Experimental dynamos

Mechanical energy contained in the motion of a liquid conductor is transformed into magnetic energy by the dynamo effect. The magnetic fields of the earth, the planets, the sun, the stars and numerous other celestial objects including galaxies are believed to be generated by a dynamo effect. The theoretical description of the basic phenomenon relies entirely on classical physics, i.e. Maxwell's equations and the Navier-Stokes equation which governs the fluid flow. Despite its central importance for geophysics and astrophysics, the dynamo effect has been demonstrated experimentally in only three different experiments because of the technological problems involved. A flow capable of sustaining a magnetic field needs a high enough magnetic Reynolds number Rm. Rm is proportional to the size of the experiment and the typical velocity of the fluid flow. The most favorable material is liquid sodium. Even using a cell of 1 m size filled with liquid sodium, the fluid needs to move at velocities of 1-10 m/s.



Several groups have engaged in building experimental dynamos. All experiments require careful planning and extensive numerical simulation in order to optimize the design in the sense of achieving a low Rm. One dynamo is located at the Forschungszentrum Karlsruhe. In this experiment, liquid sodium is forced into a helical flow by pumping it through an arrangement of pipes and blades sketched below. The actual dimensions are a=0.21 m, r0=0.94 m and d=0.9 m. The experiment has generated a self-sustained magnetic field for the first time in November 1999. Precise descriptions of the apparatus and a review of dynamo experiments in general, together with an account of the design calculations for the Karlsruhe experiment and simulations of other experiments are found in the references.