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Dr. Frank Stefani

Head Geo- and Astrophysics
f.stefaniAthzdr.de
Phone: +49 351 260 3069

Magnetorotational instability (PROMISE Experiment)

Cosmic magnetic fields play a surprisingly active role in cosmic structure formation by fostering outward angular momentum transport and inward mass accretion onto central objects, like protostars or black holes, by means of the magnetorotational instability (MRI). At the PROMISE experiment, two special versions of MRI, the helical MRI and the azimuthal MRI, are investigated.

PROMISE is basically a Taylor-Couette experiment with a gap width of 4 cm and a height of 40 cm (Figure 1). The utilized fluid is the eutectic alloy GaInSn which is liquid at room temperatures. Figure 2 shows the present set-up, including a 20 kA power supply for the central rod and a pentagon-shaped return-current system for the homogenization of the azimuthal field component.

The Taylor-Couette experiment is usually carried out in the hydrodynamically stable regime, when the angular momentum is increasing outward. The helical MRI (HMRI) appears as an axisymmetric travelling wave when the azimuthal field acquires an amplitude that is comparable with that of the axial field (Figure 3). For purely or strongly dominant azimuthal field, the non-axisymmtric azimuthal MRI (AMRI) shows up (Figure 4).

Figure 1: Sketch of the PROMISE facility, comprising a Taylor-Couette cell with inner radius of 4 cm, outer radius of 8 cm, and a height of 40 cm, filled with GaInSn, an external coil and a central copper rod for the generation of the axial and the azimuthal magnetic field, respectively.
Figure 2: Present configuration of PROMISE with a 20 kA power supply (right) for the central rod and a pentagon-shaped return-current system for the homogenization of the azimuthal field component.
Figure 3: Results for HMRI. Measured velocity structure when increasing the current through the central rod, for a fixed current of 76 A in the coil (top). Approximately at 5000 A, HMRI emerges as a wave trevallinf upward. Experimentally and numerically determined dependence of the mean squared of the velocity perturbations on the applied rod current (bottom).
Figure 4: Results for AMRI. Axial velocity perturbation for Re = 1480 and Ha = 124: (a) simulation for an idealized axisymmetric field. (b) Simulation for the realistic field geometry. (c) Measured velocity. (d) Numerically predicted growth rate. (e) Simulated and measured mean squared velocity perturbation. (f) Angular drift frequency.

Interesting effects emerge when examining AMRI under the influence of radial temperature gradients. These arise, e.g., by the thermal radiation of the central current-carrying copper tube. In this case, the symmetry between the upward and downward travelling AMRI waves is broken, so that the instability acquires the form of a "one-winged butterfly".

In connection with PROMISE, two new forms of MRI were also found that apply for the case that the rotation of the outer cylinder is faster than that of the inner one. The variant called “Super-HMRI”, which might be relevant for the destabilization of the equatorial regions of the solar tachocline, is to be investigated in the MATISSE experiment in frame of the DRESDYN project.

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