- [Narrator] World War II was one of the most traumatic events
in the history of the world. But on the other hand, it also resulted in 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 magnetrons are also
used in microwave ovens, where they are responsible for producing high-powered microwaves. In this video, we will explore the physics behind the cavity magnetron. Cavity magnetrons work on the
principle of LC oscillation. LC oscillation occurs when a
charged capacitor is placed along an inductor. This simple arrangement creates back and forth motion of electrons. To find out more about oscillations, please click on the I button. When an antenna with an inductor attached to it is placed near to the
inductor of an LC circuit, the antenna radiates
electromagnetic waves. This is the theory behind
the cavity magnetron. Obviously, the energy oscillation
and associated radiation of this theoretical
device will die out fast, since it loses energy in
the form of radiation. How can this theoretical
device be converted into a practical one? Let's look at this in the coming sessions. Consider this configuration,
a cathode and a filament. The current flow through
the filament will heat up the cathode, and due to this, electrons
will be emitted from it. This phenomenon is known
as thermionic emission. Interestingly, in this case,
the electrons come back to the cathode. If we place an anode
with positive potential, the emitted electrons accelerate
and move towards the anode. As the theory of radiation states, the charges produce radiation
when they accelerate. However, in this arrangement, the electrons radiate inefficiently as they spend very little
time in the interaction space. In order to increase the
time spent by the electrons in this space, a permanent
magnet is introduced into the structure. The magnetic field forces the electrons to take a curved path. Since the path of the
electrons is now curved, the time that the electrons spend in the interaction space is increased. The final structure thus formed is known as a hull magnetron. Hull magnetrons are more efficient than the previously explained technology, however, its efficiency
can be further improved with the help of the LC oscillations, which we saw in 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 huge differences in the physics of magnetrons. To understand this, let's
consider a simple case. Let's consider a metal bar with a cavity. Assume a negative charge is
passing near to the metal. The negative charge will
obviously repel the electrons near to it, as shown in this animation. Similarly, when the
negative charge passes near to the cavity, the electrons around the
cavity surface are disturbed. You can see that an
accumulation of positive and negative charges occurs
across the cavity surfaces due to this disturbance. In short, the cavity surfaces
acts like capacitor plates. If you connect an inductor
across the cavity surface, the charges will start oscillating. 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 track the effect of the
very first electron ejected into these cavities. As explained above, this
electron will induce positive and negatives charges
on the cavity surfaces. Here, the cavities are
arranged in a circular manner. This means the charged cavity surface pair cannot stay in isolation. To keep the electric
field zero in the metal, all the cavity pairs have to be charged with the opposite polarity. One interesting thing to note here is that the curved surface of the
cavity acts like an inductor. This means that the
charges accumulated will go for a simultaneous LC oscillation. With the help of a metal
loop and an antenna, this oscillation is extracted
and converted into EM waves. These oscillations will be
sustained in the magnetron, since the electrons continually
flow from cathode to anode and transfer their energy. Now let's see what happens
to the remaining electrons in the interaction space. The very first electron that
reached the cavity surface has already created a charge
pattern on the cavities. This means the remaining
electrons will be attracted to the positive charge
regions, and they will form an interesting spoke
wheel pattern like this. Since the charges on the
cavities are oscillating, the spoke wheel has to
spin 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 its reach. As you must have noticed,
the antenna is connected only to a single cavity, since the magnetic field lines generated in one cavity also link
with the other cavities. This phenomenon is called mutual coupling. This means the extraction
of magnetic energy from one cavity would be
the same as the extraction from all of the cavities combined. The cavity magnetron was developed in the UK during World War II
to enhance radar technology. Cavity magnetrons are able to
produce high-powered pulses at a shorter wavelength, and
this led to the detection of smaller objects being possible. The compact size of the
cavity magnetron made the radar size smaller. This UK technology was transferred to the US during World War II, and initially, the US
scientists had a difficult time in understanding the physics
behind cavity magnetrons. This means that the technology
you now understand is one of the most complicated
engineering technologies.
