Magnetron Microwave | Construction and Applications | It’s 4+ Important Phases

Magnetron Microwave

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Points of Discussion: Magnetron Microwave

Introduction to Magnetic Microwave | What is Magnetron?

A magnetron is a kind of Microwave Tube. Before discussing magnetron and its related topics, let us find out some of the basic definitions.

Microwave Tubes: Microwave tubes are devices which generate microwaves. They are the electron guns which produces linear beam tubes.

Now, the definition of Magnetron is given as –

Magnetron: Magnetron is a type of vacuum tube which generates signals of the microwave frequency range, with the help of interactions of a magnetic field and electron beams.

Magnetron tube consumes high-power, and its frequency depends on the physical dimension of the tubes’ cavities. There is a primary difference between a Magnetron and other types of Microwave Tubes. A magnetron works only as an Oscillator but not an amplifier, but a Klystron (a Microwave Tube) can work as an amplifier and as an Oscillator.

Magnetron Microwave
A typical Magnetron Microwave, Image Credit: HCRS Home Labor Page, Magnetron1CC BY-SA 2.0 AT

A Brief History of Magnetron Microwave

The Siemens Corporation developed the very first magnetron in the year 1910 with the guidance from scientist Hans Gerdien. Swiss physicist Heinrich Greinacher finds out the idea of electrons’ motion in the crossed electric and magnetic field from his own failed experiments of calculation of the mass of electrons. He developed the mathematical model around the year 1912.

In the United States, Albert Hull started working to control electrons’ motions using a magnetic field rather than using the conventional electrostatic field. The experiment was initiated to bypass the patent of ‘triode’ of Western’s Electric.

Hull developed a device almost like a Magnetron, but it had no intention to generate signals of microwave frequencies. Czech physicist August Žáček and German physicist Erich Habann independently discovered that Magnetron could generate signals having frequencies of Microwave range.

The invention and increased popularity of RADAR increased the demand for devices which can produce microwave at shorter wavelengths.

In the year 1940, Sir John Randall and Harry Boot of University of Birmingham developed a working prototype of a cavity magnetron. In the beginning, the device produced around 400 Watts of power. Further development like water cooling and several other improvements hiked the produced power from 400 W to 1 kW and then up to 25 kW.

There was a problem related to the frequency instability in the magnetron developed by British scientists. In 1941, James Sayers solved that problem.

Magnetron Microwave
The cavity magnetron developed by Sir John Randall and Harry Boot of University of Birmingham, Magnetron Microwave, Image Credit: Elektrik FanneR&B MagnetronCC BY-SA 4.0

Applications of Magnetron

A magnetron is a beneficial device, has several applications in various fields. Let us discuss some of them.

  • Magnetrons in Radar: The use of Magnetron for a Radar used to generate short pulses of high-power Microwave frequencies. A magnetron’s waveguide is attached with any of the antennae inside a Radar.
    • There are several factors of Magnetron which causes complexity to the Radar. One of them is the problem related to the frequency instability. This factor generates the problem of frequency shifts.
    • The second characteristics are that a magnetron produces signals with the power of broader bandwidth. So, the receiver should have a broader bandwidth to accept them. Now, having a wider bandwidth, the receiver also receives some sort of noise which is not desired.
Magnetron Microwave
An early commercial radar for airport, Magnetron Microwave, Image by: Unknown authorUnknown author, Magnetron radar assembly 1947, marked as public domain, more details on Wikimedia Commons
  • Magnetron Heating | Magnetron Microwave Ovens: Magnetrons are used to generate microwaves that are further used for heating. Inside a microwave oven, at first, the magnetron produces the microwave signals. Then, the waveguide transmits the signals to an RF transparent port into the food chamber. The chamber is of a fixed dimension, and also close to the magnetron.  That is why standing wave patterns are randomized by the revolving motor, which rotates the food inside the chamber.
Magnetron Microwave
Microwave Oven, Magnetron Microwave applications, Image Credit: The original uploader was 吉恩 at Chinese Wikipedia., WeiboluCC BY-SA 3.0
  • Magnetron Lighting: There are plenty of devices available which lights up using the Magnetron excitation. Devices like the sulfur lamp is a prime example of such light. Inside the devices, magnetron generates the microwave field, which is carried out by a waveguide. Then the signal is passed through the light-emitting cavity. These types of devices are complex. Nowadays, they are not used instead of more superficial elements like Gallium Nitride (GaN), or HEMTs are used.

