Magneto rotational 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 an upward travelling wave. 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.|
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