X-ray tube having focusing cup with non-emitting coating

Abstract

An x-ray tube is described including a focusing cup electrode coated with a high work function material, such as platinum or gold, to prevent the field emission of electrons from such cup. The method of applying the non-emitting coating is preferably sputtering or ion plating, but may also be electroplating followed by vacuum fusion in the case of gold or other low melting point metals.

We claim: 
     
       1. An x-ray tube in which the improvement comprises: an anode;   a focusing electrode  .Iadd.including a body having a focusing recess therein which defines a focusing surface.Iaddend.;   a thermionic cathode mounted .Iadd.in said recess .Iaddend.adjacent .Iadd.said focusing surface of .Iaddend.said focusing electrode so that said focusing .[.electrode.]. .Iadd.surface .Iaddend.focuses the electrons emitted by said cathode onto said anode to cause x-rays to be emitted from said anode;   an evacuated envelope containing said cathode, anode and focusing electrode; and   a layer of non-electron emissive material provided on at least a portion of .Iadd.said focusing surface .Iaddend.of the focusing electrode and having a higher work function than the underlying base material of said focusing electrode .[.substantially.]. to prevent the .Iadd.field .Iaddend.emission of electrons from said focusing electrode.   
     
     
       2. A tube in accordance with claim 1 in which the non-emissive material is taken from the group consisting of gold and platinum. 
     
     
       3. A tube in accordance with claim 2 in which the cathode is a coiled filament. 
     
     
       4. A tube in accordance with claim 3 in which the filament contains thoriated tungsten. 
     
     
       5. A tube in accordance with claim 1 in which the layer of non-emissive material is a sputtered layer. 
     
     
       6. A tube in accordance with claim 5 in which the sputtered layer contains platinum. 
     
     
       7. A tube in accordance with claim 1 in which the .[.forming.]. .Iadd.focusing .Iaddend.electrode includes a focusing cup within which a filament cathode is mounted, and the layer of non-emissive material is provided on the surface of said focusing cup. .Iadd.8. The x-ray tube of claim 1 wherein said focusing surface is a curved surface. .Iaddend. .Iadd.9. An x-ray tube comprising: an evacuated envelope;   an anode mounted in the envelope and including an x-ray generating target area;   a cathode mounted in the envelope in spaced relationship with the anode and adapted to emit electrons toward the anode;   an electron focusing electrode mounted in the envelope and including a focusing surface shaped and positioned to focus electrons emitted by the cathode onto said target area;   conductor means connected to the anode, the cathode, and the focusing electrode for connecting each of them to sources of electric energy whereby to cause the focused flow of electrons from the cathode to said target area; and,   said focusing electrode being comprised of a base material and a coating of a nonelectron emissive inhibiting material having a higher work function than the base material, said coating forming at least a portion of said focusing surface whereby to prevent electron emission by the focusing   
     
     
        electrode. .Iaddend. .Iadd.10.  The X-ray tube of claim 9 wherein the nonelectron emissive material is sufficiently smooth that arcing between said cathode and said focusing surface is minimized. .Iaddend. .Iadd.11. The X-ray tube of claim 10 wherein said nonelectron emissive material is taken from the group consisting of gold and platinum. .Iaddend.