Excerpt from A Helical Wave Guide II

In recent years a great deal of research has been done on an ultra-high frequency amplifier known as the traveling-wave It has been found that this tube possesses the desirable properties of high gain, broad bandwidth, and low noise level. The essential part of the tube is a helical transmission line which is designed to pass an electromagnetic wave with wave length along the guide, and hence phase velocity along the guide, smaller than that of the free-space wave by a factor of about thirteen. An electron beam with a velocity approximately equal to the phase velocity of the electromagnetic wave is shot down the center of the helical guide. The electromagnetic wave and the electron beam interact, and as a result the wave amplitude is magnified.

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