Klystron: Tube for Outer Space
February 1961 Radio Electronics

February 1961 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Klystron - tube for outer space

Not only for outer space, but for its usefulness wherever microwaves must be generated, the importance of this tube grows with the industry's use of higher and higher frequencies

Less romantic, but even more practical, are other applications for klystrons. Large-power klystrons are used widely in Europe for UHF television transmitters. Here UHF television has not become common enough to need many power klystrons. Klystrons are also the heart of the new "scatter" communications systems in which the line-of-sight rule about microwave transmission is violated simply by using very high-power transmitters, large antennas and ultra-sensitive receivers.

Another major use of klystrons is in experiments with food sterilization. These use high-speed electrons issuing from linear electron accelerators, and these in turn are powered by large klystron tubes.

In linear accelerators, the klystrons provide a mighty push to the electrons passing through successive drift tubes, eventually speeding them up to almost the speed of light.

What then are these klystrons, what do they look like and how do they operate?

Klystrons were invented just before the start of World War II by the Varian brothers, then graduate students at Stanford University. I remember their little shack behind the Ryan laboratory in the hills behind the university, and the excited talk of a resident in the area who had seen the barbed-wire fence around this little shack develop a mysterious red-hot glowing section of wire, True or not, the klystron has played an enormously important role in the development of radar and microwave communications, and is now on the verge of taking over industrial jobs from other tubes.

Resonant Cavities

To start the explanation of klystrons, let us first look into another item, resonant cavities. Understanding cavities is essential to understanding klystrons. All RF oscillating circuits contain resonant elements (Fig. 1-a). As frequency increases, we must decrease the inductance and capacitance of the resonant circuits. We decrease the inductance by decreasing the turns until we have nothing left but a straight wire or even a flat strip of metal. The capacitance is reduced by lowering the number of plates in our capacitor and finally by further separating the plates (Fig, 1-b). Eventually we get to paralleling inductances (Fig. 1-c) since paralleling two inductors halves their inductance, and the entire process winds up as in Fig. 1-d or 1-e. The final product is a box or cavity, the top and bottom representing the capacitor plates and the sides the paralleled inductors.

Cavities follow certain hard and fast rules, which can be determined easily from common-sense observation. For example, regarding the top and bottom plates of the cavity as plates of a capacitor, we see that they are virtually short-circuited at the edges. This means that at the edges of the plates we cannot have a charge, and therefore no field. From this follows our first rule about cavities: the electric field parallel to a wall must be zero at that wall. Now to maintain any charge which has a field in the center of the plate and none at the edges, the voltage distribution must look something like a sine-wave half-cycle from wall to wall. In fact, this is the simplest way we can maintain a field in a cavity, the simplest "mode" in which we can operate it. It follows that the width of the cavity should be just about a half-wavelength of the microwave energy, or any multiple of that. And the same goes for the length, if the cavity is rectangular.

The magnetic field always associated with an electric field, and always at right angles to it, will then be parallel to the top and bottom of the cavity. Thus it would cut the end plates. But since it is a changing magnetic field, it will induce a current in any conductor within the field, and the end plates have currents induced in them which set up counter-magnetic fields equal to and thus cancelling the first fields.

Here we have the second rule about cavities: the magnetic field must be zero at any wall which it cuts at right angles. Thus the magnetic field is confined to the box as well. But with the magnetic field we do not have the same dimensional problem, for we can swap density for space. Therefore, the top-to-bottom dimension of the cavity is not as critical, but does determine the capacity of the cavity to maintain a certain field amplitude. For just as a capacitor dielectric would break down if it were too thin for the voltage on the plates, so a cavity can break down, dielectrically speaking, when the voltage gets too high between top and bottom plates. Because we design the cavity carefully as far as dimensions are concerned, we can then set up standing waves in it, and the cavity can easily be excited with small charges on the top and bottom plates.

If we make the cavity an integral part of a vacuum tube, and make part of the top and bottom into a grid area (punch holes in it or slot it), this does not drastically change the properties of the cavity. It can still be excited easily by charge differences between top and bottom plate. The klystron incorporates one or more of these cavities with grids in top and bottom. Fig. 2 is a cutaway representation of a typical two-cavity klystron.

