To fulfill the need of a plasma ion source for SPIRAL-2 and EURISOL, capable of producing radioactive ion beams under strong radiation, the first prototype of the IRENA (Ionization by Radial Electrons Neat Adaptation) ion source has been designed. For designing an optimized prototype based on the first one, the influence of the geometrical parameters on the electron trajectories is investigated by means of two-dimensional/three-dimensional (3D) simulations. Due to the strong space charge effect in this kind of ion source, 3D simulations help particularly to better estimate effects on ion confinement and extraction. Simulation constructions will be presented and results discussed.