  The effect of a dynamo-generated mean magnetic field of Beltrami type on the
mean electromotive force is studied. In the absence of the mean magnetic field
the turbulence is assumed to be homogeneous and isotropic, but it becomes
inhomogeneous and anisotropic with this field. Using the testfield method the
dependence of the alpha and turbulent diffusivity tensors on the magnetic
Reynolds number Rm is determined for magnetic fields that have reached
approximate equipartition with the velocity field. The tensor components are
characterized by a pseudoscalar alpha and a scalar turbulent magnetic
diffusivity etat. Increasing Rm from 2 to 600 reduces etat by a factor ~5,
suggesting that the quenching of etat is, in contrast to the 2-dimensional
case, only weakly dependent on Rm. Over the same range of Rm, however, alpha is
reduced by a factor ~14, which can qualitatively be explained by a
corresponding increase of a magnetic contribution to the alpha effect with
opposite sign. The level of fluctuations of alpha and etat is only 10% and 20%
of the respective kinematic reference values.