The Bunching Action

At the bottom of the tube we have an electron gun that produces a narrow beam of electrons. This beam leaves the gun under the influence of the accelerating grid, which you can see just below the first cavity. Then the electrons travel on through the two cavities, and the space between them - called the drift space - to the collector, which can collect the electrons because of a positive charge on it. As the electrons travel through the first cavity grids, they constitute a current through these grids, from one grid to the next - after all, a current is nothing more than a flow of electrons. But, since this is a steady flow of electrons, the best that we could expect would be a steady potential difference on the grids.

If we manage to excite the cavity between the grids in some way creating an alternating potential between these grids, we will affect the electrons between them. An electron traveling toward a grid that is positive will be attracted and speed up and one traveling toward a negative grid will slow down. If the bottom grid of the lower cavity is momentarily negative, and the top grid positive, the electrons approaching the bottom grid from the cathode will be retarded, while those between the grids approaching the top grid of the first cavity will be accelerated.

In the next half-cycle of applied RF, the lower grid will be positive and the top one negative. Thus electrons which then approach the lower grid will be accelerated, and the electrons which are then between the two grids will be retarded. In this way, the grids and cavity with applied RF will form bunches of electrons, some of which move faster than when they left the cathode and some of which move a bit slower.

When the RF applied to the cavity goes through zero, the electrons then passing through the grids will not. be affected, and will just travel on at the same velocity. The lower cavity and grid assembly, forming the bunches, is appropriately called the buncher. (The Varians named this a rhumbatron.) In the space between the cavities, the drift space, the electrons that are moving at the original "from-the-cathode" velocity will join some of those which were slowed down. They in turn will be joined by some of those that speeded up. Thus the bunches of electrons in the drift space become denser, and the space between bunches has fewer and fewer electrons.

Were we to let the bunches drift too long, the repulsion between electrons would again scatter them. But we don't give them time to do that. The denser bunches, now with more electrons, pass through the second set of grids. Through these grids then pass alternately dense bunches of electrons and spaces .with none or just a few. This is, in effect, a pulsed dc. Pulsed dc can look very much like ac if we shift the base line (different zero level).

The bunches then constitute a periodically changing current capable of inducing an RF voltage in the second cavity. Note that the acceleration and deceleration of electrons between the buncher grids lasted nearly a half-cycle. The bunches which reach the "catcher" grid are also about a half-cycle long. They will induce in the catcher cavity an RF of the same frequency as was applied to the buncher.

Getting Power from a Klystron

To induce a field in the second cavity, the electrons must give up energy. It is easy to see how this happens after the field has built up. Electrons approaching a negative grid are retarded and impart energy to the grid. Electrons leaving a positive grid are also retarded, giving off energy. Thus if we time the bunches (by regulating the initial velocity of the electrons) to be between the catcher grids only when the first catcher grid is positive and the second catcher grid is negative, while we make sure that we have virtually no electrons between the grids when this situation is reversed, then we draw the maximum energy from our bunches of electrons. This is the way a klystron is operated. The collector and accelerator voltages must be precisely adjusted to get this kind of timing.

If we feed back a portion of the catcher energy to the buncher, the tube will oscillate. If our timing is correct, the phase of the RF will of course be exactly right for the feedback situation, for the bunching occurs when the second buncher grid is negative, and we get the most energy when the second catcher grid is also negative. Amplification is obtained, because the bunches going through the catcher contain many more electrons, thanks to the time spent in the drift space, than the bunches coming out of the buncher.

The energy is coupled into the buncher and out of the catcher cavities with a small loop, which will contain some of the magnetic lines of force of the fields and will thus have a current induced in them.

We can of course use the energy in one of the catcher cavities to excite additional cavities and grids, and this we do many times to increase the energy produced by large klystrons. Fig. 3 shows such a large multicavity klystron made by Eimac, capable of producing 10,000 watts output in the 720-985 mc range.

