Discussion
The primary of the helix in a travelling wave tube is to
Answer: B
The helix reduces the axial velocity of the RF field. The electron beam travels at a speed that is a small fraction of the speed of light, so the RF wave must be slowed to about the beam velocity for them to interact over a distance. The helix does this because the wave follows the turns of the wire and so advances along the axis much more slowly.
Preventing the beam from spreading is the job of the focusing magnet. Broadband operation and the noise figure are features of the tube, but they are not the primary purpose of the helix.
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