Computer simulations are performed to investigate beam-plasma physics in the Space Experiment With Particle Accelerators, which is planned for active and interactive experiments in the earth's upper atmosphere and magnetosphere using a high-power electron gun on board the space shuttle. Using a two-dimensional electromagnetic particle simulation code (EM2), we study the divergence and propagation of the electron beam through the ambient magnetoactive plasma as well as the plasma wave excitation by the electron beam. First, we apply the EM2 code to a one-dimensional model where the x axis is taken perpendicular to the static magnetic field. We find a strong excitation of a slow extraordinary wave around the electron beam and a propagation of a pulselike ordinary wave packet excited as an impulse response to the onset of the beam injection. Second, we apply the EM2 code to a two-dimensional model where the x-y plane is taken transverse to the static magnetic field. The detailed behavior of the beam as well as the background cold plasma is studied. The electron beam is locally injected in the x-y plane with a drift velocity Vz. The electron beam initially expands in the x-y plane owing to an electrostatic field produced by the excess charge of the beam electrons, and it subsequently oscillates with the frequency of the slow extraordinary wave which is excited around the beam. Uniformity in the z; direction is assumed, producing a self-consistent two-dimensional simulation as a preliminary to a later three-dimensional study where this condition can be relaxed.