Autofocus Tube

Autofocus tube, an X-ray tube, the focal spot area of which is automatically changed in accordance with changes in the tube load. The automatic change in focal spot area is achieved with the help of a device, schematically shown in the figure. In the middle of the turns of an ordinary flat spiral cathode, perpendicular to its plane, a rod S is insulated from the turns. The high voltage from the transformer is applied directly to rod S, while to the spiral turns through a large resistance R. During tube operation, the rod will always have a slightly greater negative potential relative to the spiral, and this potential difference must depend on the strength of the current flowing through the tube. Let resistance R = 10,000 ohms; then, with a tube current of 100 mA, the potential drop in resistance R will be (according to Ohm's law) 1,000 volts. By this amount, the potential of the rod will be higher than the potential of the spiral. With a tube current of 1 mA, the difference between the negative potentials of the rod and the spiral will only be 10 volts. Rod S, having a more negative potential than the spiral, will always influence the path of the electrons from the spiral such that they will be somewhat scattered in their movement toward the anticathode, forming a focus with a larger area. This increase in focal spot area will be greater the more the potential of the rod exceeds the potential of the spiral, i.e., it will be, as indicated above, at higher tube loads. The increase in focal spot size at higher loads protects the tube's anticathode from rapid destruction, since the energy lost by the electrons is distributed over a larger surface.

v. Karchagin.