The Legendary Magnetron: How It Really Works

[00:00:00]Hello everyone. I've already released a couple of popular science videos about the tunnel diode and the gun diode. The videos turned out to be really interesting and a lot of people appreciated them. You can watch these videos by following the links in the description.

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[00:00:33]This video will be long but very engaging. So, make yourself comfortable.

[00:00:37]Today, we're going to talk about an electronic device that was invented a long time ago. A unique device that was actively used for military purposes and now helps you cook your food. A mysterious device that shoots electrons.

[00:00:51]A weapon that disables electronics and disperses crowds of people. Meet His Majesty, the great and terrible magnetron.

[00:01:01]The very term magnetron was proposed in 1921 by Albert Hull, who went down in history as the creator of the magnetron.

[00:01:09]Hull's magnetron was not intended for generating high-frequency radiation.

[00:01:13]Hull simply wanted to create an alternative to the vacuum triode with a control grid bypassing Western Electric's patents.

[00:01:21]He planned to control the anode current of the tube not with a grid but with a magnetic field. Hull's magnetron was created as a magnetically controlled vacuum device which could be used, for example, as a switch or an amplifier.

[00:01:33]Three years later, two scientists, August Žáček and Eric Habann, independently discovered that the magnetron could generate decimeter waves with frequencies up to 1 GHz. In the 1920s and 1930s, there were projects to develop magnetrons in many countries.

[00:01:48]So, it's impossible to say that the magnetron is the achievement of any one country or individual.

[00:01:54]In 1940, at the University of Birmingham in Britain, scientist John Randall and Harry Boot created a prototype of a multi-cavity magnetron with a high pulse power of about 400 watts. And from that moment, the era of the magnetron began.

[00:02:08]It was this version of the device that became the heart of centimeter wave radars during World War II.

[00:02:13]And the first microwave oven was created by the Raytheon company after engineer Percy Spencer noticed that magnetron radiation could heat food.

[00:02:21]Mass production of these ovens began 2 years later in 1947. They were large in size and were primarily intended for restaurants, while the familiar household microwave ovens only began to be produced in the 1960s. Design and operating principle. The operation of the magnetron is based on classical phenomena, and in fact, it's all simpler than it might seem. But nevertheless, let's start from the very beginning. In front of you is a classic oscillating LC circuit consisting of an inductor and a capacitor. The resonant frequency of the circuit is calculated using the formula shown.

[00:02:55]The smaller the inductance of the coil and the capacitance of the capacitor, the higher the frequency, and vice versa. An LC circuit can be constructed in different ways. A piece of copper wire, even if it doesn't have the shape of a coil, still has some, albeit tiny, inductance. Any piece of metal, regardless of its shape, has inductance.

[00:03:12]The same can be said about a capacitor.

[00:03:14]There is always a certain amount of electrical capacitance between any two conductors. For this reason, at very high frequencies, even ordinary wires, component leads, PCB traces, and metal structural elements start to behave like resonant circuit elements with parasitic inductance and capacitance.

[00:03:31]An LC circuit by itself is not a source of energy and can't generate sustained oscillations out of nothing, aside from the small noises that are present in any real system.

[00:03:42]In order for oscillations to occur in the circuit, energy must first be introduced into it by charging the capacitor, passing current through the coil, or exciting the circuit with an external electromagnetic field. In a real resonant circuit, due to losses, these oscillations gradually die out, so continuous generation requires constant external energy input.

[00:04:02]Let's assemble a simple circuit using a power source, an inductor, and a capacitor. The circuit also includes a couple of resistors, which can be neglected.

[00:04:11]In the first position of the switch, the capacitor will start charging from the power source. Initially, before the power is applied, the voltage across it is about zero.

[00:04:21]A standalone acts as a capacitive load.

[00:04:24]When connected to the power source, at the initial moment, the charging current is at its maximum, while the voltage is at its minimum. As the capacitor charges, the current will start to decrease, and the voltage will increase.

[00:04:34]In the case of an inductor, it's the opposite. It acts as an inductive load, and the voltage will lead the current.

[00:04:41]Next, when we flip the switch, the external power source is disconnected, and all the energy stored in the capacitor goes to the inductor. At this point, the circuit becomes a closed system without any supply.

