Backward wave oscillator

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A backward wave oscillator (BWO), also called carcinotron or backward wave tube, is a device that is used to generate microwaves and terahertz radiation. It consists of a vacuum tube that provides 1 mW to 50 mW of power. Some types can produce frequencies in a range of 200 GHz to 1.1 THz. It sustains the oscillations by reflecting the traveling wave backwards against the beam. It is derived from the traveling wave tube. It has two main subtypes, the M-type (M-BWO) and O-type (O-BWO). Carcinotrons are used as powerful and stable microwave sources. Due to the good quality wavefront they produce, they find use as illuminators in terahertz imaging.

It is an amplifier with a wide tuning range. The electron gun generates an electron beam that passes through a slow-wave structure. The generated electromagnetic wave moves in the direction opposite to the direction of the electrons. The output signal is coupled out near the electron gun.

Carcinotrons were designed in 1950s. They are a voltage-controlled version of magnetrons, tunable over a wide range of frequencies by varying the accelerating voltage. Initially they had a low power, but later as their output power grew they found military use. 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 effective radar 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 use barrage 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 and Ramona passive sensor employed carcinotrons in their receiver systems.

[edit] M-type carcinotron

A M-type carcinotron, or M-type backward wave oscillator, uses focusing and interaction with the beam via magnetic field, similar to a magnetron.

[edit] O-type carcinotron

An O-type carcinotron, or O-type backward wave oscillator, uses an electron beam longitudally focused in the tube, with a slow-wave circuit interacting with the beam. A collector terminates the beam at the end of the tube.