"Electron Gun"

The most common form of an electronic "commutator" used in

television receiving and transmitting devices is shown schematically

In the Figure. The source of the electrons is a small flat thermionic

cathode, consisting of a nickel disc coated with a relatively stable

low work function¹ material, such as a mixture of barium and

strontium oxides. As the material is heated, a minute fraction of

the electrons within it attains sufficient energy to overcome the

potential barrier at the surface which prevents the bulk of the

electrons from escaping. These electrons are accelerated toward a

positively biased first anode² and at the same time deflected toward

the axis by the negatively biased control grid³. They form a pencil

of minimum cross section at the point where the principal paths

(i. e. the paths of electrons leaving the cathode with zero velocity)

cross the axis and diverge from this point on toward a final

electron lens, which serves to image the crossover on the image

screen or target. The final lens illustrated is that formed between

two cylinders at different potentials; the sequence of curved lines

represents the equipotential surfaces which can be thought of as

refracting the electron paths in the same manner as a boundary

surface between two media of slightly different refractive index⁷.

The magnetic field of a short solenoid can be similarly employed for imaging the crossover on the screen although the detailed interac­tion between field and electron is quite different. The intensity of the electron beam is varied by changing the potential of the control grid; as the potential of the latter is reduced a smaller fraction of the elec­trons emitted by the cathode passes through the grid aperture⁸, the remainder being turned back toward the cathode. At the same time, the crossover moves toward the cathode; this has, however, only a mi­nor effect on the sharpness of focus on the screen since the crossover displacement is small compared to the distance between the crossover and final lens.

The system shown in the figure, designated as an electron gun, serves merely to form a sharply focused electron spot of controllable intensity at one point on the screen or target. To effect the "commuta­tion" the beam is subjected to a pair of transverse magnetic (or elec­tric) fields just beyond the final lens. The exciting currents (or volta­ges) for the horizontal and vertical deflector⁹ exhibit a sawtooth-shaped¹⁰ variation with periods corresponding to the time required for describing a single scanning line (about 60 microseconds) and a complete picture field (1/60 second) respectively, the electron emission being suppressed during the short return times of the sawtooth. As a result, the electron spot covers the screen and target area with a clo­sely spaced raster¹¹ of horizontal lines; the deflections at the transmit­ter and receiver are synchronized so that the beam scanning a certain point of the image area in the receiver is modulated by the signal deri­ved from the corresponding point of the scene at the transmitter. The design of deflection systems becomes a complex electron-optical prob­lem when practical considerations (such as large viewing screens and small receiver depth) demand large angular ranges of deflection, commonly of the order of 110°.