Riga Dynamo Experiment

It is widely believed that almost all magnetic fields in a natural
environment are a result of the dynamo process -- field generation in a
moving nearly homogeneous fluid electroconducting material of celestial
bodies. Such are fields of the Earth, most of the planets, Sun, another
stars and even galaxies. The believe is based on the theory and numerical
simulation. Until very recently no direct laboratory experiment was
supporting this important point. We are not going to model in the laboratory
any particular celestial body. Our aim is to demonstrate the very idea -- by
intense stirring in a large volume of good electroconducting liquid one can
generate a magnetic field. As the working fluid we are using 2 qm of molten
sodium -- the best electroconducting liquid available. The fluid part of
celestial bodies is stirred by thermal and other types of natural
convection. In laboratory circumstances such stirring is much too slow.
Hence we are stirring sodium by an outside forced propeller consuming 150kW
of power. The propeller is producing an axisymmetric swirling flow inside a
prolongated annular vessel. When flowrate reaches 0.6 qmps or so the device
starts to generate magnetic field. The field pattern is nonaxisymmetric and
slowly (1.2 --1.5hz) rotates round symmetry axis. The overall rotation is
superimposed by turbulent fluctuations. The generation process is stable in
operational sense and results are reproducible. We report on two experiments
at the Riga dynamo facility. In November 1999, a slowly growing magnetic
field eigenmode was observed for the first time in a liquid metal
experiment. In July 2000, the magnetic field saturation regime was studied.
We sketch also the underlying theory in order to illuminate the background
of the Riga dynamo experiment.