Klystron - What is it? Working Principe.
A klystron is a vacuum tube that uses a microwave RF input to vary the velocity of electrons flowing through the tube.
Figure 1 shows a simplified klystron that uses two cavities, one for the input and one for the output. The cathode is used to generate an electron beam by applying a very high voltage between the cathode and anode. Those electrons then travel to the first cavity which is called the buncher cavity which either speeds up or slows down electrons based on the microwave input.
The electrons then go through the drift space which is long enough to let the electrons gather into groups by letting faster electrons catch up to slower moving ones. The catcher cavity then absorbs the electron groups and transmits them out of the microwave output. Any electrons that aren't absorbed by the catcher cavity are absorbed by the collector and are dissipated as heat. Modern klystrons usually use electromagnets to help focus the beam and utilize more than two cavities to increase the tube gain.
Another type of klystron is called a reflex klystron. It also modulates electron velocity but is constructed differently using a resonant cavity. The cavity allows the reflex klystron to oscillate and it's those oscillations that modulate the electron velocity. With some exceptions modern semiconductors have replaced reflex klystrons.
Large radar systems and high power microwave transmitters use large klystrons like in Figure 2, some of which are capable of up to tens of megawatts of output power.
But sometimes even that isn't enough power. For example, Stanford University is working on a linear particle accelerator that needs thousands of 75-MW klystrons to power it. Those types of systems are out of reach for hobbyists, but fortunately for us there's the 2K25 klystron.
2K25 klystrons are readily available and their low power output makes them much more practical to experiment with. They are a reflex klystron design and were typically used as the local oscillator in 3-cm (9.6-GHz) radar receivers. They came in two different versions, one where you tune the oscillation frequency using a screw (Figure 3a) and the other which is tuned electronically using a tuning diode (Figure 3b).
The best part about them though is that there's a lot of information online available about how to use them, so why not give it a try?
Elektor Electronics, 2015, March & April