Basic structure, parameters and correct use of microwave magnetron

JasonYoo

Basic structure, parameters and correct use of microwave magnetron [Copy link]

Basic structure, parameters and correct use of microwave magnetrons
　
Contents
0 Overview
1 Anode
2 Cathode and its leads
3 Energy output device
4 Magnetic circuit system
5 Correct use of magnetrons
　
0 Overview
Microwave energy is generated by a microwave generator, which includes two parts: a microwave tube and a microwave tube power supply.
The function of the microwave tube power supply (abbreviated as power supply or microwave source) is to
convert the commonly used AC power into DC power, creating conditions for the operation of the microwave tube. The microwave tube is the core of the microwave generator,
which converts DC power into microwave energy.
Microwave tubes are divided into two categories: microwave transistors and microwave electron tubes. Microwave transistors have low output power and are generally used in
measurement and communication. There are many types of microwave electron tubes, and commonly used ones include
magnetrons, klystrons, traveling wave tubes, etc. They have different working principles, structures, and performances, and
are widely used in radar, navigation, communication, electronic countermeasures and heating, scientific research, etc.
Due to the simple structure, high efficiency, low operating voltage, simple power supply, and strong ability to adapt to load changes, magnetrons
are particularly suitable for microwave heating and other applications of microwave energy.
Magnetrons can be divided into two categories: pulse magnetrons and continuous wave magnetrons due to different working states. Microwave heating equipment mainly
works in a continuous wave state, so continuous wave magnetrons are mostly used.
Magnetron is an electric vacuum device used to generate microwave energy. It is essentially a
diode .
Under the control of mutually perpendicular constant magnetic fields and constant electric fields, the electrons in the tube interact with the high-frequency electromagnetic field, converting the energy obtained from the constant electric field into microwave energy, thereby achieving
the purpose of generating microwave energy.
There are many types of magnetrons, and here we mainly introduce multi-cavity continuous wave magnetrons.
The magnetron consists of a tube core and magnetic steel (or electromagnet). The structure of the tube core includes four parts: anode, cathode, energy output device
and magnetic circuit system. The inside of the tube maintains a high vacuum state.
The structure and function of each part are introduced below.
1 Anode
The anode is one of the main components of the magnetron. Together with the cathode, it forms
a space . Under the action of a constant magnetic field and a constant electric field, electrons
complete the task of energy conversion in this space. In addition to collecting electrons like the anode of an ordinary diode, the anode of a magnetron also plays a decisive role in the oscillation frequency of
a high-frequency electromagnetic field.
The anode is made of a metal material with good conductivity (such as oxygen-free copper) and is provided with multiple resonant cavities. The number of resonant cavities
must be an even number. The higher the operating frequency of the tube, the more cavities there are.
The types of anode resonant cavities are often hole-slot, fan-shaped, and slot-fan-shaped. Each small resonant cavity on the anode is equivalent to a parallel 2C
oscillation circuit. Taking the slot-fan cavity as an example, it can be considered that the slot part of the cavity mainly
constitutes the capacitance of the oscillation circuit, while its fan-shaped part mainly constitutes the inductance of the oscillation circuit. It can be known from the microwave technology theory that the resonant frequency of the
resonant cavity is inversely proportional to the geometric size of the cavity. The larger the cavity
, the lower its operating frequency. Therefore, we can estimate its operating frequency band based on the size of the cavity. The anode of a magnetron
is coupled together by many resonant cavities to form a complex resonant system. The resonant cavity frequency of
this system is mainly determined by the resonant frequency of each small resonant cavity. We can also
estimate the operating frequency band of the magnetron based on the size of the small resonant cavity.
In addition to generating the required electromagnetic oscillations, the anode resonance system of the magnetron can also generate a variety of
electromagnetic . In order to make the magnetron work stably in the required mode, "
isolation tape" is often used to isolate the interference mode. The isolation tape connects the anode fins one by one to increase the
frequency interval between the working mode and the adjacent interference mode.
In addition, since the electrons after energy exchange still have a certain amount of energy, these electrons hit the anode to
increase . The more electrons the anode collects (that is, the greater the current), or
the greater the energy of the electrons (the lower the energy conversion rate), the higher the anode temperature. Therefore, the anode needs to have good heat dissipation capabilities. In general, the power tube
adopts forced air cooling, and the anode has a heat sink. High-power tubes are mostly water-cooled, and there is a cooling water jacket on
the anode .
2 Cathode and its leads
The cathode of the magnetron is the emitter of electrons and a component of the interaction space. The performance of the cathode has a great influence on the working characteristics and life of
the tube , and is regarded as
the heart of the entire tube.
There are many types of cathodes with different performances. Directly heated cathodes are commonly used in continuous wave magnetrons. They are made of tungsten wire or pure tungsten wire
wound into a spiral shape. After being heated to a specified temperature by current, they have the ability to emit
electrons . This cathode has the advantages of short heating time and strong resistance to electron bombardment, and
is widely used in continuous wave magnetrons.
This type of cathode has a large heating current, requiring the cathode lead to be short and thick, and the connection part to have good contact. The
cathode lead of a high-power tube has a very high temperature during operation, and forced air cooling is often used to dissipate heat.
When the magnetron is working, the cathode is connected to negative high voltage, so the lead part should have good insulation performance and meet the requirements of vacuum sealing. In order to
prevent the anode from overheating due to electron back bombardment,
