- [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.