Klystron

The klystron is an electron tube that takes advantage of the maturity of the electrons for generation or amplification of high frequency signals ( traveling wave ). It was invented in 1937 by brothers Russell and Sigurd Varian with the assistance of William Webster Hansen at Stanford University ( California).

In klystron undergoes a generated in a vacuum and accelerated by high voltage electron current through a high-frequency alternating electric field, a velocity modulation. He passes to one fed with a high-frequency signal cavity. After a certain period of time, the velocity modulation causes a density modulation. The modulated electron stream may be passed through one or more additional cavities ( Mehrkammerklystron ), and the last resonator, a portion of its energy will be taken as the high frequency energy.

Two cavity klystron

This amplifier for very high frequencies is not used in practice, but is well suited to describe the basic operation of a klystron. The high-frequency part to form two cavities: the first cavity ( control chamber, coupling- resonator) is excited by the supplied signal, an electromagnetic wave whose electric component in the center of the resonator will interact with the electron beam and the speed thereof is modulated ( alternately acceleration and deceleration, depending on the phase in which the controlling signal is ). The faster electron can now overtake the previously emitted, slower electrons. After a certain period of time are braked, uninfluenced and accelerated electrons at the same site, it creates an electron bunch ( bunch, density modulation). Due to the speed differences continue to exist this package dissolves again to form again later. That along the running path of the electron, the density modulated Make repeated periodically; it creates a space-charge wave.

In the two- cavity klystron, the center of the second cavity ( Auskoppelresonator ) is at the first maximum of the density modulation. Induction by wall currents caused in the second cavity resonator, the cavity is formed an electromagnetic wave whose electric component is directed so that it brakes the electron bunches. Thereby, the electron transfers part of its kinetic energy to the electromagnetic field. Of the energy of a portion in turn can be coupled. The coupled- wave has a result of energy transfer from the electron beam a larger amplitude than the coupled, that is, the two- cavity klystron works as an amplifier.

The electron beam is then collected by a collector.

Multi-chamber klystron

For the outputs required in practice high power electron beams are needed. The resulting high charge density in the beam due to the Coulomb forces between the required electron to form a package with high-speed modulation. This is achieved between the input and output resonator by use of additional cavities ( intermediate resonators). Because of them, no power is coupled out usually develop in them cascading higher electric fields in the Resonatorzentren that eventually lead to the required package formation. The method has the advantage that a substantially higher relative to one another in comparison with the dual chamber klystron bandwidth (up to about 1% ) is achieved by a detuning of the resonance frequencies of the resonators between the same.

Klystron amplifier can be used as multi- cavity klystron for short-pulse performance ( microsecond range ) are built up to about 100 megawatts, in continuous operation ( " continuous wave power " ) to well over 1 MW. The frequency range extends from a few 100 MHz to several GHz 10

Typical areas of application are strong UHF and microwave transmitters, radar ( Reflexklystron as local oscillator ), microwave heating ( eg in particle board production ), medical and scientific particle accelerators as well as the broadband satellite communications.

Until some years ago the klystron in terrestrial UHF stations was widespread. Meanwhile, there is but there is increasingly superseding of IOT ( Inductive Output Tube) or semiconductor amplifiers.

Reflexklystron

When Reflexklystron are the modulating and the evulsed cavity identical - the electron current is reflected in this by a negatively biased electrode. The Reflexklystron can therefore act as an oscillator. The operation is explained with reference of the accompanying image: Some of the electrons that are emitted from the thermionic cathode and accelerated by the anode, pass through the resonator chamber and generate therein by induction a weak electromagnetic field. After a certain period of time, they are forced by the negative electrical potential of the reflector to turn back and go through the resonator in the reverse direction. If at this time the previously induced current direction is just the reverse in this resonant circuit, this vibration is amplified again, it creates an oscillator. A part of the generated RF energy can by a wire (green marked ) are inductively coupled. Since most of the electrons emitted from the cathode directly to land on the anode, the efficiency is only a few percent. The decisive factor for the function is a sufficient compliance of the electron transit time with a multiple of the oscillation period. With little change in the reflector voltage will result in a frequency modulation. The frequency can often be changed or adjusted by mechanically deforming the resonator.

Reflexklystrons are no longer in use - they have been replaced by oscillators with semiconductor devices ( Gunn diodes ), which are considerably smaller and operate with lower operating voltage of only 5 V.

Reflexklystron for connecting an external resonator ( Type K -11, Soviet Union 1963)