The Reflex Principle

But there are also klystrons with but one cavity, The principle is illustrated in Fig. 4. These we call reflex klystrons because the collector at the end of the tube is given a negative voltage, thus repelling the electrons. This electrode is usually called a repeller, What happens here is that the electrons, after being bunched in the grids, travel on into the drift space above the cavity for a time, then are repelled back toward the grids. If we repel them with exactly the right velocity to make them arrive at the grids when the voltages on these grids are of the correct phase to obtain energy from the electron bunches, the original field is augmented, and we have oscillation. So the reflex klystron is used primarily as an oscillator.

Reflex klystrons come in many shapes. Fig. 5 shows three of World War II vintage, the 417A made by Westinghouse for the S-band (10 cm), the 707B with an external cavity, also for the same frequency range, and the 2K25 used most often as the' local oscillator in 3-cm (10,000-mc) radar receivers.

All three are tunable to a certain extent (Fig. 6), The 417A is tuned by changing the cavity dimensions with

a tuning lever and screws, the 707B by modifying the electric fields in the cavity with slugs projecting into it, and the 25K5 by changing the cavity dimensions with the tuning "bow". The tuning bow is flexed by the screw. This alters the position of the more or less flexible top portion of the metal enclosure, and the top cavity grid with it.

A more modern version of the reflex klystron, using ceramic insulation, is shown in the head photo. Such ceramic klystrons are now produced and regularly oscillate at 25 kmc, while some laboratory models have been used to generate frequencies as high as 100 kmc. The latter are not in production, but are strictly experimental tubes.

Fig. 6 - Three klystron tuning methods.

Klystrons can be modulated in various ways. One is to vary somewhat the reflector voltage or, in the power klystron, the collector voltage. This has the effect of changing the velocity of the electrons, and thus the frequency of oscillation in the klystron is affected. This kind of modulation is limited within very narrow ranges. Klystrons specially built with a modulating anode near the electron gun can be amplitude-modulated by the simple mechanism of making the electron beam vary in density. Since the amplification of the tube depends on increasing the density of the electron bunches in the drift space, the effect of the bunching will be more pronounced when a lot of electrons are available than when only a few are traveling through the cavity grids. These anode-modulated klystrons are so constructed that the total voltage between the cathode and the tube structure (including the cavities) remains the same. Thus the velocity of the electrons is constant, but the voltage between the modulating anode and the cathode can vary and the quantity of electrons with it.

Very often, particularly in television transmitters, it is actually unnecessary to modulate the klystron. Here it acts as a power amplifier, and the modulation can be introduced at an earlier stage. Thus the klystron amplifies the already modulated signal.

The klystron can be pulse-modulated by the anode in the types which have this separately insulated anode, and by turning the collector voltage on and off in the types that do not.

Except when we want to modulate the klystron, the voltages supplied to the elements must be very stable. Usually they are supplied from well regulated power supplies. The reasons are fairly obvious. If the dc voltages on the cavities and collector or reflector varies, the velocity of the electrons also varies. And, since the speed with which the electrons travel through the buncher determines the frequency of the generated rf, this too would vary.

In the reflex klystron the situation is even more critical. The path the electrons travel must be exactly the right length to allow the electrons on their return voyage to reinforce the original bunching action. If the path should be altered, by a varying voltage, the electrons would arrive at the wrong time and might partly cancel the bunching. The oscillation would then soon die out.

As a matter of fact, this device is used to allow the reflex klystron to operate in different "modes". The path of the electrons, for oscillation, must always be a multiple of a quarter-wave-length. But whether the tube has a path of 3 3/4. or 4 1/4 wavelengths for the electrons, the action is the same. However, with the longer path, caused by a lower (less negative) reflector voltage, the density of the beam is somewhat affected, and the klystron produces less power. By selecting one or the other modes the klystron can be made to put out at different levels of power. The 25K5 for example can operate in about five modes, all producing the same frequency, but with different power levels.

As UHF television becomes more popular, the klystron will be used increasingly for high-power amplification in the transmitters. Further increases in UHF scatter communication and in microwave applications as we progress in the space age is also to be expected. The klystron, which has proven its mettle in bouncing signals off our neighboring planets, will most certainly be the power amplifier for space telephony, once man takes the big jump and starts traveling between planets in the solar system and to distant stars. It is a special vacuum tube to be reckoned with for the next few centuries of man's technological development.

Posted December 19, 2018
(updated from original post on 4/29/2013)