  A conceptual design for beamed core antimatter propulsion is reported, where
electrically charged annihilation products directly generate thrust after being
deflected and collimated by a magnetic nozzle. Simulations were carried out
using the Geant4 (Geometry and tracking) software toolkit released by the CERN
accelerator laboratory for Monte Carlo simulation of the interaction of
particles with matter and fields. Geant permits a more sophisticated and
comprehensive design and optimization of antimatter engines than the software
environment for simulations reported by prior researchers. The main finding is
that effective exhaust speeds Ve ~ 0.69c (where c is the speed of light) are
feasible for charged pions in beamed core propulsion, a major improvement over
the Ve ~ 0.33c estimate based on prior simulations. The improvement resulted
from optimization of the geometry and the field configuration of the magnetic
nozzle. Moreover, this improved performance is realized using a magnetic field
on the order of 10 T at the location of its highest magnitude. Such a field
could be produced with today's technology, whereas prior nozzle designs
anticipated and required major advances in this area. The paper also briefly
reviews prospects for production of the fuel needed for a beamed core engine.
