Gridded convergent flow electron gun

Claim 1

In an electron gun: thermionic cathode emitter means having, a base member with a concave emitting surface, said concave emitting surface being constituted of a plurality of lesser individual concave oxide coated cathode emitter surfaces, said lesser oxide coated emitter surfaces being concave in each of two orthogonal directions with radii of curvature substantially less than that of said composite cathode emitter surface; an accelerating electrode means having a central aperture of substantially smaller cross-sectional area than the area of said emitting surface of said cathode emitter, said central aperture being disposed in axial alignment with the center of said concave emitting surface of said cathode emitter for accelerating and converging a stream of electrons through said central aperture of said accelerating electrode into a unitary beam of electrons; control grid means comprising a multiapertured concave control grid structure disposed overlaying the concave emitting surface of said cathode emitter with the centers of individual apertures in said control grid structure being disposed in alignment along the convergent beam path with the centers of individual lesser concave cathode emitting surfaces to form a plurality of individual convergent beam electron guns projecting the beam through said control grid in a plurality of nonintercepting beamlets which converge into the unitary beam after passage through said control grid; shadow grid means comprising a second multiapertured grid structure of substantially identical hole pattern to that of said control grid structure, said shadow grid structure being carried from said cathode emitter and operated at the same potential as said cathode emitter for inhibiting thermionic emission from the underlying regions of said cathode emitter shadowed by said shadow grid and for focusing the lesser beamlets through the respective aligned apertures in said control grid structure, and said shadow grid being made of a materail having essentially the same coefficient of thermal expansion as that of said cathode emitter, said shadow grid being joined to the adjacent surface of said cathode emitter at a plurality of discrete points spaced over the surface of the cathode emitter, the remaining portions of said shadow grid being adjacent, but free of attachment to, said cathode emitter.