[00:04:54]In our circuit, the current can't instantly reach its maximum, because the inductor resists sudden changes. Then, a free oscillatory process begins. The energy stored in the electric field of the capacitor begins to transfer into the magnetic field of the inductor. The voltage across the capacitor decreases, while the current through the inductor increases.

[00:05:15]When the capacitor is almost completely discharged, its voltage becomes minimal, and the current through the inductor reaches its maximum. The energy from the capacitor has almost entirely transferred into the magnetic field of the inductor. The inductor has an important property. It stores energy in its magnetic field, and resists sudden changes in current. This phenomenon is related to self-induction. Pay attention to the initial polarity of the voltage across the capacitor.

[00:05:41]At first, one of its plates was charged positively, and the other negatively.

[00:05:45]But, when the capacitor discharges, the current in the circuit doesn't disappear instantly. Thanks to the energy stored in its magnetic field, the inductor continues to maintain the current in the same direction. The magnetic field of the inductor begins to decrease, and due to the phenomenon of self-induction, an EMF is generated in the inductor that keeps the current flowing in the same direction.

[00:06:04]In fact, for a short time, the inductor itself becomes a source of voltage and returns the stored energy back to the circuit.

[00:06:12]This current continues to flow through the capacitor and charges it, but now with the opposite polarity. Then, once again, energy is accumulated and released, and each time the polarity of the voltage will reverse, which results in sinusoidal oscillations. With one polarity, one half wave is formed. With the opposite polarity, the other half wave is formed. With each such cycle, the oscillations will fade until the energy is completely depleted. This is the operating principle of an oscillatory circuit. Oscillations in the system can also be excited without interfering with the LC circuit itself by using an does all of the above have to do with a magnetron?

[00:06:51]Actually, it has a direct connection, even though it might not be obvious yet, so let's keep going. In front of me is an incandescent lamp. When it's off, it's just the glass bulb with a vacuum inside and a filament. When power is supplied, the filament heats up then emits light and heat.

[00:07:09]This is also a lamp, but it's an electron tube, or simply a vacuum tube.

[00:07:13]It also has a filament, and in the simplest design, it also has an anode and a cathode. The filament is heated, and in turn, it heats the cathode.

[00:07:22]If you heat the cathode to a certain temperature, some of the electrons in the cathode will gain enough energy to leave the cathode. This is called thermionic emission. The electrons that leave the surface of the cathode begin to drift near it, forming an electron cloud. When a high voltage is applied between the anode and the cathode, an electric field is created. The cathode is negative, the anode is positive, so the electrons, being negatively charged particles, will move toward the positively charged anode.

[00:07:52]You can visualize this process more clearly on an analog oscilloscope with a cathode ray tube.

[00:07:58]Simply put, inside such a tube, there's also a heated cathode that emits electrons.

[00:08:04]Then, a high voltage is applied between the cathode and the anodes. The electrons are accelerated toward the screen, strike the phosphor coating, and we see a glowing dot or a beam.

[00:08:15]So, this beam is the result of electrons hitting the screen. By bringing a permanent magnet close to the screen, we can deflect the electron beam, and as a result, the beam on the screen. And now, knowing all this, we can move on to studying the magnetron itself and understanding how it works. A magnetron is essentially just another vacuum tube.

[00:08:33]Structurally, it's quite simple. Here is a magnetron from a microwave oven, a copper bulb with aluminum plates pressed onto it, serving as a cooling radiator.

[00:08:43]Permanent magnets are located on the top and bottom. There are filament leads, as well as an antenna or an output for the ultra high frequency radiation. That's at least what you can see from the outside, but all the magic happens inside. In cross-section, a magnetron looks like this. Inside the copper bulb, of course, is a vacuum. The entire body of the magnetron is nothing more than a massive anode. The anode isn't just a piece of metal. It has resonant cavities built into it. Basically, these are three-dimensional oscillating circuits, but instead of being made from a coil and a capacitor, they're formed as metal cavities.

[00:09:15]In the center, there's a cathode assembly with a heater, also known as the filament. This here is the antenna or the output for the useful radiation.

[00:09:22]A magnetron is essentially a type of directly heated vacuum tube. When power is supplied to the filament, the cathode heats up and emits electrons. When high voltage is applied between the cathode and the anode, the electrons start moving toward the positively charged anode. And this is where things get really interesting.

