Laboratory Astrophysics: PROMISE
PROMISE Gets Cosmic Magnetism into the LaboratoryAIP and Forschungszentrum Dresden-Rossendorf at breakthrough in laboratory astrophysics The first experimental proof of the existence of a magnetic effect was furnished which plays an essential role in the cosmos, for example during the formation of stars. Magnetic fields can generate turbulent motions in rotating, electrically conducting fluids and gases. The name of the laboratory experiment is PROMISE, the 'Potsdam-ROssendorf Magnetic InStability Experiment'. The effect known from celestial bodies is called magnetorotational instability. Different rotation periods at different distances from the center cause shear in the fluid which is the main driver of the instability. The effect was hitherto unknown in terrestrial laboratories. Theory however predicted the instability for the electrically conducting gases in the cosmos. The idea and the physical computations come from the experts for magnetic fields, Professor Günther Rüdiger at Astrophysikalisches Institut Potsdam and Dr. Rainer Hollerbach at the University of Leeds. The magnetohydrodynamics department of Forschungszentrum Dresden-Rossendorf possessed the technical experience with fluid-metal experiments for the implementation of the proof of the instability, headed by Dr. Gunter Gerbeth and Dr. Frank Stefani. The project was one of the awarded proposals for financial support from the Leibniz-Gemeinschaft. Two coaxial cylinders rotate about their common axis. The inner one rotated faster than the outer one. Gallium-Indium-Tin (GaInSn) is filled in between the cylinders which is fluid at room temperatures. Electrical currents generate the magnetic fields. A large coil (yellow windings in Figure 1) yields a magnetic field with vertically aligned field lines, a strong current of a few thousand Ampere through the cylinder axis causes a field with ring-like field lines threading the GaInSn. The phenomenon of magnetorotational instability emerges even for weak magnetic fields. A combination of rather strong fields has to be used in the experiment though, since the required rotation speeds would be much to high otherwise.
Many astronomical objects rotate faster in their interior than at the surface. Because even weak magnetic fields are sufficient for the onset of the instability, theorists suggest that phenomena like star formation or the enormous luminosity of quasars can be explained by the magnetorotational instability. While astronomers usually get their information only from the light of distant worlds, PROMISE is giving a new meaning to the term 'laboratory astrophysics'. The onset of the instability is first observed in the emergence of rolls in the fluid which drift with a certain velocity through the GaInSn. Figure 3 shows the drift velocity of the rolls in dependence on the ratio of rotation speeds of the two cylinders. At a rational ratio of 0.25, one rotation of the outer cylinder lasts four times longer than a rotation of the inner cylinder. The measurements by theory were predicted to lie in the yellow area. Two drift velocities were determined which indeed lie in the predicted area (crosses). Ultrasound probes "look" into the gap between the two cylinders from above and measure the vertical motions of the fluid metal. If the roll-type motions emerge, a certain pattern of alternating velocity directions is observed, depending on depth. These drift vertically with time and give the drift speed. If the current through the axis has 1000 Ampere, the motions are still weak. At 5000 Ampere, the drifting rolls are indicated by inclined areas marked in red in Figure 4. |
Figure 1: The experiment PROMISE.
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PI PROMISE
Prof. Günther Rüdiger
Astrophysikalisches Institut Potsdam
An der Sternwarte 16
D-14482 Potsdam
(0331) 7499 512
gruediger@aip.de
Press contact
presse@aip.de
Publications
G. Rüdiger, R. Hollerbach, F. Stefani, T. Gundrum, G. Gerbeth, R. Rosner: The traveling wave MRI in cylindrical Taylor-Couette flow: comparing wavelengths and speeds in theory and experiment. Astrophysical Journal Letters, in press. astro-ph/0607041
