Which microwave device employs internal positive feedback to sustain oscillations through electron bunching and reflection? (Assume standard device configurations and biasing.)

Introduction:
Positive feedback is a defining feature of oscillators: part of the output is fed back in phase to sustain a self-excited RF signal. In microwave tubes, different mechanisms produce or avoid feedback. This item asks you to identify which device uses internal positive feedback by design.

Given Data / Assumptions:

• Standard two-cavity and multi-cavity klystrons are configured as amplifiers with external input drive.
• A reflex klystron uses a single resonant cavity and a repeller electrode.
• We consider normal bias and operating modes.

Concept / Approach:

The reflex klystron is an oscillator. Electrons are accelerated toward a cavity, pass through its gap, and are then turned back by a negatively biased repeller. Proper repeller voltage and transit time cause bunched electrons to return to the cavity gap in phase with the RF field, reinforcing it. This constitutes internal positive feedback. By contrast, two-cavity and multi-cavity klystrons are amplifiers; they rely on an external RF input and do not intentionally create the regenerative loop necessary for self-oscillation.

Step-by-Step Solution:

Verification / Alternative check:

Operationally, the reflex klystron can start from noise and build RF amplitude without an external signal, a hallmark of regenerative feedback oscillators.

Why Other Options Are Wrong:

• Two-cavity and multi-cavity klystron amplifiers: designed for gain with external drive; feedback is minimized to avoid oscillation.
• TWT with helix: distributed interaction amplifier; typically not a self-oscillator without additional feedback.
• “All of the above”: incorrect because only the reflex klystron is inherently regenerative.

Common Pitfalls:

Assuming any device with bunching “has feedback.” Bunching alone does not imply the regenerative loop required for oscillation.

Final Answer:

Reflex klystron (oscillator)