Electron guns and Ion guns are handled by the LORENTZ module. This is a trajectory calculation module that is added to any of our field analysis programs, or combinations thereof. The LORENTZ product page presents more specific options.
Typically, electron guns and ion guns are first modeled with LORENTZ 2D-E (i.e. 2D electric analysis program) in either RS or 2D mode to get the initial emission and focus characteristics. Devices further along the beam path may require an electric-magnetic coupled version. Any grids present tend to require a full 3D analysis, hence LORENTZ 3D-E