Science Fair Project Encyclopedia
A klystron is a specialized vacuum tube (evacuated electron tube) called a linear-beam tube. The pseudo-Greek word klystron comes from the stem form κλυσ- (klys) of a Greek verb referring to the action of waves breaking against a shore, and the end of the word electron. Klystrons are used as an oscillator or amplifier at microwave and radio frequencies to produce both low power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for linear accelerators. It has the advantage (over the magnetron) of coherently amplifying a reference signal and so its output may be precisely controlled in amplitude, frequency and phase.
Russell and Sigurd Varian of Stanford University are generally considered to be the inventors of the klystron. Their prototype was completed in August 1937. Upon publication in 1939, news of the klystron immediately influenced the work of US and UK researchers working on radar equipment. (The Varians went on to found Varian Associates to commercialize the technology.)
In the two-chamber klystron, an electron beam from the cathode of an electron gun is injected into a resonant cavity. The beam is held together by a parallel magnetic field and is attracted through a connecting passage (called a drift tube) to a second resonant chamber containing a positively charged anode. While passing through the connecting chamber the electron beam is velocity modulated (periodically bunched) by the weaker RF signal. The electrons are attracted to a positive anode contained in a second resonant chamber. As the bunched electrons enter the second chamber they induce standing waves at the same frequency as input signal. The signal induced in the second chamber is much stronger than that in the first.
In the reflex klystron, the electron beam passes through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun. After passing through the resonant cavity they are reflected by a negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity. The formation of electron bunches takes place in the drift space between the reflector and the cavity. The voltage on the reflector must be adjusted so that the bunching is at a maximum as the electron beam re enters the resonant cavity, thus ensuring a maximum of energy is transferred from the electron beam to the RF oscillations in the cavity. The reflector voltage may be varied slightly from the optimum value, which results in some loss of output power, but also in a variation in frequency. This effect is used to good advantage for automatic frequency control in receivers, and in frequency modulation for transmitters. The level of modulation applied for transmission is small enough that the power output essentially remains constant. At regions far from the optimum voltage, no oscillations are obtained at all. There are often several regions of reflector voltage where the reflex klysron will oscillate; these are referred to as modes. The electronic tuning range of the reflex klystron is usually referred to as the variation in frequency between half power points—the points in the oscillating mode where the power output is half the maximum output in the mode. Modern semiconductor technology has effectively replaced the reflex klystron in most applications.
In the multicavity klystron, multiple toroidal cavities surround a cylindrical acceleration tube.
Floating drift tube klystron
The floating drift tube klystron has a single cylindrical chamber containing an electrically isolated central tube. Electrically, this similar to the two cavity oscillator klystron with a lot of feedback between the two cavities. Electrons exiting the source cavity are velocity modulated by the electric field as they travel through the drift tube and emerge at the destination chamber in bunches, delivering power to the oscillation in the cavity. This type of oscillator klystron has the advantage over the two-cavity klystron on which it is based of only needing one tuning element to effect changes in frequency. The drift tube is electrically insulated from the cavity walls, and DC bias is applied separately. The DC bias on the drift tube may be adjusted to alter the transit time through it, thus allowing some electronic tuning of the oscillating frequency. The amount of tuning in this manner is not large, and is normally used for frequency modulation when transmitting.
These amplifiers are used to produce HF, VHF, UHF, and EHF signals where such high amplitude (power) is required that solid-state devices remain inadequate. Klystrons can be found at work in radar, satellite and wideband high-power communication (very common in television broadcasting and EHF satellite terminals), and high-power physics (particle accelerators and experimental reactors).
The two-cavity amplifier klystron is readily turned into an oscillator klystron by providing a feedback loop between the input and output cavities. Two-cavity oscillator klystrons have the advantage of being some of the lowest-noise microwave sources available, and for that reason have often been used in the illuminator system of missile targeting radars. The two-cavity oscillator klystron normally generates more power than the reflex klystron—typically watts of output rather than milliwatts. As there is no reflector, only one high-voltage supply is required, but the voltage must be adjusted to a particular value for the tube to oscillate. This is because the electron beam must produce the bunched electrons in the second cavity in order to generate output power. as the location of the second cavity is physically fixed with respect to the first, this must be done by varying the velocity of the electron beam to a suitable level. Often several 'modes' of oscillation can be observed in a given klystron.
- http://www.tpub.com/neets/book11/45b.htm (two cavity klystron)
- http://www.tpub.com/neets/book11/45c.htm (multicavity klystron)
- http://www.tpub.com/neets/book11/45d.htm (reflex klystron)
- http://www2.slac.stanford.edu/vvc/accelerators/klystron.html (high power for linear accelerator)
- http://www.tubecollector.org/list.php?L=-K&M=Y&H=Klystrons - Klystron collection in the Virtual Valve Museum
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