  In this paper we present results of using parallel supercomputers to simulate
beam dynamics in next-generation high intensity ion linacs. Our approach uses a
three-dimensional space charge calculation with six types of boundary
conditions. The simulations use a hybrid approach involving transfer maps to
treat externally applied fields (including rf cavities) and parallel
particle-in-cell techniques to treat the space-charge fields. The large-scale
simulation results presented here represent a three order of magnitude
improvement in simulation capability, in terms of problem size and speed of
execution, compared with typical two-dimensional serial simulations. Specific
examples will be presented, including simulation of the spallation neutron
source (SNS) linac and the Low Energy Demonstrator Accelerator (LEDA) beam halo
experiment.
