Correction of spherical aberration for an electrostatic gridded lens

Two methods to correct spherical aberration in electrostatic gridded lenses have been studied using ray-tracing simulations. Both methods are based on modifying the electrostatic field on the radial periphery of the lens. In the simplest case, the modification is done by extending the grid support axially. In the second method, the electric field on a radial periphery of the lens is modified by applying optimum voltage on an isolated correcting electrode. It is demonstrated that, for a given focal length, the voltage on this lens can be optimized for minimum aberration, and also that these lenses reduce the emittance growth of the ion beam.