Magnetron, How does it work?

- World War II was one of the most traumatic events in world history. But, on the other hand, it also gave rise to several inventions that have completely changed the world. One of the key inventions of this era was the cavity

magnetron

, which made radars super efficient. Cavity

magnetron

s are also used in microwave ovens, where they are responsible for producing high-power microwaves. In this video, we will explore the physics behind the cavity magnetron. Cavity magnetrons

work

on the LC oscillation principle. LC oscillation occurs when a charged capacitor is placed across an inductor. This simple arrangement creates a back and forth motion of the electrons.

To learn more about oscillations, click the I button. When an antenna with an inductor attached is placed near the inductor of an LC circuit, the antenna radiates electromagnetic waves. This is the theory behind the cavity magnetron. Obviously, the power swing and associated radiation from this theoretical device will disappear quickly as it loses energy as radiation. How to convert this theoretical device into a practical one? Let's look at this in the next sessions. Consider this configuration, a cathode and a filament. The flow of current through the filament will heat the cathode and due to this electrons will be emitted.

This phenomenon is known as thermionic emission. Interestingly, in this case the electrons return to the cathode. If we place an anode with positive potential, the emitted electrons accelerate and move towards the anode. As radiation theory says, charges produce radiation when they are accelerated. However, in this arrangement, the electrons radiate inefficiently as they spend very little time in the interaction space. To increase the time that electrons spend in this space, a permanent magnet is introduced into the structure. The magnetic field forces the electrons to follow a curved path. Since the electron path is now curved, the time the electrons spend in the interaction space increases.

The final structure thus formed is known as a hull magnetron. Hull magnetrons are more efficient than the technology explained above; However, its efficiency can be further improved with the help of LC oscillations, which we saw at the beginning of this video. Let's see how we achieve oscillation in a magnetron. To achieve oscillation, the anode is designed with cavities. These cavities cause enormous differences in the physics of magnetrons. To understand this, let's consider a simple case. Let's consider a metal bar with a cavity. Suppose a negative charge passes near the metal. The negative charge will obviously repel electrons close to it, as shown in this animation.

Similarly, when the negative charge passes near the cavity, the electrons around the surface of the cavity are disturbed. You can see that a buildup of positive and negative charges occurs on the surfaces of the cavity due to this disturbance. In short, the cavity surfaces act as capacitor plates. If you connect an inductor across the surface of the cavity, the charges will start to oscillate. This simple physics is the basis of the cavity magnetron. A magnetron has many such cavities. Many electrons are ejected from the cathode by thermionic emission. Let's follow the effect of the first electron ejected into these cavities.

As explained above, this electron will induce positive and negative charges on the cavity surfaces. In this case, the cavities are arranged in a circular shape. This means that the pair of charged cavity surfaces cannot remain isolated. To maintain zero electric field in the metal, all pairs of cavities must be charged with opposite polarity. One interesting thing to note here is that the curved surface of the cavity acts as an inductor. This means that the accumulated charges will go through a simultaneous LC oscillation. With the help of a metal loop and antenna, this oscillation is extracted and converted into EM waves.

These oscillations will be maintained in the magnetron, as electrons continually flow from the cathode to the anode and transfer their energy. Now let's see what happens to the remaining electrons in the interaction space. The first electron that reached the cavity surface already created a charge pattern in the cavities. This means that the remaining electrons will be attracted to the positively charged regions and form an interesting spoked wheel pattern like this. As the loads in the cavities oscillate, the spoke wheel has to rotate as illustrated. This phenomenon could be related to the analogy of a donkey, a carrot and a stick.

Here, no matter how many steps the donkey takes to reach the carrot, the carrot always remains out of reach. As you may have noticed, the antenna is connected only to a single cavity, since the magnetic field lines generated in one cavity also connect to the other cavities. This phenomenon is called mutual coupling. This means that the extraction of magnetic energy from one cavity would be the same as the extraction from all the cavities combined. The cavity magnetron was developed in the United Kingdom during World War II to improve radar technology. Cavity magnetrons are capable of producing high-power pulses at a shorter wavelength, which made it possible to detect smaller objects.

The compact size of the cavity magnetron made the size of the radar smaller. This UK technology was transferred to the US during World War II and initially American scientists had difficulty understanding the physics behind cavity magnetrons. This means that the technology you now understand is one of the most complicated engineering technologies.