The resulting disadvantages are heating of the control grid and reducing beam current, thereby reducing radiation output of the X-ray tube.Īn object of the present invention is to provide a Pierce type electron gun in an X-ray generator with grid control capability without experiencing the above mentioned disadvantages. However, if the grid is at a positive potential with respect to the electron emitter, the grid will collect electron beam current. However, the beam current magnitude of such guns is not ordinarily controllable with a grid because the grid would have to be at a positive potential to avoid distortion of the electric field produced between the accelerating anode and the cathode. Pierce guns can be designed for producing high electron beam currents and are desirable for use in X-ray tubes for that reason. Further along the electron beam path there is an apertured accelerating anode which increases the energy of the electrons such that most of the electrons in the convergent beam pass through the aperture and impinge on the X-ray target anode. The field causes the emitted electrons to converge. This type of gun comprises an electron emissive cathode having a curved emitting surface and electric field shaping or focusing electrodes connected to the cathode and located near it. The well-known Pierce type of electron gun has been used in various electron tubes including X-ray tubes for producing a focused electron beam that impinges on a traget anode. The invention is particularly concerned with controlling the electron beam current magnitude and with selecting beam focal spot size in an X-ray tube. The invention pertains to X-ray generators or X-ray tubes as they are commonly designated.
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