Tuesday, November 30, 2010

Traveling Wave Tube

A traveling wave tube is an elongated vacuum tube with an electron gun used as a microwave amplifier. A broadband traveling wave tube can have a bandwidth that exceeds an octave, being capable of gains greater than 40 dB. A magnetic containment field around the tube focuses the electrons into a beam, which then passes down the middle of a wire helix that stretches from the RF input to the RF output, the electron beam finally striking a collector at the other end. A directional coupler, which can be either a waveguide or an electromagnetic coil, fed with the low-powered radio signal that is to be amplified, is positioned near the emitter, and induces a current into the helix.

The helix acts as a delay line, in which the RF signal travels at near the same speed along the tube as the electron beam. The electromagnetic field due to the RF signal in the helix interacts with the electron beam, causing bunching of the electrons (an effect called velocity modulation), and the electromagnetic field due to the beam current then induces more current back into the helix (i.e. the current builds up and thus is amplified as it passes down). A second directional coupler, positioned near the collector, receives an amplified version of the input signal from the far end of the helix. An attenuator placed on the helix, usually between the input and output helices, prevents reflected wave from traveling back to the cathode.

The essential principle of operation of a traveling wave tube lies in the interaction between an electron beam and an radio frequency signal. The velocity, v, of an electron beam is given by:

An anode voltage of 5 kV gives an electron velocity of 4.2 x 10*7 mso*-1. The signal would normally travel at c, the velocity of light (3x10*8 ms*-1), which is much faster than any 'reasonable' electron beam (relativistic effects mean that the electron mass actually increases as its velocity approaches c, so that achieving electron velocities approaching c is a complicated business), If, however, the signal can be slowed down to the same velocity as the electron beam, it is possible to obtain amplification of the signal by virtue of its interaction with the beam. This is usually achieved using the helix electrode, which is simply a spiral of wire around the electron beam.

Without the helix, the signal would travel at a velocity c. With the helix, the axial signal velocity is approximately c x (p /2pa) where a, p are shown above, so the signal is slowed by the factor p/2pa. Note that this is independent of signal frequency. The signal travelling along the helix is known as a slow wave, and the helix is referred to as a slow-wave structure, The condition for equal slow-wave and electron-beam velocities is therefore approximately.