Patent Number: 043119123
Section: claims

1. In a neutron accelerator tube having a replenisher section for supplying accelerator gas, an ionization section adjacent said replenisher section, and a target section adjacent said ionization section comprising: (a) means forming a target chamber in said tube,  (b) a target in said chamber having an active surface facing said ionization section formulated of a material which produces neutrons in response to bombardment by ionized accelerator gas,  (c) a cathode member interposed between ionization section and said target chamber, said cathode member having a recessed convergent surface exposed to said target chamber which converges down to a point at which an aperture in said cathode member extends from said target chamber to said ionization section to provide for the flow of ionized gas from said ionization section into said target chamber, said cathode thereby giving a concave-like appearance when viewed from said target chamber, and  (d) an extraction electrode in said target chamber interposed between said target and said cathode member and having a divergent projecting surface facing said recessed cathode surface which projects outward to a point at which an aperture therein extends from said target chamber toward said target, said electrode thereby giving a convex-like appearance when viewed from said cathode, whereby the relationship between said cathode member and said extraction electrode surfaces concentrates the electric field in said target chamber through the central portion thereof so as to enhance the acceleration and extraction of ions through said cathode aperture to said target, and  whereby the field strength in the peripheral regions of said target chamber is maintained at an acceptable level without internal arcing associated with field emission breakdown. 2. The system of claim 1 wherein the ratio of the diameter of said cathode aperture to the length thereof is at least 0.75. 3. The system of claim 1 wherein the diameter of the aperture in said electrode is greater than the diameter of the aperture in said cathode. 4. The system of claim 1 wherein the diameter of the aperture in said electrode is greater than the length thereof. 5. The system of claim 1 wherein the slope of the projecting surface of said electrode in the direction of said target is at least as great as the slope of the recessed surface cathode in the direction of said target. 6. The system of claim 5 wherein the slope of said electrode surface in the direction of said target is greater than the slope of said cathode surface in the direction of said target. 7. The system of claim 6 wherein the diameter of said electrode aperture is greater than the diameter of said cathode aperture.