In a multi-cavity klystron amplifier, does the RF signal to be amplified create an alternating (AC) gap voltage at the signal frequency across the buncher cavity? (Assume standard klystron operation with input coupling into the buncher.)

Introduction:
Klystron amplifiers use velocity modulation and subsequent bunching of an electron beam to achieve RF amplification. The buncher cavity is where the incoming RF signal is coupled to the beam. This question checks whether you understand that the applied signal appears as an alternating gap voltage across the buncher cavity, initiating velocity modulation.

Given Data / Assumptions:

• Multi-cavity klystron amplifier with at least a buncher and a catcher (output) cavity.
• Small-signal, steady-state RF excitation at the input port.
• Normal operating bias on the electron gun and beam transport through drift spaces.

Concept / Approach:

The incoming RF power is coupled into the buncher cavity so that an AC electric field exists across its gap. Electrons traversing this gap experience time-varying acceleration or deceleration, producing velocity modulation. In the subsequent drift region, faster electrons catch up with slower ones, forming density bunches. These bunches then excite RF fields in the output (catcher) cavity, transferring kinetic energy to the RF signal and providing gain.

Step-by-Step Solution:

Verification / Alternative check:

Small-signal klystron theory shows that the induced RF voltage in the buncher is proportional to the applied drive; without an AC gap voltage there is no velocity modulation and therefore no gain.

Why Other Options Are Wrong:

• False (and variants): contradicts fundamental klystron operation where the buncher must have an RF electric field.
• “Only at very low frequencies” or “only when over-coupled”: coupling conditions and frequency affect efficiency/bandwidth, not the existence of the AC gap voltage itself.

Common Pitfalls:

Confusing DC focusing/beam acceleration with the RF gap field. The buncher has both: DC beam acceleration elsewhere and a superimposed RF field across its gap from the drive.

Final Answer:

True