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{{More footnotes|date=July 2011}}
[[Image:Backward wave oscillator.jpg|thumb|Miniature O-type backward-wave oscillator tube produced by Varian in 1956. It could be voltage-tuned over an 8.2 - 12.4 GHz range and required a supply voltage of
[[File:Backward-wave Oszillator-Stockholm.jpg|thumb|Backward wave oscillator at Stockholm University operating in the
A '''backward wave oscillator''' ('''BWO'''), also called '''carcinotron'''
An [[electron gun]] generates an [[electron beam]] that
It has two main subtypes, the '''M-type''' ('''M-BWO'''), the most powerful, and the '''O-type''' ('''O-BWO'''). The output [[Power (physics)|power]] of the O-type is typically in the range of 1 [[Watt|mW]] at 1000 GHz to 50 mW at 200 [[Hertz|GHz]]. Carcinotrons are used as powerful and stable microwave sources. Due to the good quality [[wavefront]] they produce (see below), they find use as illuminators in [[Terahertz radiation|terahertz]] imaging.
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The backward wave oscillators were demonstrated in 1951, '''M-type''' by [[Bernard Epsztein]]
<ref>{{citeref patent|country=FR|number= 1035379 |inventor=Bernard Epsztein|pubdate=1959-03-31|title=Backward flow travelling wave devices|status=patent}}</ref>
and '''O-type''' by [[Rudolf Kompfner]]. The M-type BWO is a voltage-controlled non-resonant extrapolation of [[magnetron]] interaction
Carcinotrons are used in research, civilian and military applications. For example, the Czechoslovak
==Basic concept==
[[File:
All travelling-wave tubes operate in the same general fashion, and differ primarily in details of their construction.
The concept is dependent on a steady stream of [[electron]]s from an [[electron gun]] that travel down the center of the tube (see
As the electrons travel down the tube, they
▲The concept is dependent on a steady stream of [[electron]]s from an [[electron gun]] that travel down the center of the tube (see [Basic Concept Figure] on the left). Surrounding the electron beam is some sort of [[radio frequency]] source signal; in the case of the traditional [[klystron]] this is a resonant cavity fed with an external signal, whereas in more modern devices there are a series of these cavities or a helical metal wire fed with the same signal.<ref name=NEET/>
As the electrons are moving, they
▲As the electrons travel down the tube, they will interact with the RF signal (see [Basic Concept Figure] on the left). The electrons will be attracted to areas with maximum positive bias and repelled from negative areas. This causes the electrons to "bunch up" as they are repelled or attracted along the length of the tube, a process known as ''velocity modulation''. This process makes the electron beam take on the same general structure as the original signal; the density of the electrons in the beam matches the relative amplitude of the RF signal in the induction system. The result is a signal in the electron beam that is an amplified version of the original RF signal.<ref name=NEET/>
▲As the electrons are moving, they will induce a magnetic field in any nearby conductor as illustrated in the Basic Concept Figure. This allows the now-amplified signal to be extracted. In systems like the magnetron or klystron, this is accomplished with another resonant cavity. In the helical designs, this process occurs along the entire length of the tube, reinforcing the original signal in the helical conductor. The "problem" with traditional designs is that they have relatively narrow bandwidths; designs based on resonators will work with signals within 10% or 20% of their design, as this is physically built into the resonator design, while the helix designs have a much wider [[bandwidth (signal processing)|bandwidth]], perhaps 100% on either side of the design peak.<ref name="Gilmour">{{cite book
| last1 = Gilmour
| first1 = A. S.
| title = Klystrons, Traveling Wave Tubes, Magnetrons, Crossed-Field Amplifiers, and Gyrotrons
| publisher = Artech House
| date = 2011
| ___location =
| pages = 317–18
| language =
| url = https://books.google.com/books?id=l_1egQKKWe4C&
| doi =
| id =
| isbn =
}}</ref>
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The original RF signal enters from what would be the far end of the TWT, where the energy would be extracted. The effect of the signal on the passing beam causes the same velocity modulation effect, but because of the direction of the RF signal and specifics of the waveguide, this modulation travels backward along the beam, instead of forward. This propagation, the ''slow-wave'', reaches the next hole in the folded waveguide just as the same phase of the RF signal does. This causes amplification just like the traditional TWT.<ref name=NEET/>
This is not the case in the BWO, where the electrons pass the signal at right angles and their speed of propagation is independent of that of the input signal. The complex serpentine waveguide places strict limits on the bandwidth of the input signal, such that a standing wave is formed within the guide. But the velocity of the electrons is limited only by the allowable voltages applied to the electron gun, which can be easily and rapidly changed. Thus the BWO takes a single input frequency and produces a wide range of output frequencies.<ref name=NEET/>
==Carcinotron==
[[File:Carcinotron jamming a pulse radar unit.png|thumb|This image shows the effect of four carcinotron-carrying aircraft on a typical 1950s pulse radar. The aircraft are located at roughly the
The device was originally given the name "carcinotron",
Previously, jamming a radar was a complex and time-consuming operation. Operators had to listen for potential frequencies being used, set up one of a bank of amplifiers on that frequency, and then begin broadcasting. When the radar station realized what was happening, they would change their frequencies and the process would begin again. In contrast, the carcinotron could sweep through all the possible frequencies so rapidly that it appeared to be a constant signal on all of the frequencies at once. Typical designs could generate hundreds or low thousands of watts, so at any one frequency, there might be a few watts of power that is received by the radar station. However, at long range the amount of energy from the original radar broadcast that reaches the aircraft is only a few watts at most, so the carcinotron can overpower them.<ref name=NEET/>
