The design and performance of a low‐energy ion gun system suitable for surface analysis is described. The ion gun system is capable of delivering up to a 0.72 &mgr;A beam of Ar+ions at a potential of 500 eV into a spot diameter of 1 mm. This performance is accomplished by using ion optics to refocus the space‐charge diverging beam along the 44‐cm path to the target. The ion optics have been optimized by the use of a new simulation program,chden, which can model a series of lenses and the effects of space‐charge forces.