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.

Experimental evidence for a transient Tayler instability in a cylindrical liquid-metal column.

In the current-driven, kink-type Tayler instability (TI) a sufficiently strong azimuthal magnetic field becomes unstable against nonaxisymmetric perturbations. The TI has been discussed as a possible ingredient of the solar dynamo mechanism and a source of the helical structures in cosmic jets. It is also considered as a size-limiting factor for liquid metal batteries. We report on a liquid metal TI experiment using a cylindrical column of the eutectic alloy GaInSn to which electrical currents of up to 8 kA are applied. We present results of external magnetic field measurements that indicate the transient occurrence of the TI in good agreement with numerical predictions. The interference of TI with the competing large-scale convection, resulting from Joule heating, is also discussed.

Eddy diffusivity from hydromagnetic Taylor-Couette flow experiments.

The stability problem of hydromagnetic Taylor-Couette flows with toroidal magnetic fields is considered for various magnetic Prandtl numbers. Only the most uniform (but not current-free) field has been treated. For high enough Hartmann numbers, the toroidal field is always unstable due to the magnetic kink-type instability, which is stabilized by rigid basic rotation. The axial electric current, which drives the instability, is reduced by the electromotive force induced by the instability itself. Numerical simulations show that this electromotive force only depends on the molecular magnetic diffusivity rather than the viscosity. The resulting eddy diffusivity should be on the order of the molecular diffusivity for all the considered magnetic Prandtl numbers. If this is true also for very small magnetic Prandtl numbers (not possible to simulate) then one can use this effect to measure the eddy diffusivity eta(T) in a laboratory. In a sodium experiment (without rotation), a detectable potential difference of approximately 16 mV between top and bottom will result for a container of 1 m length and a gap width of 10 cm.