The system was so powerful that it was found that a carcinotron operating on an aircraft would begin to be effective even before it rose above the [[radar horizon]]. As it swept through the frequencies it would broadcast on the radar's
The concept was so powerful as a [[Radar jamming and deception|jammer]] that there were serious concerns that ground-based radars were obsolete. Airborne radars had the advantage that they could approach the aircraft carrying the jammer, and, eventually, the huge output from their transmitter would "burn through" the jamming. However, interceptors of the era relied on [[ground controlled interception|ground direction]] to get into range, using ground-based radars. This represented an enormous threat to air defense operations.<ref name=CandR/>▼
▲The system was so powerful that it was found that a carcinotron operating on an aircraft would begin to be effective even before it rose above the [[radar horizon]]. As it swept through the frequencies it would broadcast on the radar's own pulse frequency at what were effectively random times, filling the display with random dots any time the antenna was pointed near it, perhaps 3 degrees on either side of the target. There were so many dots that the display simply filled with white noise in that area. As it approached the station, the signal would also begin to appear in the antennas [[sidelobe]]s, creating further areas that were blanked out by noise. At close range, on the order of {{convert|100|miles}}, the entire [[radar display]] would be completely filled with noise, rending it useless.<ref name=NEET/>
For ground radars, the threat was eventually solved in two ways. The first was that radars were upgraded to operate on many different frequencies and switch among them randomly from pulse to pulse, a concept now known as [[frequency agility]]. Some of these frequencies were never used in peacetime, and highly secret, with the hope that they would not be known to the jammer in wartime. The carcinotron could still sweep through the entire band, but then it would be broadcasting on the same frequency as the radar only at random times, reducing its effectiveness. The other solution was to add passive receivers that triangulated on the carcinotron broadcasts, allowing the ground stations to produce accurate tracking information on the ___location of the jammer and allowing them to be attacked.<ref name=CandR>{{cite encyclopedia
▲The concept was so powerful as a [[Radar jamming and deception|jammer]] that there were serious concerns that ground-based radars were obsolete. Airborne radars had the advantage that they could approach the aircraft carrying the jammer, and, eventually, the huge output from their transmitter would "burn through" the jamming. However, interceptors of the era relied on ground direction to get into range. This represented an enormous threat to air defense operations.<ref name=CandR/>
▲For ground radars, the threat was eventually solved in two ways. The first was that radars were upgraded to operate on many different frequencies and switch among them randomly from pulse to pulse, a concept now known as [[frequency agility]]. Some of these frequencies were never used in peacetime, and highly secret, with the hope that they would not be known to the jammer in wartime. The carcinotron could still sweep through the entire band, but then it would be broadcasting on the same frequency as the radar only at random times, reducing its effectiveness. The other solution was to add passive receivers that triangulated on the carcinotron broadcasts, allowing the ground stations to produce accurate tracking information on the ___location of the jammer and allowing them to be attacked.<ref name=CandR>{{cite encyclopedia
|editor-first=Sandy |editor-last=Hunter
|first=Alec |last=Morris
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|publisher=Royal Air Force Historical Society
|date=1996
|pages=
== The slow-wave structure ==
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Two examples of slow-wave circuit characteristics are shown, in the ω-k or [[Léon Brillouin|Brillouin]] diagram:
* on figure (a), the fundamental n=0 is a forward space harmonic (the [[phase velocity]] v<sub>n</sub>=ω/k<sub>n</sub> has the same sign as the [[group velocity]] v<sub>g</sub>=dω/dk<sub>n</sub>), synchronism condition for backward interaction is at point B, intersection of the line of slope v<sub>e</sub> - the beam velocity - with the first backward (n = -1) space harmonic,
* on figure (b) the fundamental (n=0) is backward
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The '''O-type carcinotron''', or '''O-type backward wave oscillator''', uses an electron beam longitudinally focused by a magnetic field, and a slow-wave circuit interacting with the beam. A collector collects the beam at the end of the tube.
===O-BWO spectral purity and noise
The BWO is a voltage tunable oscillator, whose voltage tuning rate is directly related to the propagation characteristics of the circuit. The oscillation starts at a frequency where the wave propagating on the circuit is synchronous with the slow space charge wave of the beam. Inherently the BWO is more sensitive than other oscillators to external fluctuations. Nevertheless, its ability to be phase- or frequency-locked has been demonstrated, leading to successful operation as a heterodyne local oscillator.
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==References==
▲* Johnson, H. R. (1955). Backward-wave oscillators. Proceedings of the IRE, 43(6), 684-697.
* Ramo S., Whinnery J. R., Van Duzer T. - Fields and Waves in Communication Electronics (3rd ed.1994) John Wiley & Sons
* Kantorowicz G., Palluel P. - Backward Wave Oscillators, ''in'' Infrared and Millimeter Waves, Vol 1, Chap. 4, K. Button ed., Academic Press 1979
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==External links==
{{Commons category|Backward wave oscillator}}
* [https://web.archive.org/web/20160313064838/http://www.tubecollector.org/cv6124.htm Virtual Valve Museum] Thomson CSF CV6124 (Wayback Machine)
{{Electronic components}}
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