Computational studies and experimental measurements of plasma injection into a Malmberg‐Penning trap reveal that the number of trapped particles can be an order of magnitude higher than predicted by a simple estimates based on a ballistic trapping model. Enhanced trapping is associated with a rich nonlinear dynamics generated by the space‐charge forces of the evolving trapped electron density. A particle‐in‐cell simulation is used to identify the physical mechanisms that lead to the increase in trapped electrons. The simulations initially show strong two‐stream interactions between the electrons emitted from the cathode and those reflected off the end plug of the trap. This is followed by virtual cathode oscillations near the injection region. As electrons are trapped, the initially hollow longitudinal phase‐space is filled, and the transverse radial density profile evolves so that the plasma potential matches that of the cathode. Simple theoretical arguments are given that describe the different dynamical regimes. Good agreement is found between simulation and theory. © 2003 American Institute of Physics