What Is A Klystron? And How Does It Work?If we have a device, which generates a beam of electrons, we notice that the electrons flow in a smooth steady stream at a particular uniform velocity. The area of the tube that the electron beam travels down is known as the DRIFT TUBE. If we insert, within the beam a grid, we can use this grid to control the beam. As we increase the positive potential on the grid, (assuming that we do not go over a certain potential which is less than the anode voltage), the electrons will be attracted to the grid, and by means of attraction, will be accellerated. On the other hand, should we decrease the potential, making it more negative, it will have the opposite effect on the beam, and try to slow down the electrons. Now using this theory, we attempt an experiment: 
We insert two grids, properly spaced for our experiment, and apply an alternating current source to the grids, such that as one grid swings positive, the other swings negative. This would mean that the electrons which are aproaching the positive going grid will be speeding up, as the ones aproaching the negative going grid will be slowing down. As the phase of the AC cycle changes 180 degrees, we have the same effect, only backwards. The result would be a sort of "slinky" effect, where the electron beam is interrupted, and moves along in bursts. This effect is known as VELOCITY MODULATION. In German, they say that electrons are moving in "Klystern". (Klyster is the German word for CLUSTER or BUNCH). Hence, the name Klystron.
On the other end of our experimental Klystron, we have two more grids installed. The purpose of these are to "feel" the now pulsing beam of electrons as it passes by them on their way to the anode. Note that an electron does not have to come into direct contact with a wire in order to induce an electric current in it. All it has to do is pass near enough to generate the current in the wire via mutual inductance. 
In reality, the grids in our klystron would be replaced by cavity resonators. Klystrons commonly used in Television work today typically have 4 to 5 cavities. Each cavity is individually tuned, and electromagnets are placed between cavities for focusing purposes. 
Klystrons can be either of the EXTERNAL CAVITY or of the INTEGRAL CAVITY type. They can be air, water, or vapor (steam) cooled, or some combination of the three.
Note to the left is a photograph of a klystron with 4 external cavities which is water cooled. To the right is a graphical representation of a 4 cavity klystron tube. In the first cavity (the input cavity), the beam is excited by the microwave signal intended to be amplified. This generates an alternating signal across the gap of the cavity. The velocity of the electrons passing through the beam will be modulated with the RF input signal. Each of the cavities are sucessively tuned in such a way as to reproduce a linear amplified input signal. In the output cavity, otherwise known as the ultimate cavity, the RF output signal is coupled to the transmission line and load. Below is a row of integral (internal) cavity klystrons (5 cavities) which are vapor (steam) cooled. Also notice that these Klystrons are shown in the standing position. 
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This Course was written by Ray Dall © All Rights Reserved.
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