Klystron - Radartutorial

What is a Klystron?

• Klystrons are high-power microwave vacuum tubes.
• Used in radars as amplifiers or oscillators.
• They utilize the kinetic energy of an electron beam for high-frequency signal amplification.
• Operate on the principle of velocity modulation and transit-time effect.
• Linear beam devices with electron flow in a straight line, focused by magnetic fields.
• Utilize resonant cavities and are classified as two-cavity, multi-cavity, and repeller klystrons.
• High gain (over 60 dB) and output power (up to tens of megawatts) but with limited bandwidth (less than 2%).
• Require high supply voltages (hundreds of kilovolts) and have limited reliability (mean time between failure: 5000 to 75000 hours).

Types of Klystrons

Two-Cavity Klystron

• Functions as a microwave amplifier through velocity and density modulation.
• Can also be used as an oscillator with feedback between cavities.
  • Oscillator tuning is complex, better suited for repeller klystron.

Design

• Consists of an electron emitter, two resonant cavities, a solenoid magnetic circuit, and a collector.
• First cavity (buncher) modulates the electron velocity, second cavity (catcher) absorbs energy.
• High beam voltage applied; collector generates X-ray interference and must be shielded.

Function

• Electrons accelerated by DC voltage through buncher grids.
• Velocity modulation occurs as electron velocity varies with signal frequency.
• Bunches form in drift space, energy absorbed by catcher cavity.

Characteristics

• Efficiency around 40%, with losses due to non-ideal electron density bundling.
• Average output power up to 500 kW, pulsed power up to 30 MW at 10 GHz.
• Power amplification up to 30 dB.

Multicavity Power Klystron

• Improved amplification, power output, and efficiency through intermediate (gain) cavities.
• Allows for increased gain or bandwidth (up to 8% with stagger-tuning).
• Kladistron is a high-performance variant with many cavities, not used in radar.

Multi-beam Klystron (MBK)

• Comprises multiple electron beams with common input/output cavities.
• Delivers high microwave power at lower voltages; shorter circuit lengths.
• Efficiency of 60 to 80%, commonly used for accelerator applications.

Sheet-beam Klystron

• Utilizes a flat electron beam (sheet beam) instead of a round one.
• Offers longer life due to lighter cathode loading and simplified construction.
• Better cooling possible compared to pencil-beam tubes.

Repeller Klystron (Reflex Klystron)

• Uses repeller instead of output cavity; feedback achieved by reversing the beam.
• Requires three power sources: filament power, beam voltage, and negative repeller voltage.
• Variation in reflector voltage can adjust frequency slightly.

Applications

• Widely used in high-frequency technology where semiconductor components fall short.
• Used in weather radars and accelerators for research, requiring massive high-frequency power.
• Applied in terrestrial TV transmitters, measuring amplifiers, and receivers.
• Ongoing research on W band klystrons with high precision manufacturing processes (LIGA).