After the dynamo experiment in November 1999 [A. Gailitis et al., Phys. Rev. Lett. 84, 4365 (2000)] had shown magnetic field self-excitation in a spiraling liquid metal flow, in a second series of experiments emphasis was placed on the magnetic field saturation regime as the next principal step in the dynamo process. The dependence of the strength of the magnetic field on the rotation rate is studied. Various features of the saturated magnetic field are outlined and possible saturation mechanisms are discussed.

In an experiment at the Riga sodium dynamo facility, a slowly growing magnetic field eigenmode has been detected over a period of about 15 seconds. For a slightly decreased propeller rotation rate, additional measurements showed a slow decay of this mode. The measured results correspond satisfactory with numerical predictions for the growth rates and frequencies.

A recent paper [R. Hollerbach and G. Rüdiger, Phys. Rev. Lett. 95, 124501 (2005)] has shown that the threshold for the onset of the magnetorotational instability (MRI) in a Taylor-Couette flow is dramatically reduced if both axial and azimuthal magnetic fields are imposed. In agreement with this prediction, we present results of a Taylor-Couette experiment with the liquid metal alloy GaInSn, showing evidence for the existence of the MRI at Reynolds numbers of order 1000 and Hartmann numbers of order 10.The role of magnetic fields in the cosmos is two-fold: First, planetary, stellar and galactic fields are a product of the homogeneous dynamo effect in electrically conducting fluids. Second, magnetic fields are also believed to play an active role in cosmic structure formation, by enabling outward transport of angular momentum in accretion disks via the magnetorotational instability (MRI) [1]. Considerable theoretical and computational progress has been made in understanding both processes. The dynamo effect has even been verified experimentally, in large-scale liquid sodium facilities in Riga and Karlsruhe, and continues to be studied in laboratories around the world [2]. In contrast, obtaining the MRI experimentally has been less successful thus far [3]. ([4] claim to have observed it, but their background state was already fully turbulent, thereby defeating the original idea that the MRI would destabilize an otherwise stable flow.)If only an axial magnetic field is externally applied, the azimuthal field that is necessary for the occurrence of the MRI must be produced by induction effects, which are proportional to the magnetic Reynolds number (Rm) of the flow. But why not substitute this induction process simply by externally applying an azimuthal magnetic field as well ? This question was at the heart of the paper [5], where it was shown that the MRI is then possible with far smaller Reynolds (Re) and Hartmann (Ha) numbers. In this paper we report experimental verification of this idea, presenting evidence of the MRI in a liquid metal Taylor-Couette (TC) flow with externally imposed axial and azimuthal (i.e., helical) magnetic fields.

The three-dimensional velocity field of a propeller driven liquid metal flow is reconstructed by a contactless inductive flow tomography (CIFT). The underlying theory is presented within the framework of an integral equation system that governs the magnetic field distribution in a moving electrically conducting fluid. For small magnetic Reynolds numbers this integral equation system can be cast into a linear inverse problem for the determination of the velocity field from externally measured magnetic fields. A robust reconstruction of the large scale velocity field is already achieved by applying the external magnetic field alternately in two orthogonal directions and measuring the corresponding sets of induced magnetic fields. Kelvin's theorem is exploited to regularize the resulting velocity field by using the kinetic energy of the flow as a regularizing functional. The results of the new technique are shown to be in satisfactory agreement with ultrasonic measurements.

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.