Patent Number: 041918886
Section: claims

1. In an ion extraction system utilizing small hole accel grid optics and comprising within a discharge chamber a neutral highly ionized plasma source at a first voltage V.sub.1, V.sub.1 being positive, a plurality of electrodes downstream of said plasma source each having apertures in axial alignment and including in the following order a screen grid at said first voltage V.sub.1 and an accel grid at a second voltage V.sub.2, V.sub.2 being negative, said accel grid having a relatively small aperture in the range of less than 50% to approximately 10% the size of the aperture of said screen grid, the electric field set up by said screen grid and said accel grid serving to extract an ion beam from the plasma source and focus the ion beam through the accel grid aperture, the improvement wherein said accel grid is provided with a relatively large aperture and a metallic foil on at least the downstream face thereof covering said large aperture, said relatively small aperture being etched in the metallic foil by the ion beam. 2. The improved accel grid structure recited in claim 1 further comprising a metallic foil on the upstream face of the accel grid and covering said large aperture, a relatively small aperture also being etched in the metallic foil on the upstream face by the ion beam. 3. The improved accel grid structure recited in claims 1 or 2 wherein the metallic foil is a thin foil of tantalum. 4. The improved accel grid structure recited in claim 1 wherein the accel grid is copper and the metallic foil is nickel.