Experiments on the helical magnetorotational instability in a Taylor-Couette flow with reduced Ekman pumping


Experiments on the helical magnetorotational instability in a Taylor-Couette flow with reduced Ekman pumping

Gundrum, T.; Gerbeth, G.; Stefani, F.; Hollerbach, R.; Priede, J.; Rüdiger, G.; Szklarski, J.

The magnetorotational instability (MRI) is believed to play a key role in the formation of stars and black holes by triggering turbulence in hydrodynamically stable Keplerian accretion discs. Although the MRI had been discovered by Velikhov as early as 1959, its key role for the understanding of accretion disks was revealed only in 1991 by Balbus and Hawley. While thousands of papers have been written on the astrophysical relevance of MRI since that time, experimental work on MRI is still rare.
In 2004, Sisan et al. had observed a new coupled mode of velocity and magnetic field perturbations in a spherical Couette flow, whose parameter dependencies resemble those of the MRI. However, the background flow in this experiment was already fully turbulent so that the original goal to investigate MRI as the first instability on an otherwise stable flow could not be met. Another MRI experiment, on the basis of a short Taylor-Couette flow, is presently pursued at Princeton University.
Both experiments are carried out with a purely axial magnetic field being applied. In this standard MRI (SMRI) configuration, the governing parameters for the onset of the instability are the magnetic Reynolds number and the Lundquist number. Surprisingly, by adding an azimuthal magnetic field to the axial magnetic field, the governing role is taken over by the hydrodynamic Reynolds number and the Hartmann number. Consequently, this helical MRI (HMRI) can be observed with much less experimental effort than the SMRI.
In previous experiments at the facility PROMISE (Potsdam ROssendorf Magnetic InStability Experiment), the HMRI was observed in a liquid metal Taylor-Couette flow at moderate Reynolds and Hartmann numbers. However, the observation of this HMRI was disrupted by a significant Ekman pumping driven by solid end-caps that confined the instability only to a part of the Taylor-Couette cell. We present the observation of the HMRI in an improved Taylor-Couette setup in which the Ekman pumping is strongly reduced by using split end-caps. The HMRI wave, 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.

  • Lecture (Conference)
    16th Couette-Taylor Workshop, 09.-11.09.2009, Princeton, USA

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Publ.-Id: 12756