the rotation axis, the measured radial magnetic field $B_r$ 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 $B_r$ beyond a critical $Rm$ in both experiments and nonlinear numerical simulations. The axisymmetric SMRI exists only at sufficiently large $Rm$ and intermediate $B_i$, 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.