Reflex klystron oscillator – Number of resonant cavities used In a reflex klystron microwave oscillator, how many resonant cavities are employed in the RF interaction structure?

Introduction:
Klystron families include multi-cavity klystron amplifiers and reflex klystron oscillators. The reflex type achieves oscillation by reflecting the electron beam back through the same gap so that it interacts twice with a single resonator, establishing the correct phase for sustained oscillations.

Given Data / Assumptions:

• Reflex klystron with cathode, input/output cavity (single gap), and negatively biased reflector (repeller).
• Beam traverses the cavity region, is turned around by the reflector, and re-traverses the gap.
• Proper reflector voltage sets the transit-time condition (n + 3/4 mode) for oscillation.

Concept / Approach:

Unlike multi-cavity amplifiers (separate buncher and catcher), the reflex klystron uses one resonant cavity. Energy exchange occurs twice with the same cavity as the bunched beam returns. Frequency is primarily determined by the cavity resonance and the reflector bias that fixes transit time.

Step-by-Step Solution:

Verification / Alternative check:

Standard reflex klystron schematics label a single re-entrant cavity and a repeller electrode; additional cavities are not used.

Why Other Options Are Wrong:

Two/three cavities describe other klystron types; “none” is impossible since a resonator is required to define frequency and store RF energy.

Common Pitfalls:

Confusing reflex klystrons with multi-cavity klystron amplifiers; misattributing multiple gaps to oscillators.

Final Answer:

one cavity resonator.