The invention relates to a system for focussing and/or for collimating an ion beam (7, 10), particularly of a beam of accelerated protons (7, 10). To this end, the system has a lens with a lens body (8) that can be passed through by the ion beam. Means are provided for generating an, in particular, electrostatic field, which propagates inside the lens body and which focuses the ion beam. The lens body has a wall of a small thickness. The field-generating means have a source for electromagnetic radiation, i.e. a laser, whose emitted beam (9) is oriented toward the outside of the wall of the lens body. The thickness of the wall and the quality of the electromagnetic radiation is selected in such a manner that the radiation generates free electrons, which exit the wall and accumulate in an electron cloud on the exit side of the wall.