[00:09:40]We remember that we have permanent magnets and the magnetic field prevents the electrons from reaching the anode directly and starts to make them spiral.

[00:09:48]The electrons are affected by the Lorentz force, which bends their trajectory.

[00:09:53]So, we're seeing almost the same phenomenon as with the oscilloscope beam.

[00:10:00]As a result, the electrons no longer fly straight to the anode, but move along a twisted trajectory. In the end, the electrons will spiral in the space between the anode and the cathode. This creates a kind of rotating electron cloud.

[00:10:14]Under the influence of the high-frequency field of the resonators, the electron cloud becomes uneven, forming rotating electron bunches or so-called spokes.

[00:10:25]Electrons passing by the slots of the resonators at the right phase are slowed down and transfer part of their kinetic energy to the electromagnetic oscillations in the resonators. After some of the electrons have given their energy to the resonators, high-frequency electromagnetic oscillations arise and intensify within them. We remember that the smaller the inductance and capacitance, the higher the resonant frequency of the circuit. As you can imagine, the inductance of the circuit coil and the capacitance of the capacitor in this case are extremely small and the oscillations occur at ultra-high frequencies.

[00:10:56]The extraction of useful ultra-high frequency energy takes place from the resonator system of the anode block through a coupling loop into the waveguide. The resonant frequency of the magnetron is determined by the physical dimensions of the cavities. The smaller the size, the higher the frequency and vice versa.

[00:11:11]In short, at the very beginning, weak random oscillations arise in the resonators due to noise from transient processes and the unevenness of the electron flow. But the resonant cavities are tuned to their own specific frequency, so it is these oscillations that subsequently begin to intensify.

[00:11:28]The resulting high-frequency field starts to group the electron cloud into bunches or spokes. And these electron bunches, as they pass by the slots of the resonators at the right phase, transfer part of their energy to them.

[00:11:40]And this is exactly how self-sustaining generation occurs.

[00:11:47]Startup. Before we continue, an important warning. Never try to repeat what you see here. A magnetron is dangerous in every sense of the word.

[00:11:56]The powerful microwave radiation it generates can cause irreversible vision loss and other injuries such as burns.

[00:12:04]The high anode voltage of several kilovolts is no less dangerous to your life. Everything shown next is done solely for educational purposes.

[00:12:13]Experiments were done in laboratory conditions under safety protocols, almost all of them. But that's not important. Bottom line, do not try this.

[00:12:21]So, we're going to start up the magnetron. For this, you need the standard set of devices found in a microwave oven.

[00:12:27]A high voltage mains transformer, which has one secondary winding to power the magnetron's filament, and a second secondary winding that outputs, on average, 2 kilovolts.

[00:12:39]In addition to that, you need a capacitor and a high voltage diode stack from the same microwave. The wiring's done according to the specified diagram.

[00:12:47]The filament receives the heating power.

[00:12:49]The filament voltage is usually 3 to 4 volts.

[00:12:52]The high voltage goes to a halfway voltage doubler, which generates about 4,000 volts of pulsating DC voltage between the anode and the cathode. At the same time, the cathode is at a high negative potential relative to the anode. In my case, the magnetron is powered by a programmable AC power supply with all the necessary protections and isolation from the mains. The radiation from the magnetron is so powerful that it causes ionization of gases and gas-filled devices like fluorescent lamps at quite a considerable distance. Microwave radiation can also damage some electronic devices.

[00:13:29]Danger, the magnetron is probably one of the most dangerous electronic devices out there, if not the most dangerous.

[00:13:36]First of all, the powerful stream of microwave radiation generated by a magnetron from a household microwave oven is highly penetrating.

[00:13:44]It can penetrate organic materials by several centimeters and cause them to heat up from the inside. This is especially dangerous for our eyes, which is why household microwave ovens have several safety features to prevent the magnetron from operating when the door is open.

[00:13:59]The ovens themselves are safe. Yes, a small amount of radiation does leak out, but it's a negligible amount. And as long as you don't spend your life next to the microwave with your head pressed against the glass door, nothing bad will happen. The high voltage source is also a hazard. That's the very transformer that generates the anode filament voltage.

[00:14:20]It's a fairly powerful transformer rated at several hundred watts and it's low frequency, which is important.

