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The self-excitation of magnetic field by a spiral Couette flow between twocoaxial cylinders is considered. We solve numerically the fully nonlinear,three-dimensional MHD equations for magnetic Prandtl numbers Pm (ratio ofkinematic viscosity to magnetic diffusivity) between 0.14 and 10 and kinematicand magnetic Reynolds numbers up to about 2000. In the initial stage ofexponential field growth (kinematic dynamo regime), we find that the dynamoswitches from one distinct regime to another as the radial width \delta r_B$ ofthe magnetic field distribution becomes smaller than the separation of thefield maximum from the flow boundary. The saturation of magnetic field growthis due to a reduction in the velocity shear resulting mainly from theazimuthally and axially averaged part of the Lorentz force, which agrees withan asymptotic result for the limit of Pm<<1. In the parameter regimeconsidered, the magnetic energy decreases with kinematic Reynolds number asRe^{-0.84}, which is approximately as predicted by the nonlinear asymptotictheory (\sim Re^{-1}). However, when the velocity field is maintained by avolume force (rather than by viscous stress) the dependence of magnetic energyon the kinematic Reynolds number is much weaker.Comment: 14 pages RevTe

Nonlinear states of the screw dynamo