Short note: Two-cavity klystron

A two-cavity klystron is an O-type (linear-beam) microwave amplifier. It has two cavities, a buncher and a catcher, separated by a field-free drift space.

 cathode buncher   drift space L   catcher
 |>==== [ gap 1 ] ============= [ gap 2 ] ==| collector
         ^ RF in   bunches form ->   v RF out

Working

1. The beam is accelerated by V₀ to v₀ = √(2eV₀/m) ≈ 0.593×10⁶ √V₀ m/s.
2. Velocity modulation: the input RF sets the buncher gap voltage V₁ sin ωt. An electron crossing at t₁ leaves with v = v₀[1 + (β_i V₁/2V₀) sin ωt₁].
3. Bunching: in the drift space, faster electrons catch up with slower ones that crossed earlier. Bunches form around the electron that crossed when the gap voltage went from negative to positive. The bunching parameter is X = β_i V₁ θ₀ / (2V₀), with θ₀ = ωL/v₀.
4. Output: the catcher is placed where the bunches are tightest. The fundamental beam current is 2I₀J₁(X), which is largest at X = 1.841 (J₁ = 0.582). The bunches induce RF current in the catcher, and its retarding gap field takes their kinetic energy, giving amplified RF output.

Key figures

• Maximum theoretical efficiency about 58%. Practical about 15–30%.
• Gain about 10–20 dB with narrow bandwidth, set by the cavity Q.
• Can be made into an oscillator by feeding part of the output back to the input.

Uses: UHF TV and troposcatter transmitters, radar driver stages, and particle accelerators. More cavities (multi-cavity klystron) are added for higher gain.