Abstract: A lens system for a beam of energetic ions employs a focusing element in combination with a defocusing element. In a preferred embodiment, the focusing element is in the form of an electromagnetic lens, such as a solenoid, through which is propagated the beam of energetic ions. Also in the beam path is provided a defocusing element which also may be in the form of an electromagnetic element, such as a solenoid, or a magnetic element, such as a magnetic dipole. Alternatively, an electrostatic defocusing element may be employed in the practice of the invention. In a magnetic embodiment, defocusing is achieved by propagating the beam of energetic ions around the exterior of the defocusing element. The focusing element may be of the electrostatic variety, such as a radial an electric field lens. In other embodiments, a further focusing element, or a further defocusing element may be arranged in the beam path.