An apparatus (200) for accelerating an ion beam comprising: a) a first electrode (202) having a proximal side and a distal side and having at least one aperture (201) therethrough, the wall of the aperture being shaped such that the radius of the aperture on the distal side of the first electrode is greater than that on the proximal side of the electrode; b) a second electrode (204) located such that it is adjacent to but spaced from the distal side of the first electrode and having at least one aperture therethrough; and c) a third electrode (206) located such that it is adjacent to and spaced from the second electrode and having at least one aperture therethrough, said at least one apertures in each electrode being aligned with corresponding apertures in the other electrodes; wherein the electrodes are arranged such that there is a potential difference between the first and second electrodes and a potential difference between the second and third electrodes.