  The dynamo action achieved in the convective cores of main-sequence massive
stars is explored here through 3D global simulations of convective core dynamos
operating within a young $\Msun{10}$ B-type star, using the anelastic spherical
harmonic (ASH) code. These simulations capture the inner 65\% of this star by
radius, encompassing the convective nuclear-burning core (about 23\% by radius)
and a portion of the overlying radiative envelope. Eight rotation rates are
considered, ranging from 0.05\% to 16\% of the surface breakup velocity,
thereby capturing both convection that barely senses the effects of rotation
and other situations in which the Coriolis forces are prominent. The vigorous
dynamo action realized within all of these turbulent convective cores builds
magnetic fields with peak strengths exceeding a megagauss, with the overall
magnetic energy (ME) in the faster rotators reaching super-equipartition levels
compared to the convective kinetic energy (KE). The core convection typically
involves turbulent columnar velocity structures roughly aligned with the
rotation axis, with magnetic fields threading through these rolls and
possessing complex linkages throughout the core. The very strong fields are
able to coexist with the flows without quenching them through Lorentz forces.
The velocity and magnetic fields achieve such a state by being nearly
co-aligned, and with peak magnetic islands being somewhat displaced from the
fastest flows as the intricate evolution proceeds. As the rotation rate is
increased, the primary force balance shifts from nonlinear advection balancing
Lorentz forces to a magnetostrophic balance between Coriolis and Lorentz
forces.
