  The evolution of magnetic fields is studied using simulations of forced
helical turbulence with strong imposed shear. After some initial exponential
growth, the magnetic field develops a large scale travelling wave pattern. The
resulting field structure possesses magnetic helicity, which is conserved in a
periodic box by the ideal MHD equations and can hence only change on a
resistive time scale. This constrains strongly the growth time of the large
scale magnetic field, but less strongly the length of the cycle period.
Comparing with the case without shear, the time scale for large scale field
amplification is shortened by a factor Q, which depends on the relative
importance of shear and helical turbulence, and which controls also the ratio
of toroidal to poloidal field. The results of the simulations can be reproduced
qualitatively and quantitatively with a mean-field alpha-Omega dynamo model
with alpha-effect and the turbulent magnetic diffusivity coefficients that are
less strongly quenched than in the corresponding alpha^2-dynamo.
