Electrostatic lens to focus an ion beam to uniform density

An electrostatic focusing lens for focusing a point source ion beam of gaussian cross sectional distribution to a uniform cross sectional distribution at a given distance from the lens, comprising a grounded inner semi-transparent grid which is axi-symmetric with said ion beam, said grid having a minimum axial aperture that matches a central cone portion of said beam, said grid being a double conical grid including a plurality of equally spaced metallic wires with a high melting point and a low expansion coefficient and a grid ring encompassing said grid wires and defining said axial aperture and provided with a desired radius with said grid wires being threaded therethrough to thus form said double conical grid, said lens further including a first grid support ring, a second support ring, a first pair of spacer rods mounted between and to said first and second rings for providing a desired spacing therebetween, a third spring support ring, a second pair of spacer rods mounted between and to said second ring and to said third ring, and a plurality of springs equal in number to said grid wires, said grid wires being attached at first respective ends thereof to the inner edge of said first support ring with the other respective ends of said grid wires extending through said second support ring and fastened to first respective ends of said springs and the other respective ends of said springs being affixed to the inner edge of said third spring support ring, and an insulated outer anode encompassing and spaced from said double conical grid within said desired spacing between said first and second support rings, said first, second and third support rings as well as said grid being at ground potential, said anode adapted to be connected to a voltage source, said anode shaped such that said voltage applied thereto provides an electrostatic field between said anode and said grid, said grid, anode and support rings adapted to be mounted within an evacuated housing, whereby a portion of the ions from said ion beam that pass through the inside of said grid are undeflected by the zero field that exists within said grid, and the remaining portion of the ions from said ion beam that are outside of said central cone defined by said grid are deflected by said electrostatic field toward the axis of said grid an amount proportional to their minimum radial distance from said central cone such that the deflected and undeflected portions of said ion beam merge to produce said uniform distribution at said given distance from said lens.