the cathode current should be reduced as required after the magnetron is working stably to extend its service life.
3 Energy output device
The energy output device is a device that transmits the microwave energy generated in the interaction space to the load. The function of the energy output
device is to pass the microwave without loss and breakdown, ensure
the vacuum sealing of the tube, and at the same time facilitate connection with the external system. Most low-power continuous wave magnetrons use coaxial output
at the place where the high-frequency magnetic field of the anode resonant cavity is the strongest. A coupling ring is placed.
When the magnetic flux passing through the ring surface changes, a high-frequency induced current will be generated on the ring, thereby leading the high-frequency power outside the ring.
The larger the area of the coupling ring, the stronger the coupling.
High-power continuous wave magnetrons often use axial energy output devices, and the output antenna is connected to the anode wing through the pole shoe hole
. The antenna is generally made into a strip or round rod, or it can be a cone. The entire antenna
is sealed by the output window.
The output window is usually made of glass or ceramic with low loss characteristics. It does not need to ensure the lossless passage of microwave energy and
have good vacuum tightness. The output window of the high-power tube is often forced air cooling.
To reduce the heat generated by dielectric loss.
4 Magnetic circuit system
When the magnetron works normally, it requires a strong constant magnetic field, and its magnetic field induction intensity is generally thousands of gauss.
The higher the working frequency, the stronger the applied magnetic field. The magnetic circuit system of the magnetron is
a device that generates a constant magnetic field. The magnetic circuit system is divided into two categories: permanent magnet and electromagnetic. The permanent magnet system is generally used for low-power tubes, and the magnetic steel and
the tube core are firmly integrated into a so-called package type. High-power tubes often use
electromagnets to generate magnetic fields. The tube core and the electromagnet are used together. There are upper and lower pole shoes in the tube core to fix the distance of the magnetic gap.
When the magnetron is working, it is very convenient to
adjust . In addition, the anode current can also be fed into the electromagnetic coil to improve
the stability of the tube.
5 Correct use of magnetrons
Magnetrons are the heart of microwave application equipment. Therefore, the correct use of magnetrons is a necessary condition for maintaining the normal operation of microwave equipment
. The following issues should be paid attention to when using magnetrons:
1. The load must be matched.
Regardless of the equipment, the output load of the magnetron is required to be matched as much as possible, that is, its voltage standing wave ratio should be as
small as possible. A large standing wave not only reflects a large power, which reduces the
actual , but also causes the magnetron to jump mode and overheat the cathode, which can damage the tube in severe cases. When the mode jumps,
the anode current suddenly drops. In addition to the small
mode are as follows:
(1) The internal resistance of the power supply is too large, the no-load current is high, and the non-π mode is excited.
(2) The load is seriously mismatched, and the reflection of the unfavorable phase weakens the interaction between the high-frequency field and the electron flow, and normal π mode oscillation cannot
be maintained .
(3) The filament is not heated enough, resulting in insufficient emission, or the cathode is poisoned by gas discharge in the tube, resulting in insufficient emission, and
the tube current required for π mode oscillation cannot be provided.
In order to avoid the occurrence of mode jumping, the internal resistance of the power supply should not be too large, the load should be matched, and the filament heating current should meet the requirements of
the manual
. 2. Cooling.
Cooling is one of the conditions to ensure the normal operation of the magnetron. The anode of a high-power magnetron is usually water-cooled, and the cathode
filament lead-out part and the output ceramic window are forced to be air-cooled at the same time. Some
electromagnets are also air-cooled or water-cooled. Poor cooling will cause the tube to overheat and fail to work normally, and in severe cases, the tube will be burned out. It is
strictly forbidden to work under insufficient cooling conditions.
3. Reasonable adjustment of cathode heating power.
After the magnetron is oscillated, the cathode temperature rises due to the negative electron back-bombardment of the cathode and is in an overheated state. The overheating of the cathode
will intensify the evaporation of the material and shorten the life. In severe cases, the cathode will be burned out. The way to
prevent the cathode from overheating is to adjust and reduce the cathode heating power according to regulations.
4. Installation and debugging.
In the commonly used microwave heating equipment, the magnetron is placed on the excitation cavity to directly excite the transmission system. The excitation cavity is both an
energy excitation device and a part of the transmission system. Therefore
, the performance of the excitation cavity has a great influence on the operation of the magnetron. The excitation cavity should be able to effectively transmit the microwave energy generated in the tube to the load. To achieve this goal, in addition to the design of the excitation cavity itself, the assembly
of the tube on the excitation cavity has a great influence on the stability of the work.
During normal operation, a large
high-frequency current passes through the contact part between the anode of the tube and the excitation cavity. There must be good contact between the two. Poor contact
will cause high-frequency sparking. The depth of the antenna inserted into the excitation cavity directly affects the transmission of energy and the working state of the tube. It should
be carefully assembled according to the instructions.
5. Storage and transportation
The electrode materials of the magnetron are oxygen-free copper, Kovar, etc., which are easily oxidized in acid and alkaline moisture. Therefore,
the storage of the magnetron should be moisture-proof and avoid acid and alkaline atmosphere. Prevent high-temperature oxidation. Packaged
magnetrons should prevent magnetic changes of the magnetic steel because they are with magnetic steel. When there is,
there should be no ferromagnetic material within 10 cm around the tube. During transportation, the tube should be placed in a special vibration-proof packaging box to
prevent damage due to vibration and impact.