  Estimates for the nonlinear alpha effect in helical turbulence with an
applied magnetic field are presented using two different approaches: the
imposed-field method where the electromotive force owing to the applied field
is used, and the test-field method where separate evolution equations are
solved for a set of different test fields. Both approaches agree for stronger
fields, but there are apparent discrepancies for weaker fields that can be
explained by the influence of dynamo-generated magnetic fields on the scale of
the domain that are referred to as meso-scale magnetic fields. Examples are
discussed where these meso-scale fields can lead to both drastically
overestimated and underestimated values of alpha compared with the kinematic
case. It is demonstrated that the kinematic value can be recovered by resetting
the fluctuating magnetic field to zero in regular time intervals. It is
concluded that this is the preferred technique both for the imposed-field and
the test-field methods.
