TWT Slow-Wave Structure: Axial RF Phase Velocity along the Tube In a traveling-wave tube (TWT), the axial component of the RF electric field progresses along the slow-wave structure with what approximate velocity relative to the speed of light?

Introduction / Context:
TWTs use slow-wave structures (e.g., helix) to reduce the RF phase velocity so that it can synchronize with the electron beam's velocity, enabling continuous interaction and amplification.

Given Data / Assumptions:

• The question asks for a qualitative comparison of axial RF phase velocity with c.
• We consider a typical helix TWT designed for beam-wave synchronism.

Concept / Approach:

Without a slow-wave structure, the phase velocity in a waveguide is comparable to or greater than c. The helix introduces a long path for the wave, reducing axial phase velocity to a small fraction of c, closer to typical electron beam speeds.

Step-by-Step Solution:

Verification / Alternative check:

Representative TWT designs show phase velocities well below c to maintain synchronism.

Why Other Options Are Wrong:

Option A and E: Too fast for interaction. Option C: 50% of c is typically high for many helix designs. Option D: Phase velocity can exceed c in some media, but axial slow-wave velocity in a helix is deliberately reduced below c for amplification.

Common Pitfalls:

Confusing group and phase velocity; forgetting that synchronism requires comparable velocities.

Final Answer:

A small fraction of the speed of light.