The klystron is a device for amplifying microwave frequency signals that achieve high levels of power gain by applying vacuum tube principles and the concept of “electron bunching”. Klystrons are used in satellite systems, television broadcast, and radar, as well as particle accelerators and medicine.
The klystron was invented by the brothers Russell and Sigurd Varian at Stanford University. Their prototype was completed and demonstrated successfully on August 30, 1937.
Klystrons can be used in the UHF region (300 MHz to 3 GHz) up to 400 GHz. There are several flavors of klystron amplifiers. One major type is the reflex klystron, which is used primarily as an oscillator.
For this article, however, we will focus on another popular type: the two-cavity klystron.
Principles of Two-Cavity Klystrons
Two-Cavity Klystron Geometry
The two-cavity klystron utilizes an electron source (heater), an anode, and a cathode like a conventional vacuum tube. It also utilizes a collector element at the end of the electron stream. The heater boils off electrons when heated and the electrons are ejected from the cathode and accelerate towards the anode due to the high dc potential between the two elements. A focused beam of electrons is thus produced.
In the case of the two-cavity klystron, the electron beam passes through a central hole in the first toroid-shaped cavity and through a similar second cavity, terminating at the collector.
On each side of the cavity hole is a grid that the electrons pass through. It is the interaction of the cavities with the beam that provides the high levels of amplification that the device can produce.
Figure 2. Layout of klystron tube
Perhaps we can digress a moment to discuss the cavity used in the buncher and the catcher. The cavity in this story is a toroid-shaped object with the following cross section:
Figure 3. 3a) Resonant cavity; 3b) Equivalent in pseudo electrical form; 3c) Equivalent circuit; 3d) Frequency response.
This can also be shown as a resonant tank circuit with the parallel region the capacitor and the circular part a single turn inductor as shown in Figure 2b and 2c.
The cavity can be made to resonate at a narrow frequency range (Figure 2d), defined by its geometry, of course. The central part of the structure acts like a capacitor with a hole in it which is where the electron beam can pass through. This capacitor and thus the charge applied to anything passing through the central hole will flip charge at the resonant frequency.
From an electrical perspective, the capacitance and inductance define the electrical resonant frequency of the structure. An exciting signal is fed into the resonator externally via a coax connection shown at the top of Figure 2a. This coax connection excites the cavity at the resonant frequency.
The klystron utilizes a phenomenon called electron bunching which goes as follows:
Electrons in a beam leaving a source at high velocity all have a roughly equal velocity in the direction of travel. With no applied interaction along the path, the electrons in the beam will continue this way until terminating at the collector. If, however, there exists a structure along the path that can oppose the movement of the electrons, it can cause some of them to reduce their velocity. This occurs when the left side grid is negative.
The grid’s negative charge pushes back on the electrons as they pass through the negative left grid slowing them down. As they pass through the space between grids and past the rightmost positive grid, the electrons are further slowed down by the positive grid as it pulls on them as they exit the opening.
On the opposite electrical cycle of the plates, the electrons encounter a positive grid initially, which pulls on them and accelerates them through the buncher grids. The now negative rightmost grid pushes them faster as it repels them on exit.
Imagine you are an electron going through the buncher and you are slowed down by the buncher. You would be cruising along and gently slowing down so all the other electrons around you would be spreading out (in the direction of travel). Life is good—lots of room up front. But wait! A whole bunch of electrons behind you got accelerated to a higher velocity and now they are catching up to you as you slow down into them! Now we are in a big bunch traveling down the drift space.
Figure 3. This diagram shows the electron bunching behavior as electrons traverse the drift space. A shows a snapshot at the start of the transit. As we progress through B to D, the slower electron group (blue) is progressively overtaken by the faster electron group (red) resulting in a period of high electron density at frame D.
The result is a density modulation or bunching proportional to the applied force on the electrons as imparted by the buncher resonant cavity (does that start to sound useful?). The end result is effectively an RF-modulated current (current is just charge flow over time, after all) between the buncher and the catcher.
Operation: Putting It All Together
So now we have a method to apply an alternating electrical signal to a capacitor that has an electron beam passing through it. As you have probably surmised (being the uber-clever engineer that you are), the alternating signal (read: polarity) on the capacitor either slows the electrons down or accelerates them. Thus electron bunching occurs at the frequency of the applied signal!
"Ok, so what?“ you may be saying. Well, if we take a similar resonator and place it, say, the exact distance for optimum bunching, and make a coax connection to the resonator and extract the signal as an output instead of an input, we can now get a signal out that is a copy of the input signal (the bunchings) and is greatly amplified!
Now we have a microwave power amplifier based on the fact that fewer electrons entered the device and many more have been bunched proportional to the input signal and are output via the catcher cavity.