Backward wave oscillator

A backward- wave oscillator ( BWO ), also Carcinotron, is a backward wave tube and generally a drift tube for generating microwaves. It has the advantage of electrical tunability over a wide frequency range compared to, for example Reflexklystrons and magnetron.

It is primarily used in research and in the military field. BWO have been developed in the early 1950s and produced industrially since the 1960s.

With BWO terahertz range can be generated depending on the type frequencies of about 1 GHz to in the. The output power is in the common construction types a few milliwatts and decreases with increasing frequency.

Structure and function

BWO oscillator tubes are composed similarly to serving only to reinforce traveling wave tubes, among others, of a helix as a delay line. The helix is running on a shaft and interacts with an electron beam. The wave generated on the helix runs in BWO but backwards, i.e. opposite to the direction of the electron beam. This undesirable in a traveling wave tube phenomenon is exploited here. The feedback of the oscillator is realized by the electron beam by the induced velocity changes due to the run-time to changes in density at the output in it that stimulate through their alternating electric field, the end of the helix. Thus, vibrations are produced when there is a phase dispersion between electron velocity and phase velocity of the wave on the delay line.

The longitudinal magnetic field of a permanent magnet is used for bundling or guide the electron beam.

The frequency of the wave generated can be combined with the acceleration voltage (anode voltage) of the electron beam vary over wide ranges.

In the technical design of a BWO consists of the following parts:

• The electron gun, which is usually implemented as a tetrode beam system,
• The delay line extraction,
• The mounting unit with magnet and base.

To avoid unwanted oscillations, the end of the delay line is attenuated by evaporating a thin layer of iron.

Application

Applies the BWO under the name Carcinotron as a high frequency generator in older radars. Were built BWO example of the U.S. company Watkins - Johnson.