Electron Guns

Author: J.B. Hoag

In many radio tubes there is a centralized filament surrounded by the various grids and plates. The electrons spread out from the filament radially in all directions. We might say that they are " broadcast."

There is an entirely different class of tubes, namely, cathode-ray tubes, television tubes, positive-ray and atom-smashing machines, which require that the charged particles should not spread in all directions from their source but should be confined to a parallel beam, or, at most, should diverge but slightly along their entire route to the end plate.

Fig. 21 G. The electron gun of a cathode-ray tube. (From E. & N. P.)

Figure 21 G shows an electron gun used in a cathode-ray oscilloscope. The cathode, K, is of the indirectly heated type wherein a hot wire is mounted inside a small metal thimble about the size of the lead in a lead pencil. The front end of this thimble is concave and is coated with chemicals which render this portion of the structure a good emitter of electrons. This cathode is surrounded by a metal cylinder, G, having a metal diaphragm with a small hole directly in front of the cathode surface. This " grid " is charged negatively with respect to the cathode. As a result, the electrons, which are themselves negative, are prevented from diverging from the cathode and are bent inward along the axis of the tube, even to the point of crossing over and again starting to diverge on the other side of the axis. According to the potential of the grid, larger or smaller numbers of electrons are brought along the axis of the tube, or are permitted to diverge and be lost inside the gun structures. In other words, the grid serves as the chief control of the number of electrons which leave the gun. The number of electrons in the cathode ray or electron beam is easily changed by turning the knob of a rheostat which varies the voltage of the grid.

In front of the grid of Fig. 21 G, there is a cylinder A₁ containing several metal diaphragms with holes in them. This is called the first anode. It is charged positively with respect to the cathode, to a potential of several hundred volts. Beyond the first anode there is a shorter but larger diameter anode A₂, charged to a much higher potential, in some cases amounting to several thousand volts. The electric field set up between the two anodes serves, as in Fig. 21 F, to bring the electrons into a near-parallel beam. The electrostatic field between the two anodes can be shown to be the equivalent of a thick, unsymmetrical converging lens. Its focal length can be readily changed, within limits, by varying the voltage applied between the two anodes. In this respect, electron lenses are greatly superior to optical lenses. With a twist of the wrist to turn a rheostat, it is possible to change the focal length of the lens. Glass lenses cannot be so changed, but the crystalline lens of the human eye can be made of shorter focal length by the muscles which control it.

Fig. 21 H. A modern electron gun

The gun structure of Fig. 21 H is used with television pickup tubes.

Fig. 21 I. A gun for the production of intense electron beams

The gun of Fig. 21 I produces very intense electron beams (2 ma. at 10,000 volts concentrated into a spot only 0.3 mm. in diameter). It uses both electrostatic and magnetic focusing.