Dr. Frank Stefani

Head Geo- and Astrophysics
Phone: +49 351 260 3069

The Riga Dynamo Experiment

Magnetic fields of planets, stars, and galaxies are produced by motion of electrically conducting fluids. While the corresponding theory of hydromagnetic dynamos has been widely elaborated in the last decades, an experimental verification of magnetic field self-excitation in conducting fluids was missing until recently. Besides a few other experimental approaches in the world, the Riga dynamo facility is one of the large sodium facilities devoted to the investigation of this phenomenon.

Figure 1 shows a sketch and photograph of the facility, whose central module (Figure 2) comprises a propeller driven central helical flow (with a velocity up to 20 m/s), a straight back-flow and sodium at rest in additional coaxial tubes of stainless steel. In order to reach self-excitation with the limited power resources, the whole facility had been optimized in a long iterative process of pump design and numerical simulations. Figure 2 shows also a snapshot of the expected magnetic field resulting from our 2D-solver for the induction equation.

Figure 1: Sketch (a) and photograph (b) of the Riga dynamo facility, with the central dynamo module (D), two DC-motors (M), the belts (B), one of the storage tanks (T), and the external Hall sensors (H).

Figure 2: Central module of the Riga dynamo experiment and the magnetic eigenfield, computed in the kinematic regime for a propeller rotation rate of 2000 rpm. The field pattern rotates with a frequency of 1.16 Hz around the vertical axis, in the same direction as the flow. The color of the field-lines indicates the z-component of the field.

In November 1999, first dynamo experiments were carried out. After having studied the pure amplification of an externally applied 1Hz magnetic field for various propeller rotation rates, an additional exponentially growing eigenmode with a frequency of 1.3 Hz appeared at the highest rotation rate of 2150 rpm (Figure 3). Unfortunately, this campaign had to be stopped due to a technical problem, so that the saturation regime of the dynamo could only be reached in the next campaign in July 2000 (Figure 4).

Figure 3: First evidence of magnetic field self-excitation as measured on 11 November 1999. For the highest rotation rate of 2150 rpm, a superposition of the amplified excitation field (0.995 Hz) and a self-excited field (1.326 Hz) appears.

Figure 4: Reaching the saturation regime of the Riga dynamo in one of the runs of July 2003.

In Figure 5 we compile the growth rates (a) and the frequencies (b) of the eigenfield measured in a number of campaigns between 1999 and 2007, showing in general a very good agreement with our numerical simulations. After repair and recommissioning in 2016, the Riga dynamo experiment is now ready for further measurement campaigns.

Figure 5: Growth rates (a) and frequencies (b) in the kinematic and the saturated regime as measured in various campaigns. The numerical predictions for the kinematic case are based on our 2D code. For the corrected curves, the effect of the finite wall thickness (determined with a 1D code) was additionally taken into account. All rotation rates, growth rates and frequencies are re-scaled to a common reference temperature of 157 °C.

Focus story