The Klystron

Author: J.B. Hoag

Let a parallel beam of electrons, moving with constant gauzes connected to a high frequency oscillator, as in Fig. 38 I.

Fig. 38 I. Velocity modulation of a cathode ray

The high frequency electrostatic field between the grids is parallel to the electron stream and will accelerate the electrons at one moment, retard them at another. We assume that the change of velocity of the electrons produced in this manner is an appreciable, but not too large, fraction of their original velocity. The electrons which leave the grids at the higher velocity will catch up with electrons ahead of them, while those at lower velocity will be bunched with those behind. Thus the emerging electrons will be grouped together in bunches along the direction of motion. This is referred to as velocity modulation.

If a velocity-modulated beam of electrons passes through two gauzes, as at 2 in Fig. 38 J, the higher and lower speed electrons will induce different amounts of potential between the gauzes.

Fig. 38 J. The principle of the klystron

These alternating potentials may be strengthened by an LC circuit tuned to the frequency of the bunched electrons, i.e., to the frequency of the r.f. generator at 1 in this figure.

Next, it is desired that the h.f. voltages of 2 in Fig. 38 J shall be fed back to 1 in such a phase and strength that they will serve to replace the r.f. generator. Then, if energy conditions are satisfactory,¹ a self-generating device will be available. Inasmuch as the electron transit time down the tube is very small, it is obvious that the scheme is useful at the ultra-high frequencies.

We recall that, at ultra-high frequencies, the " circuits " should be of the concentric-line type in order that the losses shall not be too great, i.e., that Q shall be high. As the frequency is raised higher and higher, the distance between the inner and outer conductor of the concentric line becomes more and more nearly comparable with the wave-length involved. It is necessary to consider not only the length but also the transverse dimensions, because standing waves can exist in the air between the inner and outer conductors as well as longitudinally. In fact, the inner conductor can be removed, leaving only a hollow tube, and still have suitable resonance conditions. Not only hollow tubes, but many other shapes, such as spheres, ellipsoids, etc., have been used. They are called cavity resonators. The electromagnetic field patterns inside these systems are often complicated (they are in three dimensions). Their study follows the same lines of reasoning used in the study of the acoustics of rooms, where the sound waves reverberate back and forth from the walls, floor, and ceiling. If the physical dimensions are properly chosen for the wave-lengths involved, three-dimensional standing-wave patterns are set up which greatly strengthen the sound intensity; and similarly strengthen the electric and magnetic fields in the electrical case. Cavity resonators may be likened to three-dimensional resonant short-lines, while the wave-guides (see later section) are like long-lines.

Cavity resonators have been used as the tuning " circuits " of the buncher and catcher systems, in the manner shown in Fig. 38 K.

Fig. 38 K. A cavity resonator

A complete oscillator of this type, called a klystron, is shown in Figs. 38 L and 38 M.

Fig. 38 L. Cross-sectional view of a klystron. (Courtesy of Electronics)

Fig. 38 M. Photograph of a klystron (cutaway). (Courtesy of Electronics)

The method of feeding energy from the catcher to the buncher, and also of removing energy to a load, such as an antenna, is shown in Fig. 38 L. The distance between the grids of the buncher is made less than the distance traveled by an electron in a half-cycle, i.e., less than vλ/2c centimeters apart, where v = electron velocity, c = velocity of light, λ = wave-length generated. The grids of the catcher may be spaced vλ/4c cms. apart. The Q of the cavity resonators is approximately 1,000. Considerable power has been generated at microwave-lengths as short as 10 cms. The klystron can also be used as an amplifier and as a detector.

1	The theory by which more energy is given up by an electron to the tuned circuit than is given to it by the " buncher " is not easy to explain in non-mathematical form.