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see electron tubeelectron tube,
device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube).
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an ultrahigh-frequency electronic vacuum device in which a steady stream of electrons is converted to an alternating stream by modulating the electron velocities with an ultrahigh-frequency electric field while the electrons move through the gap of a cavity resonator. Modulating the velocities has the effect of grouping the electrons into bunches, owing to differences in velocity in a drift space, a section that is free from the ultrahigh-frequency field.

Two types of klystrons are in use: the floating drift and the reflex. In the floating drift klystron, electrons pass successively through the gaps of cavity resonators (see Figure 1). Velocity modulation occurs in the gap of the input resonator, the ultrahigh-frequency field in the gap periodically accelerating (half a cycle) and decelerating (half a cycle). Accelerated electrons catch up with retarded electrons in the drift space, resulting in the formation of electron bunches. In transit through the gap of the output resonator, the electron bunches interact with the resonator’s ultrahigh-frequency field; most are decelerated, and some of their kinetic energy is converted to the energy of ultrahigh-frequency oscillations.

Figure 1. Diagrams of floating-drift klystrons: (a) klystron amplifier, (b) klystron oscillator; (1) cathode, (2) focusing cylinder, (3) electron stream, (4) input cavity resonator, (5) input aperture for ultrahigh frequency energy, (6) resonator gap, (7) drift space, (8) output cavity resonator, (9) output aperture for ultrahigh frequency energy, (10) electron stream collectors, (11) intermediate cavity resonators, (12) anode DC power supply, (13) heater power supply, (14) first cavity resonator, (15) coupling slot through which some ultrahigh frequency energy passes from second resonator to first resonator, (16) second cavity resonator

In 1932, the Soviet physicist D. A. Rozhanskii investigated the idea of converting a steady electron stream to a stream of varying density, making use of the fact that accelerated electrons catch up with decelerated electrons. A method of producing high-power ultrahigh-frequency oscillation based on this idea was proposed by the Soviet physicist A. N. Arsen’eva, jointly with the German physicist O. Heil, in 1935. Actual floating drift klystrons were first designed and built by the American physicists W. Hahn and G. Metcalf (and, independently, by R. and Z. Varian).

Most floating drift klystrons are manufactured as multicavity klystron amplifiers (see Figure 1, a). Intermediate cavity resonators located between the input resonator and the output resonator make it possible to broaden the frequency pass band, increase efficiency, and increase gain. Klystron amplifiers are built for operation in narrow frequency ranges of the decimeter or centimeter wavelengths. Pulse-mode klystrons have an output from several hundred watts (W) to 40 megawatts (MW); continuous-mode klystrons, from a few watts to 1 MW. The gain usually runs from 35 to 60 decibels (dB). Efficiency varies from 40 to 60 percent. The pass band is less than 1 percent in the continuous mode and up to 10 percent in pulse mode. The principal areas of application of klystron amplifiers are in Doppler radar, communications with earth satellites, radioastronomy, and television (continuous-mode klystrons), as well as in linear acceleration of elementary particles and power output amplification in longdistance high-resolution radar (pulse-mode klystrons).

A small number of industrially manufactured klystrons are continuous-mode klystron oscillators, usually with two cavity resonators (see Figure l,b). A small fraction of the ultrahigh-frequency oscillatory power generated in the second resonator is transmitted through a coupling slot to the first resonator in order to modulate electron velocities. The typical output of such klystrons is from 1 to 10 W, and their efficiency is less than 10 percent. Klystron oscillators are used mainly in parametric amplifiers and in radio beacons with wavelengths in the centimeter or millimeter range.

Figure 2. Diagram of a reflex klystron: (1) cathode, (2) focusing cylinder, (3) electron stream, (4) accelerating grid, (5) cavity resonator, (6) resonator gap, (7) reflector, (8) second resonator grid, (9) first resonator grid, (10) vacuum-tight ceramic window serving as lead-out for ultrahigh frequency energy from resonator, (11) resonator voltage supply, (12) heater power supply, (13) reflector voltage supply

Reflex klystrons are those in which the electron stream, having passed through the resonator gap, arrives at the decelerating field of the reflector, to be repelled by the field and pass through the resonator gap in the opposite direction (see Figure 2). During the first transit through the gap, the ultrahigh frequency electric field of the gap modulates the electron velocities. The second time, moving in the opposite direction, the electrons arrive at the gap grouped in bunches. The ultrahigh frequency field in the gap retards these bunches and converts some of their kinetic energy to the energy of ultrahigh-frequency oscillations. Electron bunches are formed because the accelerated electrons follow a longer path in the space between cavity resonator and reflector and thus spend more time there than do the decelerated electrons. If the negative reflector voltage is changed, then the electron transit time, the arrival phase of the bunches at the gap, and the frequency of oscillations generated will also be changed (see Figure 3).

Figure 3. Reflex klystron frequency and output power as a function of reflector voltage: (a) oscillation bandwidth, (b) oscillation bandwidth at half power, (f1) oscillation frequency at center of bandwidth,(∆f) frequency deviation from f1, (c) electronic tuning range at half power

The possibility of changing the frequency of oscillation is used in electronic tuning. This makes it possible to control oscillation frequency, practically inertia-free and without power loss, in frequency modulation and automatic frequency control. Mechanical frequency tuning can be accomplished by changing the gap, either by deflecting the face (a diaphragm) of a metallic klystron (see Figure 4,a) or by moving a tuning piston of a detachable part of the cavity resonator that is joined to the edges of metallic disks protruding from the klystron’s glass or ceramic shell (see Figure 4,b). In addition to this primary cavity resonator, many reflex klystrons have a second cavity resonator located outside the vacuum envelope (see Figure 4,c). Mechanical frequency tuning is accomplished in this case by moving a stub, thereby changing the gap of the second cavity resonator. Such designs make possible an unlimited number of frequency retunings. The incorporation of a high-Q resonator improves frequency stability but reduces the klystron’s output power.