[00:14:26]Given the high voltage and high power, the anode voltage from this transformer can easily kill a person under unfortunate circumstances. If you're lucky, you might just get an electric shock you'll remember for the rest of your life along with a severe burn.

[00:14:40]On top of that, the magnetron's design includes ceramic insulators that are really hard and conduct heat well. In some cases, this can be beryllium ceramic. The dust from the ceramic is extremely toxic. If it gets into your lungs, it can cause a severe illness called berylliosis. Beryllium ceramic itself isn't dangerous as long as you don't grind, saw, or drill it. The real danger comes from the fine particles and dust of the ceramic. Otherwise, beryllium ceramic is widely used because of its excellent properties. It's thermally stable, a good dielectric, strong, and has outstanding thermal conductivity. I've already listed three dangerous factors, but that's not all.

[00:15:20]The cathode assembly in some magnetrons may contain radioactive thorium 232. The amount of thorium is tiny, but still A note here, not every household magnetron necessarily contains thorium, but it has been used in magnetrons.

[00:15:34]The magnetron is a weapon? Overall, microwave radiation has been considered a promising weapon for many decades, and the magnetron isn't the only candidate in this regard, since there are many other powerful generators of ultra-high frequency radiation besides it. If we consider a classic microwave oven magnetron, even though it has an impressive power output of several hundred watts, it only works effectively at short distances, up to a few dozen centimeters. At greater distances, from several to dozens of meters, almost all the energy dissipates into the surrounding space.

[00:16:07]A directional antenna could change the situation, but even with one, a microwave oven magnetron at best can cause interference, but it can't disable electronics at long distances.

[00:16:18]For reference, there actually are microwave sources designed to jam enemy communications over relatively long distances, but even for that, you need pulsed power in the hundreds of megawatts. I'm not even talking about disabling electronic devices or harming personnel at long distances. To make that possible, you need an enormous energy density and incredibly high power. Such devices are completely impractical because they require a very powerful power source.

[00:16:48]Microwave weapons focus not on prolonged operation, but on short pulsed bursts.

[00:16:54]One of the interesting projects is ADS, the American Active Denial System.

[00:16:59]This is a microwave weapon that generates oscillations at about 95 GHz, delivering a short-term shock effect to crowds of people.

[00:17:08]When the ADS system is used, 85% of the microwave radiation is absorbed by the upper layer of the skin. The effect it produces on a potential adversary is described, quoting Wikipedia, as an immediate and highly motivated escape behavior. In fact, this system very quickly heats up the surface layer of the skin. A person experiences pain, shock, and a reflexive urge to immediately escape the affected area.

[00:17:33]The Pentagon conducted about 10,000 certified tests. The pain threshold is reached within 2 seconds of exposure.

[00:17:40]After 5 seconds, the pain becomes unbearable. With longer exposure, burns begin to form on the skin.

[00:17:48]The main problem with microwave weapons is that it is extremely difficult and expensive to concentrate the radiation into a thin beam, like with a laser.

[00:17:56]Even the highest frequency terahertz sources are still far below lasers in terms of frequency.

[00:18:03]For example, a household magnetron with a frequency of 2.45 GHz has a wavelength of 12 cm, while terahertz sources have a wavelength of a fraction of a millimeter, whereas a typical visible laser has a wavelength of hundreds of nanometers.

[00:18:16]That's why a laser can be focused into a much finer spot. That's why there are range limitations when using a magnetron as a weapon. Conclusion, the magnetron is a relic of the past that is still widely used today. During the vacuum tube era, the magnetron was one of the most efficient ways to generate powerful ultra-high frequency radiation from hundreds of megahertz to hundreds of gigahertz.

[00:18:39]It became widespread precisely because of its compact size, high efficiency, low cost, and simple design.

[00:18:46]It can operate at high power for long periods, have a relatively long service life, and are not sensitive to the parameters of the filament and anode voltages.

[00:18:55]In radar, they were once indispensable, but times are changing. More efficient broadband generators of ultra-high frequency radiation with flexible control are being developed. The magnetron is probably the last mass-produced vacuum tube that is still in demand where a cheap and powerful microwave source is needed.

[00:19:14]The most common device in this regard is the ordinary household microwave oven.

[00:19:19]Well, friends, I hope I didn't tire you out too much and that the video is useful or at least enjoyable to watch.

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