Construction of Magnetron

In this section, we will discuss the physical construction and components of a Magnetron.

The magnetron is grouped as a diode as it is deployed on grid. The anode of the magnetron is set into a cylindrical shaped block which is made up of copper. There are filaments with filament lead and the cathode at the centre of the tube—the filaments-leads help keep the cathode and filament attached with it at the centre. The cathode is made up of high-emission material, and it is heated for the operation.

Magnetron Microwave
A Magnetron with its parts, Magnetron Microwave, Image Credit: HCRS Home Labor Page, Magnetron2CC BY-SA 2.0 AT

The tube has 8 to 20 resonant cavities which are cylindrical holes around its circumference. The internal structure is divided into several parts: the number of cavities present in the tube. The division of tube is done by the narrow slots connecting the cavities to the centre.

Each cavity functions like a parallel resonant circuit where the anode copper block’s far-wall works as an inductor. The vane tip region is considered the capacitor. Now, the resonant frequency of the circuit is dependent on the physical dimensions of the resonator circuit.  

It is evident that if a resonant cavity starts oscillation, it excites other resonant cavities and they start oscillation too. But there is one property that every cavity follows. If a cavity starts oscillation, the next cavity starts oscillation with 180 degrees delay in phase. This applies to every cavity. Now, the series of oscillation creates a slow-wave structure which is self-contained. That is why this type of Magnetron construction is also known as “Multi-Cavity Travelling Wave Magnetron”.

Magnetron Microwave
Central cathode in the middle of the magnetron microwave, Image Credit: Pingu Is SumerianMagnetron section transverse to axisCC BY-SA 3.0

The cathode supplies the electrons necessary for the energy transfer mechanism. As mentioned earlier, the cathode is in the centre of the tube, further set up by the filament leads. There is a particular open space between the cathode and anode which needs to be maintained; otherwise, it will cause malfunction to the device.

There are four types of cavity arrangement available. They are –

  • Slot-type
  • Vane-type
  • Rising Sun type
  • Hole and slot type

Operation of a Magnetron Microwave

Magnetron goes under some phases to generate signals of microwave frequency ranges. The phases are listed below.

Though the name of the phases is indicative enough to let us discuss the incidents, those occur in each phase.

Phase 1: Electron Beam generation and acceleration

The cathode inside the cavity posses the negative polarity of the voltage. The anode is kept in a radial direction from the cathode. Now, indirect heating of cathode causes the flow of electron towards the anode. At the time of generation, there is no magnetic field present in the cavity. But after the generation of the electron, a weak magnetic field bends the path of the electrons. The path of the electron gets a sharp bend if the strength of the magnetic field increases further. Now, if the velocity of the electrons gets increased, the bend becomes sharper again.

Phase 2: Velocity control and changes of Electron beam

This phase occurs inside the ac field of the cavity. The AC field is located from adjacent anode segments to the cathode region. This field accelerates the flow of the electron beam, which is flowing towards the anode segments. The electrons which flow toward the segments gets slowed down.

Phase 3: Generation of “Space Charge Wheel”

The flows of electrons in two different directions with separate velocities causes a motion known as “space charge wheel”. This helps increase the electrons’ concentration, which further delivers enough power for the radio frequency oscillations.

Phase 4: Transformation of energy

Now, after the generation of the electron beam and its acceleration, the field acquires energies. The electrons also dispense some energy to the field. While travelling from cathode electrons dispenses energy at every cavity it passes through. Loss in energy causes a decrease in speed and eventually deceleration. Now, this happens multiple times. The released energy is efficiently used, and up to 80% efficiency is reached.

Health Related Concerns from Magnetron Microwave

A magnetron microwave produces microwave signals which may cause issue to human bodies. Some magnetrons consist of thorium in their filament, which is a radioactive element and not good for humans. Elements like beryllium oxides and insulators made with ceramics are also dangerous if they are crushed and inhaled. This can affect the lungs.

There are also chances of damages from overheating of magnetron microwave ovens. Magnetrons require high voltage power supplies. So, there is a chance of electrical hazards as well.

About Sudipta Roy

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