Gridded ion sources are described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745, which is incorporated herein by reference. The ion sources described therein use a direct-current electrical discharge to generate ions. It is also possible to use a radiofrequency electrical discharge to generate ions, as shown by U.S. Pat. No. 5,274,306—Kaufman et al.
The ion optics for gridded ion sources incorporate closely spaced grids with mutually aligned pluralities of apertures, through which the ions are electrostatically accelerated. A high current density of these accelerated ions at the desired operating voltages is beneficial in that it corresponds to a high process rate in an industrial application and a high thrust in a space electric-propulsion application. The maximum current density of the accelerated ions varies inversely as the square of the distance between the grids, so that obtaining a high current density requires closely spaced grids.
A close grid spacing can be obtained easily for small ion beams with small ion current capacities, but becomes progressively more difficult as the beam diameter (assuming a circular beam) becomes larger. To include the effect of beam diameter, d, in the difficulty of maintaining a given nominal grid spacing, L, it has been found useful to use a span-to-gap ratio, d/L, as discussed in the aforesaid article by Kaufman, et al. As also described in the aforesaid article, a large span-to-gap ratio, hence a large ion beam current, can be obtained by using grids having a matching dished shape. For dished grids, the grids approximate matching segments of a sphere instead of the more obvious flat shapes used in most early ion sources. This beneficial effect of dished grids has been the motivation for development of the complicated fabrication techniques required for these grids, as described in U.S. Pat. No. 3,864,797—Banks and U.S. Pat. No. 3,914,969—Banks.
While dished grids have permitted larger span-to-gap ratios, they also have a substantial degree of curvature. This curvature can be used in some industrial applications to generate either focused or defocused ion beams, as described in a brochure by Kaufman, et al., entitled Characteristics, Capabilities, and Applications of Broad-Beam Sources, Commonwealth Scientific Corporation, Alexandria, Va. (1987). If a more collimated ion beam is desired, the curvature used in conventional dished grids presents a problem in that the grids must first be dished, then the apertures in the two grids must displaced relative to each other to obtain a more parallel beam. The trajectory deflection obtained by aperture displacement is also described in the aforesaid brochure. This displacement is obtained, however, with a reduction in maximum ion beam current.
The use of conventional dished grids in ion optics thus permits the use of a large span-to-gap ratio (a small spacing for a given beam diameter), but requires the expense of dishing the grids and at the same time makes it difficult to obtain a nearly collimated ion beam.