Short note: Multi-cavity klystron amplifier

A multi-cavity klystron is a linear-beam (O-type) microwave power amplifier. Three or more resonant cavities are placed along an electron beam to get high gain and high power.

 cathode  input    idler     idler    output
  gun    cavity   cavity    cavity   cavity  collector
 |>==== [ C1 ] == [ C2 ] == [ C3 ] == [ C4 ] ====|
  beam ->  RF in                        RF out
       <---- drift spaces with focusing B ---->

Working

1. The electron gun produces a uniform beam accelerated by V₀ to v₀ = √(2eV₀/m).
2. The weak RF input at the buncher (first) cavity velocity-modulates the beam. Some electrons are sped up and some are slowed down.
3. In the drift space the fast electrons catch up with the slow ones, which forms bunches (density modulation).
4. Each intermediate (idler) cavity is excited by the bunched beam. It builds up a larger gap voltage and modulates the beam again, so the bunching becomes tighter. Each cavity adds about 15–20 dB of gain.
5. The catcher (output) cavity is placed where the bunches are tightest. Its retarding field takes kinetic energy from the bunches, and amplified RF is coupled out.
6. The spent beam is collected by the collector.

Features

• Gain of about 40–60 dB with 3–5 cavities. Peak power up to tens of MW (pulsed). Efficiency of about 35–50%.
• Synchronous tuning (all cavities at one frequency) gives the highest gain but narrow bandwidth. Stagger tuning widens the bandwidth at the cost of gain.
• An axial magnetic field (solenoid or PPM) keeps the beam focused.

Applications: UHF TV transmitters, radar transmitters, particle accelerators (linacs), and satellite uplink stations.