# Klystrons which are used as amplifiers

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## Main Question or Discussion Point

So I was reading about klystrons which are used as amplifiers, and I've got so many questions!
1st of all , what does an amplifier like klystron do? Does it incease the frequency of the input microwave?
What does low power microwave mean? Are they low power because of lw frequency or intensity?
What is the relationship between amplitude and frequency?
Why is velocity modulation is important in klystrons? Can't we just have all electrons with the same velocity for having high frequency microwave and not a wide bandwidth of microwaves?

It seems that I almost learn nothing about waves

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Klystrons amplify the input power (watts) without changing frequency (Hertz). A low power microwave signal might be a few milliwatts (mW); while a high power klystron output might be many megawatts (MW, peak pulsed). There is no direct relation between amplitude (watts) and frequency (hertz). Klystrons have a high current electron beam with an energy (per electron) up to a few hundred kilovolts, that passes through an input cavity, and output cavity, and a beam stop (plate or anode). The input microwave signal in the klystron input cavity modulates the electron beam velocity, not the electron beam current. Velocity modulation at the input by the microwave signal causes the electrons in the electron beam to move at different velocities as they drift toward the output cavity. When the electron beam arrives at the output cavity, the velocity modulation at the input has become a time (current) modulation. Consider a bunch of automobiles waiting at a stoplight. When the light turns green, all cars start accelerating, but some cars are faster than others. At the next intersection, the faster cars arrive first, the slower last.

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Klystrons amplify the input power (watts) without changing frequency (Hertz). A low power microwave signal might be a few milliwatts (mW); while a high power klystron output might be many megawatts (MW, peak pulsed). There is no direct relation between amplitude (watts) and frequency (hertz). Klystrons have a high current electron beam with an energy (per electron) up to a few hundred kilovolts, that passes through an input cavity, and output cavity, and a beam stop (plate or anode). The input microwave signal in the klystron input cavity modulates the electron beam velocity, not the electron beam current. Velocity modulation at the input by the microwave signal causes the electrons in the electron beam to move at different velocities as they drift toward the output cavity. When the electron beam arrives at the output cavity, the velocity modulation at the input has become a time (current) modulation. Consider a bunch of automobiles waiting at a stoplight. When the light turns green, all cars start accelerating, but some cars are faster than others. At the next intersection, the faster cars arrive first, the slower last.
Thanks a million

Now another question:what does slow-wave structure mean here?

Electric field from microwaves at buncher alternately speeds and slows electron beam

This causes electrons to bunch up

Electron bunches at catcher induce microwaves with more energy

The cavities form a slow-wave structure

marcusl
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The purpose of a slow-wave structure is to retard the effective speed of an electromagnetic wave so that it matches that of a particle stream. The classic slow-wave device is a traveling wave tube (TWT) where electrons are accelerated down a tube that is surrounded by a helix. Radio waves launched on the helix travel more slowly than in a vacuum, so they match the mean electron speed. As a result, the electron beam bunches. This works as follows: most electrons end up where the electric field is smallest. Those that lag behind end up in the highest field region and are accelerated back to the bunch. Those that lead end up in the highest negative field region and are retarded. In the process, power from the energetic electrons is transferred back into the radio wave, amplifying it.

In the Klystron, a series of coupled cavities performs the function of the slow wave structure, and because each cavity is resonant, the strength of radio field inside is greater than in a comparable TWT.

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Thank you, marcus!
Can I ask another question?:shy: Is it possible to use klystron as a amplifier for the microwaves which are produced by a magnetron?

marcusl
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No, it wouldn't make sense to do that. Magnetrons are "brutes"--you can't modulate them, and they don't amplify. There's no such thing as getting partial power out of a magnetron, it's always putting out full power (note that your microwave achieves a 50% power level by turning the magnetron on for 5 seconds and off for 5 seconds). It's just a high power oscillator, and not a very stable one at that. Although it was made famous in World War II as the power source for radar, it is not often used for radar anymore due to its inherent drift.

A klystron, on the other hand, is a linear amplifier. When used to amplify a precision microwave source, it can power particle accelerators and high-performance radars where wide bandwidth and good frequency and phase stability are needed.

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Thanks a million