CATHODE FOR FOCUSING ELECTRON OF X-RAY TUBE, CATHODE ASSEMBLY FOR FOCUSING ELECTRON BEAM, AND METHOD FOR CHANGING DIMENSION OF FOCUS OF ELECTRON BEAM

摘要

PROBLEM TO BE SOLVED: To provide a cathode for focusing electrons of X-ray tube capable of regulating or varying dimensions of focuses of an X-ray tube to improve image quality while maintaining service life of a desired tube, a cathode assembly for focusing electron beams, and a method for changing dimensions of focuses of electron beams. SOLUTION: A cathode having a great cavity 21 inside thereof is used. In the cavity, electron clouds are generated, and the cavity is shielded from a main electric field between a cathode and an anode. The electron clouds flow into the main electric field via a small, narrow path 22 from the cavity. A pair of electric lattices separately provided in opposite to each other and including arrays 28 and 29 composed of individual electrode segments respectively, has selected opposite segments to which a negative bias is electrically applied. A sectional area of the flow of electrons passing between these segments is changed. The sectional area with the changed electron flow determines dimensions of focuses of electron beams colliding with a target anode.