  The role of shear in alleviating catastrophic quenching by shedding
small-scale magnetic helicity through fluxes along contours of constant shear
is discussed. The level of quenching of the dynamo effect depends on the
quenched value of the turbulent magnetic diffusivity. Earlier estimates that
might have suffered from the force-free degeneracy of Beltrami fields are now
confirmed for shear flows where this degeneracy is lifted. For a dynamo that is
saturated near equipartition field strength those estimates result in a 5-fold
decrease of the magnetic diffusivity as the magnetic Reynolds number based on
the wavenumber of the energy-carrying eddies is increased from 2 to 600.
Finally, the role of shear in driving turbulence and large-scale fields by the
magneto-rotational instability is emphasized. New simulations are presented and
the 3pi/4 phase shift between poloidal and toroidal fields is confirmed. It is
suggested that this phase shift might be a useful diagnostic tool in
identifying mean-field dynamo action in simulations and to distinguish this
from other scenarios invoking magnetic buoyancy as a means to explain migration
away from the midplane.
