Experimental observation of the standard magnetorotational instability in a modified Taylor-Couette cell

The standard magnetorotational instability (SMRI) is a promising mechanism for turbulence and rapid accretion in astrophysical disks. It is a magnetohydrodynamic (MHD) instability that destabilizes otherwise hydrodynamically stable disk flow. Unlike other fundamental plasma processes such as Alfvén waves and magnetic reconnections which have been subsequently detected in space and in the laboratory, SMRI remains unconfirmed since its proposal, despite its its astrophysical importance. Its direct detection has been hindered in observations due to its microscopic nature at astronomical distances and stringent requirements in laboratory experiments. In this talk, I will present direct evidence showing that SMRI indeed exists in a novel laboratory setup, where a uniform magnetic field is imposed along the rotation axis of a differentially rotating liquid-metal flow confined radially between two coaxial cylinders and axially by copper endrings. Through in situ measurement of the radial magnetic field B_r at different azimuths of the inner cylinder, onset of the axisymmetric SMRI is identified from the nonlinear increase of B_r beyond a critical magnetic Reynolds number. The SMRI is found to be accompanied by a nonaxisymmetric MHD instability, which has an exponential growth at its onset and a dominant m=1 mode in the azimuthal direction. Further analysis suggests that the nonaxisymmetric instability is unlikely to be the conventional hydrodynamic instability or the Stewartson-Shercliff layer instability. The experimental results are reproduced by nonlinear three-dimensional numerical simulations, which further show that 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 an accretion disk.