Patent Number: 
Section: claims

1. An electron beam focusing electrode, comprising:a plate having a single polygonal through-hole and electrically connected to a power source; anda protrusive portion formed on at least one side of the single polygonal through-hole,wherein an inner surface of the through-hole is inclined with respect to a traveling direction of an electron beam passing through the through-hole. 2. The electron beam focusing electrode according to claim 1, wherein the protrusive portion is spaced apart from both ends of the side on which the protrusive portion is formed. 3. The electron beam focusing electrode according to claim 1, wherein a length of the protrusive portion is smaller than the distance from a center of the through-hole to the side on which the protrusive portion is formed. 4. The electron beam focusing electrode according to claim 1, wherein the polygonal through-hole includesfour sides, andfour protrusive portions respectively arranged on the four sides, each protrusive portion protrudes from a center of the respective side. 5. The electron beam focusing electrode according to claim 4, wherein each of the protrusive portions has a rectangular cross section. 6. An electron beam focusing electrode, comprising:a plate having a single polygonal through-hole and electrically connected to a power source; anda protrusive portion formed on at least one side of the single polygonal through-hole,wherein the through-hole has a first area and a second area, the first area is smaller than the second area, the first area is an incident area of an electron beam, and the second area is an emission area of the electron beam with respect to a traveling direction of an electron beam passing through the through-hole. 7. The electron beam focusing electrode according to claim 6, wherein the protrusive portion is spaced apart from both ends of the side on which the protrusive portion is formed. 8. The electron beam focusing electrode according to claim 6, wherein a length of the protrusive portion is smaller than the distance from a center of the through-hole to the side on which the protrusive portion is formed. 9. The electron beam focusing electrode according to claim 6, wherein the polygonal through-hole includesfour sides, andfour protrusive portions respectively arranged on the four sides, each protrusive portion protrudes from a center of the respective side. 10. The electron beam focusing electrode according to claim 9, wherein each of the protrusive portions has a rectangular cross section. 11. An electron gun comprising:a cathode electrode radiating electrons;an anode electrode spaced apart from the cathode electrode and on which the electrons radiated from the cathode electrode are focused; andan electron beam focusing electrode, includinga plate having a single polygonal through-hole and electrically connected to a power source anda protrusive portion formed on at least one side of the single polygonal through-hole,wherein the electron beam focusing electrode is between the cathode electrode and the anode electrode. 12. The electron gun according to claim 11, wherein the electron beam focusing electrode is electrically isolated from the cathode electrode. 13. The electron gun according to claim 11, wherein the electron beam focusing electrode is connected to the cathode electrode. 14. The electron gun according to claim 11, further comprising:a gate electrode between the electron beam focusing electrode and the anode electrode to adjust a current quantity of an electron beam. 15. The electron gun according to claim 11, wherein the length of the protrusive portion is smaller than the distance from a center of the through-hole to the side on which the protrusive portion is formed. 16. The electron beam focusing electrode according to claim 11, wherein an inner surface of the through-hole is inclined with respect to a traveling direction of an electron beam passing through the through-hole. 17. The electron beam focusing electrode according to claim 11, wherein the through-hole has a first area and a second area, the first area is smaller than the second area, the first area is an incident area of an electron beam, and the second area is an emission area of the electron beam. 18. The electron gun according to claim 11, wherein the polygonal through-hole includesfour sides, andfour protrusive portions respectively formed on the four sides, each protrusive portion protrudes from a center of the respective side, and each of the protrusive portion has a rectangular cross section. 19. The electron gun according to claim 11, whereinthe anode electrode has a hole at a center of the anode electrode,the cathode electrode includes a cathode sleeve with an emission hole, an inner surface of the emission hole is formed to have an angle with respect to a surface of the cathode electrode. 20. The electron gun according to claim 11, wherein the cathode electrode is one of a cold emission cathode, a photocathode and a plasma source. 21. The electron gun according to claim 11, further comprising:a heat shield mounted around the cathode electrode to shield heat radiated from the cathode electrode, the cathode electrode is a thermionic emission device. 22. The electron beam focusing electrode according to claim 11, wherein the protrusive portion is spaced apart from both ends of the side on which the protrusive portion is formed. 23. The electron beam focusing electrode according to claim 11, wherein a length of the protrusive portion is smaller than the distance from a center of the through-hole to the side on which the protrusive portion is formed. 24. The electron beam focusing electrode according to claim 11, wherein the polygonal through-hole includesfour sides, andfour protrusive portions respectively arranged on the four sides, each protrusive portion protrudes from a center of the respective side. 25. The electron beam focusing electrode according to claim 24, wherein each of the protrusive portions has a rectangular cross section.