The present invention concerns a system for the focusing and/or a collimation of an ion beam, in particular a beam of accelerated protons, wherein the system has a lens with a lens body that is permeable to the ion beam, wherein means for the generation of an in particular electrostatic field, propagating within the lens body and focusing the ion beam, are provided, wherein the lens body has a wall of low thickness, wherein the means for the field generation comprise a source of electromagnetic radiation, whose emitted beam is directed onto the outer side of the wall of the lens body, wherein the thickness of the wall and the quality of the electromagnetic radiation is chosen such that the radiation generates free electrons that emerge from the wall and accumulate on the exit side of the wall in an electron cloud.