Backward-Wave Tube

(or backward-wave oscillator), an electrovacuum device in which the interaction of an electron beam with an electromagnetic wave traveling in a slow-wave circuit in a direction opposite to the motion of the electrons is used to generate superhigh-frequency (SHF) electromagnetic oscillations. Backward-wave tubes are used in broad-band standard-signal and sweep generators for electronic measurements and radio spectroscopy, in heterodynes of quick-tuning receivers, and in the driving oscillators of quick-tuning transmitters. The phenomenon of generations of SHF oscillations as a result of the interaction of an electron beam with a backward wave was discovered and described by the American physicist S. Millman in 1950. The designation “backward-wave tube” was proposed in 1953 by the American scientists R. Kompfner and N. Williams, who were the first to investigate the operation of tubes of this type.

In a backward-wave tube, a rectilinear electron beam generated by an electron gun passes through a slow-wave circuit (interaction structure) formed by a row of opposing plates and excites in it an electromagnetic wave that travels in a direction opposite the direction of electron motion. Under the influence of the electric field of the traveling wave, electron bunches are formed in the electron beam. Each bunch, in turn, passes through the gaps between the plates of the slow-wave circuit. In each gap such a bunch encounters a successive potential an-tinode of the traveling wave and is retarded by its electric field (a precondition for the generation of oscillations). This condition is satisfied if the transit time of the bunch from one gap to the next is slightly less than half of an oscillation period. An increase (decrease) in the voltage between the cathode of the electron gun and the slow-wave circuit decreases (increases) the transit time and, consequently, shortens (lengthens) the period of the generated oscillations. To focus the electron beam in a backward-wave tube, a stationary magnetic field directed along the axis of the electron beam is applied, or an electrostatic focusing system is used.

Backward-wave tubes are manufactured with oscillation powers from 5 to 100 milliwatts (mW) and frequencies from 1 to 1500 gigahertz (GHz). They have a frequency tuning range by voltage from 10 percent to an octave and an efficiency on the order of 1 percent.

In addition to the backward-wave tubes described above, M-type tubes are also manufactured. Most such tubes generate oscillations at frequencies from 0.5 to 18 GHz, with power outputs from 0.1 to 1.0 kW, and have a frequency tuning range of approximately 30 percent and an efficiency of 6–50 percent. M-type tubes are used to generate jamming and in measuring apparatus and communications systems.