Operation of Klystron
Klystron As Microwave Generator
Klystrons amplify RF signals by converting the kinetic energy in DC electron beam into radio frequency power. A beam of electrons is produced by a thermionic cathode (a heated pellet of low work function material), and accelerated by high voltage electrodes. (typically in the tens of kilovolts). This beam is then passed through an input cavity. RF energy is fed into the input cavity at, or near, its natural frequency to produce a voltage which acts on the electron beam. The electric field causes the electrons to bunch: electrons that pass through during an opposing electric field are accelerated and later electrons are slowed, causing the previously continuous electron beam to form bunches at the input frequency. To reinforce the bunching, a klystron may contain additional "buncher" cavities. The electron bunches increase in magnitude, as the overall drift velocity of the beam decreases, and this in effect represents the sum of an RF current in the beam along with the the DC component. An RF current of course will produce a magnetic field, and this will in turn excite a voltage across the gap of the output cavity, thus allowing the transfer of RF energy developed flows out through a waveguide. The spent electron beam, which now contains less energy than it started with, is captured in a collector.
A klystron looks and works something like an organ pipe. In an organ pipe:
- Blowing into the organ pipe produces a flow of air.
- Flowing air excites vibrations in the cavity of the whistle.
- The vibrations flow into the surrounding air as sound waves.
In a klystron:
- The electron gun (1) produces a flow of electrons.
- The bunching cavities (2) regulate the speed of the electrons so that they arrive in bunches at the output cavity.
- The bunches of electrons excite microwaves in the output cavity (3) of the klystron.
- The microwaves flow into the waveguide (4), which transports them to the accelerator.
- The electrons are absorbed in the beam stop.(5)