X-ray tube apparatus

An X-ray tube apparatus comprising:

• an X-ray tube including a vacuum envelope having a tube axis, an anode target and a cathode assembly means which are disposed within said vacuum envelope in a manner to oppose each other, said cathode assembly means including:
• a flat plate-like filament for generating an electron beam;
• beam shaping electrode means formed with an elliptical beam limiting aperture and with a focusing dimple having a shape substantially similar to said beam limiting aperture, said beam shaping electrode means being insulated from said filament, when Sy, Sx, Dy, Dn are assumed to represent the longitudinal length of said focusing dimple, the lateral length of said focusing dimple, the longitudinal length of said beam limiting aperture, and the lateral length of said beam limiting aperture, respectively, and P and Q are assumed to represent the value of the ratio Sy/Dy and the ratio Sx,Dx, respectivley, the value of the ratio (P/Q) of P to Q satisfies the inequality:
• 0.4<P/Q<2.0;
• a concave elecltrostatic lens for diverging the electron beam from said filament to the beam shaping electrode means, which is formed between said beam shaping electrode means and said filament;
• and,
• a convex electrostatic lens for convertging the diverged electron beam passing through the beam limiting electrode means to a focal point defined by the concave and convex electrostatic lenses, which is formed within the dimple of said beam shaping electrode;
• said anode target having a target surface which is disposed between the beam shaping electrode means and the focal point of the lenses and on which the converged electron beam is impinged to form an electron beam spot which has longitudinal length ly and lateral length lx, X-rays being irradiated from the electron beam spot in an X-ray irradiation direction,
• when d2 and d3 are assumed to represennt a depth of said focusing dimple and a distance between said beam shaping electrode and said target surface, respectively, the value of the ratio of d2 to d3 satisfies the equality or inequality:
• 1. 0.ltoreq.d3/d2.ltoreq.4.0,
• when .theta. is assumed to represent an anode angle defined between the target surface of said anode target and the X-ray irradiation direction, the relationship between said anode angle .theta. and the value of the ratio Sy/Sx of said longitudinal length Sy to the lateral length Sx satisfies the inequality:
• 1<Sy/Sx<2/sin .theta.
• and
• power soure means including a first power source for applying first voltage across said anode traget and said filament, a second power source for supplying an electric current to said filament so as to heat the same, and a third power source for applying a bias voltage to said beam shaping electrode means, said bias voltage being positive against said filament and the focal point being shifted away from said target as the bias voltage is increased, whereby the size of the beam spot is increased in such a manner that the ratio of ly/lx is maintained within the range of 1.4.