Patent ID: 8471452

Claim:
Apparatus for accelerating a beam of positive ions comprising: a first electrode having a proximal side and a distal side and having at least one aperture 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; a second electrode 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 a third electrode 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 said first electrode is arranged to operate at a first potential, said second electrode is arranged to operate at a second potential, and said third electrode is arranged to operate at a third potential, and wherein there is a potential difference between the first and second electrodes and a potential difference between the second and third electrodes; wherein a ratio of potential difference across a gap between the first electrode and the second electrode to a final beam energy is at least 2:1; wherein a thickness of the first electrode material divided by the aperture entrance radius is from 0.5 to 1; wherein the aperture side wall of the first electrode is beveled for a portion of the thickness of the first electrode on the proximal side and flat for a remaining portion of the thickness of the first electrode on the distal side of the first electrode, with the bevel sloping at an angle of from 20° to 60°; and wherein a ratio of the bore radius on the proximal side of the first electrode to the bore radius on the distal side of the first electrode is 1.1 to 1.7.