In a traveling-wave tube (TWT) using a helix slow-wave structure, what is the direction of the RF electric field associated with the applied signal that interacts with the electron beam?

Introduction:
In a TWT, an electron beam exchanges energy with an RF wave that is slowed to near the beam’s axial velocity by a slow-wave structure (often a helix). Understanding the field orientation is essential for grasping the beam–wave interaction that produces gain over wide bandwidths.

Given Data / Assumptions:

• Helix slow-wave structure supporting a reduced phase velocity.
• Axial electron beam passing through the helix center.
• Small-signal operation with synchronism conditions approximately satisfied.

Concept / Approach:

The slow-wave helix supports an electromagnetic wave whose dominant electric field component is axial (parallel to the beam). This axial E field modulates the beam velocity, enabling continuous interaction along the tube. Although the full field has transverse components, the component that couples strongly to the beam for energy exchange is along the axis.

Step-by-Step Solution:

Verification / Alternative check:

Small-signal TWT theory derives gain from the axial field component Ez interacting with beam current; without Ez the coupling would be negligible.

Why Other Options Are Wrong:

• Pure radial, circumferential, or fixed oblique angles do not describe the dominant coupling field in a helix TWT.

Common Pitfalls:

Confusing field visualization around the helix with the component that performs work on the beam; it is the axial E field that governs velocity modulation.

Final Answer:

is directed along the helix axis