Figure 4. Mechanical frequency tuning methods in a reflex klystron: (a) by deflecting diaphragm, (b) by moving piston in detachable part of cavity resonator, (c) by moving stub in cavity resonator outside vacuum envelope; (1) diaphragm whose deflection changes resonator gap (increasing the gap increases oscillation frequency), (2) edges of metal disks to which detachable part of cavity resonator is joined, (3) detachable part of resonator, (4) piston within cavity resonator (lowering decreases length of resonator and increases oscillation frequency), (5) vacuum-tight ceramic coupling window between cavity resonators, (6) stub (raising stub increases resonator gap and oscillation frequency), (7) output aperture for ultrahigh frequency energy

Reflex klystrons were developed in 1940 by the Soviet engineers N. D. Deviatkov, E. N. Danil’tsev, and I. V. Piskunov, working as a group, and, independently, by the Soviet engineer V. F. Kovalenko. The first papers on the theory of the reflex klystron were published by the Soviet physicists Ia. P. Terletskii in 1943 and S. D. Gvozdover in 1944.

Reflex klystrons are the most widely used ultrahigh-frequency device. They are manufactured for operation in the decimeter, centimeter, and millimeter wave bands. Their output power ranges from 5 mW to 5 W. Their mechanical frequency-tuning range is as much as 10 percent (for klystrons with detachable cavity resonators, several dozen percent). Their electronic tuning range is usually less than 1 percent. Their efficiency is about 1 percent. Reflex klystrons are used as heterodynes in superheterodyne radio receivers, as driving oscillators in radio transmitters, as low-power oscillators in radar, in radio navigation, and in measurement engineering.


Kovalenko, V. F. Vvedenie v elektroniku sverkhvysokikh chastot, 2nd ed. Moscow, 1955.
Lebedev, I. V. Tekhnika i pribory SVCh, 2nd ed., vol. 2. Moscow, 1972.
Gaiduk, V. I., K. I. Palatov, and D. M. Petrov. Fizicheskie osnovy electroniki sverkhvysokikh chastot. Moscow, 1971.
Microwave Tube DATA Book, 28th ed. [New Jersey] 1972.



An evacuated electron-beam tube in which an initial velocity modulation imparted to electrons in the beam results subsequently in density modulation of the beam; used as an amplifier in the microwave region or as an oscillator.


An evacuated electron-beam tube in which an initial velocity modulation imparted to electrons in the beam results subsequently in density modulation of the beam. A klystron is used either as an amplifier in the microwave region or as an oscillator.

For use as an amplifier, a klystron receives microwave energy at an input cavity through which the electron beam passes. The microwave energy modulates the velocities of electrons in the beam, which then enters a drift space. Here the faster electrons overtake the slower to form bunches. In this manner, the uniform current density of the initial beam is converted to an alternating current. The bunched beam with its significant component of alternating current then passes through an output cavity to which the beam transfers its ac energy.

Klystrons may be operated as oscillators by feeding some of the output back into the input circuit. More widely used is the reflex oscillator in which the electron beam itself provides the feedback. The beam is focused through a cavity and is velocity-modulated there, as in the amplifier. The cavity usually has grids to concentrate the electric field in a short space so that the field can interact with a slow, low-voltage electron beam. Leaving the cavity, the beam enters a region of dc electric field opposing its motion, produced by a reflector electrode operating at a potential negative with respect to the cathode. The electrons do not have enough energy to reach the electrode, but are reflected in space and return to pass through the cavity again. The points of reflection are determined by electron velocities, the faster electrons going farther against the field and hence taking longer to get back than the slower ones. Reflex oscillators are used as signal sources from 3 to 200 GHz. They are also used as the transmitter tubes in line-of-sight radio relay systems and in low-power radars.


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Electrons leave the heated cathode, and are accelerated and focus by the focusing elements. They are decelerated and bunched by the deceleration grid. They U-turn at the repeller. Their frequency is dependent on size.
A form of electron tube used for generation and amplification of microwave electromagnetic energy. It is a linear-beam tube; it incorporates an electron gun, one or more cavities, and an apparatus for modulating the beam produced by the electron gun. The most commonly used klystron tubes are the two-cavity, the multicavity, and the reflex klystron.


A type of vacuum tube used as an amplifier and/or oscillator for UHF and microwave signals. It is typically used as a high-power frequency source in such applications as particle accelerators, UHF TV transmission and satellite earth stations. The klystron was invented at Stanford University in 1937 and originally used as the oscillator in radar receivers during World War II.

A klystron tube makes use of speed-controlled streams of electrons that pass through a resonating cavity. Electrons in a klystron are accelerated to a controlled speed by the application of several hundred volts. As the electrons leave the heated cathode of the tube, they are directed through a narrow gap into a resonating chamber, where they are acted upon by an RF signal. The electrons bunch together and are directed into one or more additional chambers that are tuned at or near the tube's operating frequency. Strong RF fields are induced in the chambers as the electron bunches give up energy. These fields are ultimately collected at the output resonating chamber. See magnetron and diode.