  We use three-dimensional direct numerical simulations of the helically forced
magnetohydrodynamic equations in spherical shell segments in order to study the
effects of changes in the geometrical shape and size of the domain on the
growth and saturation of large-scale magnetic fields. We inject kinetic energy
along with kinetic helicity in spherical domains via helical forcing using
Chandrasekhar-Kendall functions. We take perfect conductor boundary conditions
for the magnetic field to ensure that no magnetic helicity escapes the domain
boundaries. We find dynamo action giving rise to magnetic fields at scales
larger than the characteristic scale of the forcing. The magnetic energy
exceeds the kinetic energy over dissipative time scales, similar to that seen
earlier in Cartesian simulations in periodic boxes. As we increase the size of
the domain in the azimuthal direction we find that the nonlinearly saturated
magnetic field organizes itself in long-lived cellular structures with aspect
ratios close to unity. These structures tile the domain along the azimuthal
direction, thus resulting in very small longitudinally averaged magnetic fields
for large domain sizes. The scales of these structures are determined by the
smallest scales of the domain, which in our simulations is usually the radial
scale. We also find that increasing the meridional extent of the domains
produces little qualitative change, except a marginal increase in the
large-scale field. We obtain qualitatively similar results in Cartesian domains
with similar aspect ratios.
