Reflex klystron oscillator: The frequency of oscillation is controlled primarily by which parameter (assuming the cavity is near its design resonance)?

Introduction / Context:
A reflex klystron uses a single resonant cavity and a repeller electrode to bunch electrons and sustain oscillations. Small-signal frequency tuning is achieved by altering the electron transit time between the cavity gaps, which is controlled by the repeller voltage.

Given Data / Assumptions:

• Single-cavity reflex design with electron beam turned back by a repeller.
• Operation near the cavity’s natural resonance.
• Adjustments are within the specified tuning range.

Concept / Approach:

Oscillation occurs when returning electron bunches deliver energy in the correct phase to the cavity fields. The transit time depends on beam energy and turning distance, both set by the repeller voltage. Varying this voltage shifts the phase condition and thereby the oscillation frequency, while the cavity sets the central frequency.

Step-by-Step Solution:

Verification / Alternative check:

Tuning curves from reflex klystron datasheets show frequency versus repeller voltage with multiple mode lobes; coarse center frequency is fixed by the cavity geometry.

Why Other Options Are Wrong:

• Cavity resonance alone fixes the baseline but does not provide agile tuning.
• Focusing voltage and cathode characteristics primarily affect beam quality and starting conditions, not the primary tuning.
• Magnetic focusing field is not the main frequency knob in a reflex klystron.

Common Pitfalls:

Confusing reflex klystron tuning with multicavity klystrons, where cavity frequencies dominate.

Final Answer:

Repeller (reflector) voltage