Magnetic turbulence in the Riga dynamo experiment

The Riga dynamo experiment demonstrates that a strong enough and appropriately directed flow of a fluid electroconductor generates a magnetic field very similar as the Earth and other celestial bodies do. Two 100 kW motors are driving a propeller which forces molten sodium to circulate inside an annular vessel, a part of which is located in the basement of the sodium lab. The sodium flow is directed by two thin coaxial electroconducting cylindrical partition walls. In the central channel sodium is swirling down from the propeller. In the coaxial counter-flow channel the flow is raising straight up to the propeller. In an outer part of the vessel the sodium is at rest, it serves only for better electric boundary conditions. Depending on sodium temperature at a propeller speed of 1800-2000 rpm (flow-rate about 0.6 qm/s) the zero state for the magnetic field is becoming unstable and a field appears seemingly from nothing. The magnetic field values are recovered from coil voltage records by means of Fast Fourier processing. For finer spectral resolution two small coils were inserted alternately in a narrow channel tip penetrating deep inside the central flow. Examples for recorded signals and FFT processed fields are presented.