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 magnetorotational instability (MRI) is thought to play a key role in the formation of stars and black holes by sustaining the turbulence in hydrodynamically stable Keplerian accretion disks. In previous experiments the MRI was observed in a liquid metal Taylor-Couette flow at moderate Reynolds numbers by applying a helical magnetic field. The observation of this helical MRI (HMRI) was interfered with a significant Ekman pumping driven by solid end caps that confined the instability only to a part of the Taylor-Couette cell. This paper describes the observation of the HMRI in an improved Taylor-Couette setup with the Ekman pumping significantly reduced by using split end caps. The HMRI, which now spreads over the whole height of the cell, appears much sharper and in better agreement with numerical predictions. By analyzing various parameter dependencies we conclude that the observed HMRI represents a self-sustained global instability rather than a noise-sustained convective one.

The magnetorotational instability (MRI) plays a key role in the formation of stars and black holes, by enabling outward angular momentum transport in accretion disks. The use of combined axial and azimuthal magnetic fields allows the investigation of this effect in liquid metal flows at moderate Reynolds and Hartmann numbers. A variety of experimental results is presented showing evidence for the occurrence of the MRI in a Taylor-Couette flow using the liquid metal alloy GaInSn. PACS numbers: 47.20.-k, 47.65.+a, 95.30.Qd

Magnetorotational instability (MRI) is one of the most important and most common instabilities in astrophysics. It is widely accepted that it serves as a source of turbulent viscosity in accretion disks -the most energy efficient objects in the Universe. However it is very difficult to bring this process down on earth and model it in a laboratory experiment. Several different approaches have been proposed, one of the most recent is PROMISE (Potsdam-ROssendorf Magnetorotational InStability Experiment). It consists of a flow of a liquid metal between two rotating cylinders under applied current-free spiral magnetic field. The cylinders must be covered with plates which introduce additional end-effects which alter the flow and make it more difficult to clearly distinguish between MRI stable and unstable state. In this paper we propose simple and inexpensive improvement to the PROMISE experiment which would reduce those undesirable effects.