In a reflex klystron oscillator, the repeller electrode is maintained at which potential?

Introduction:
Reflex klystrons are single-cavity microwave oscillators that use velocity modulation and a repeller electrode to bunch electrons and sustain oscillations. Understanding the repeller bias is fundamental to device operation.

Given Data / Assumptions:

• Device: reflex klystron (single cavity)
• Key electrodes: cathode, anode cavity (accelerating gap), repeller
• Objective: identify correct repeller potential

Concept / Approach:
The electron beam is emitted from the cathode and accelerated through the cavity gap. The repeller electrode is biased negative relative to the cavity so that electrons are turned back toward the gap after a controlled transit time, causing bunching and energy transfer to the RF field.

Step-by-Step Solution:

Verification / Alternative check:
Standard reflex klystron diagrams show V_repeller negative with respect to the cavity/anode to effect reflection and bunching timing.

Why Other Options Are Wrong:

• A/B: Positive potential would attract electrons away, not reflect them back.
• D: Zero potential would not provide adequate reflection control or stable oscillation conditions.

Common Pitfalls:
Confusing the multi-cavity klystron (with catcher) with the reflex type; assuming the repeller accelerates rather than reflects electrons.

Final Answer:
Negative potential