traveling-wave tube

a vacuum tube in which prolonged interaction between a traveling electromagnetic wave and an electron flux moving in the same direction is used to amplify SHF electromagnetic oscillations. The main use of traveling-wave tubes is the amplification of SHF oscillations (300 megahertz [MHz] to 300 gigahertz [GHz]) in receiving and transmitting devices; they are also used for frequency conversion and multiplication. A vacuum tube whose operation was based on the interaction between an electron flux and a traveling wave was first proposed and patented by the American engineer A. Hoeff in 1936, and the first traveling-wave tube was built by the American scientist R. Kompfner in 1943. The first theoretical work dealing with the traveling-wave tube was published by the American physicist J. Pierce in 1947.

The main components of a traveling-wave tube (see Figure 1) are an electron gun, which generates and forms the electron beam, and a slow-wave system, which reduces the velocity of the traveling wave along the axis of the tube to a velocity approaching that of the electrons, thus achieving a wave motion that is synchronous with the electron beam. (The system is usually a metal helix that is rigidly attached by longitudinal dielectric supports; the velocity of the wave traveling along the helix depends only weakly on the frequency. As a result, effective interaction of the wave with the electron beam can be achieved over a wide frequency band.) Other components include a focusing system (a periodic system of permanent magnets, solenoids, and so on) to confine the electron beam within the required cross-sectional limits along the entire length of the beam by means of a magnetic field; a collector, which is used to capture electrons; a radio-frequency input and output; and an absorber of SHF oscillatory energy. The absorber is placed in a small region of the slow-wave system to eliminate self-excitation of the traveling-wave tube caused by reflection of the wave at the ends of the slow-wave system.

Figure 1. Diagram of a traveling-wave tube: (1) electron gun, (2) slow-wave system, (3) solenoid focusing system, (4) collector, (5) output, (6) absorber of SHF oscillation energy, (7) input

The mechanism of interaction between the electron flux and the electromagnetic wave may be explained as follows. Under the action of the accelerating and retarding regions of the wave’s electric field (the positive and negative half-wave, respectively), the electrons moving synchronously with the wave are grouped into bunches. The bunches are located in the regions of the field where an accelerating half-wave gives way to a decelerating half-wave. If the velocities of the wave and the electrons are equal, no energy interchange occurs between them, and no amplification takes place. If the velocity of the electrons is slightly higher than the velocity of the wave, the bunches of electrons overtake the wave and enter into the retarding field regions and therefore are decelerated. The kinetic energy lost by the electrons during deceleration is converted to the energy of the traveling wave.

Traveling-wave tubes are broad-band devices; the frequency pass band exceeds an octave in many types of tubes. Traveling-wave tubes are manufactured with an output power from fractions of a milliwatt (input-stage low-power and low-noise tubes in SHF amplifiers) to dozens of kilowatts (output-stage high-power tubes in SHF transmitters) in the continuous mode and up to several megawatts for pulsed operation.

Traveling-wave tubes provide high amplification, usually from 30 to 60 decibels (dB). The efficiency of medium-power and high-power tubes is not great (usually about 30 percent). For input amplification stages in a broad frequency band traveling-wave tubes are manufactured with an output power of 10−4 to 10 W and with low noise factors (3-20 dB). In addition to the tubes already discussed, tubes of the magnetron type are also in use.