Experimental Evidence for Nonaxisymmetric Magnetorotational Instability in a Rotating Liquid Metal Exposed to an Azimuthal Magnetic Field

The azimuthal version of the magnetorotational instability (MRI) is a nonaxisymmetric instability of a hydrodynamically stable differentially rotating flow under the influence of a purely or predominantly azimuthal magnetic field. It may be of considerable importance for destabilizing accretion disks, and plays a central role in the concept of the MRI dynamo. We report the results of a liquid metal Taylor-Couette experiment that shows the occurrence of an azimuthal MRI in the expected range of Hartmann numbers.

Figure 1

Sketch of the experimental setup.

Figure 2

Velocity perturbation $v_z(m=1,z,t)$ for $\mu =0.26$, $\mathrm{Re}=1480$, and $\mathrm{Ha}=77$. (a) 3D simulation for ideal axisymmetric field. (b) 3D simulation for realistic field. (c) Experimental results.

Figure 3

As in Fig. 2, but for $\mathrm{Ha}=110$.

Figure 4

Results for $\mathrm{Re}=1480$. (a) Growth rate from a 1D linear stability code, (b) mean square velocity perturbation from experiment and two different 3D simulations for idealized and real magnetic field geometry, (c) angular drift frequency, and (d) wave number from experiment, from the 1D linear stability code, and from the 3D simulation with real field geometry. The error bar of the velocity perturbation corresponds to an 85 percent confidence level. The “up” and “down” values in (c) and (d), which refer to the travel direction of the velocity perturbations as exemplified in Figs. 2 and 3, are determined by a center-of-gravity method applied to the 2D FFT of the data.

Figure 5

As in Fig. 4 (except the 3D simulation data), but for $\mathrm{Re}=2960$.