- Backward wave oscillator
A

**backward wave oscillator**(**BWO**), also called**carcinotron**(a trade name for tubes manufactured by CSF, now Thales) or**backward wave tube**, is avacuum tube that is used to generatemicrowaves up to the terahertz range. It belongs to thetraveling-wave tube family. It is an oscillator with a wide electronic tuning range.An

electron gun generates anelectron beam that is interacting with a slow-wave structure. It sustains theoscillation s by propagating a traveling wave backwards against the beam. The generatedelectromagnetic wave power has itsgroup velocity directed oppositely to the direction of motion of the electrons. The output power is coupled out near the electron gun.It has two main subtypes, the

**M-type**, the most powerful, (**M-BWO**) and the**O-type**(**O-BWO**). The O-type delivers typically power in the range of 1 mW at 1000 GHz to 50 mW at 200 GHz. Carcinotrons are used as powerful and stable microwave sources. Due to the good qualitywavefront they produce, they find use as illuminators in terahertz imaging.The backward wave oscillators were demonstrated in 1951,

**M-type**by Bernard Epsztein, (French patent 1,035,379; British patent 699,893; US patent 2,880,355) and**O-type**by Rudolf Kompfner. The M-type BWO is a voltage-controlled non-resonant extrapolation ofmagnetron interaction, both types are tunable over a wide range of frequencies by varying the acceleratingvoltage . They can be swept through the band fast enough to be appearing to radiate over all the band at once, which makes them suitable for effectiveradar jamming , quickly tuning into the radar frequency. Carcinotrons allowed airborne radar jammers to be highly effective. However, frequency-agile radars can hop frequencies fast enough to force the jammer to usebarrage jamming , diluting its output power over a wide band and significantly impairing its efficiency.Carcinotrons are used in research, civilian and military applications. For example, the

Kopac passive sensor andRamona passive sensor employed carcinotrons in their receiver systems.**The Slow-wave structure**

thumb|250px|(a) Forward fundamental space harmonic (n=0),(b) Backward fundamentalThe needed slow-wave structures must support a Radio Frequency (RF) electric field with a longitudinal component; the structures are periodic in the direction of the beam and behave like microwave filters with passbands and stopbands. Due to the periodicity of the geometry, the fields are identical from cell to cell except for a constant phase shift Φ.This phase shift, a purely real number in a passband of a lossless structure, varies with frequency.According to Floquet's theorem (see

Floquet theory ), the RF electric field E(z,t) can be described at anangular frequency ω, by a sum of an infinity of "spatial or space harmonics" E_{n}E(z,t)=$sum\_\{n=-infty\}^\{+infty\}\; \{E\_n\}e^\{j(\{omega\}t-\{k\_n\}z)\}$

where the wave number or propagation constant k

_{n}of each harmonic is expressed as:k

_{n}=(Φ+2nπ)/p (-π<Φ<+п)z being the direction of propagation, p the pitch of the circuit and n an integer.

Two examples of slow-wave circuit characteristics are shown, in the ω-k or Brillouin diagram:

* on figure (a), the fundamental n=0 is a forward space harmonic (the

phase velocity v_{n}=ω/k_{n}has the same sign as thegroup velocity v_{g}=dω/dk_{n}), synchronism condition for backward interaction is at point B, intersection of the line of slope v_{e}- the beam velocity - with the first backward (n = -1) space harmonic,* on figure (b) the fundamental (n=0) is backward

A periodic structure can support both forward and backward space harmonics, which are not modes of the field, and cannot exist independently, even if a beam can be coupled to only one of them.

As the magnitude of the space harmonics decreases rapidly when the value of n is large, the interaction can be significant only with the fundamental or the first space harmonic.

**M-type BWO**The

**M-type carcinotron**, or**M-type backward wave oscillator**, uses crossed static electric field E andmagnetic field B, similar to themagnetron , for focussing an electron sheet beam drifting perpendicularly to E and B, along a slow-wave circuit, with a velocity E/B. Strong interaction occurs when thephase velocity of one space harmonic of the wave is equal to the electron velocity. Both E_{z}and E_{y}components of the RF field are involved in the interaction (E_{y}parallel to the static E field). Electrons which are in a decelerating E_{z}electric field of the slow-wave, lose thepotential energy they have in the static electric field E and reach the circuit. The sole electrode is more negative than the cathode, in order to avoid collecting those electrons having gained energy while interacting with the slow-wave space harmonic.**O-type BWO**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.**References*** 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

; Patents

FR-1,035,379 - Bernard Epsztein (filed 1951)

[

*http://www.wikipatents.com/gb/699893.html GB-699,893*] - Bernard Epsztein (1952 - filed 1951)[

*http://www.pat2pdf.org/patents/pat2880355.pdf US-2,880,355*] - Bernard Epsztein (1959 - filed 1952)**See also***

Klystron

*Magnetron

*Crossed-field amplifier

*Traveling-wave tube

*Terahertz radiation * [

*http://www.tubecollector.org/cv6124.htm Virtual Valve Museum*] Thomson CSF CV6124

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