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Microwave energy is generated by microwave generator, which consists of microwave tube and microwave tube power supply. The function of microwave tube power supply (power supply or microwave source for short) is to change the commonly used AC energy into DC energy, creating conditions for the work of microwave tubes. Microwave tube is the core of microwave generator, which converts DC electric energy into microwave energy.

Microwave tubes include microwave transistors and microwave electron tubes. The output power of microwave transistor is small, which is generally used in measurement, communication and other fields. There are many kinds of microwave electron tubes, such as magnetron, klystron, traveling wave tube, etc. They have different working principles, structures and performances, and are widely used in radar, navigation, communication, electronic countermeasure, heating, scientific research and so on. Due to its simple structure, high efficiency, low operating voltage, simple power supply and strong ability to adapt to load changes, magnetron is especially suitable for microwave heating and other applications of microwave energy. Magnetrons can be divided into pulse magnetrons and continuous wave magnetrons due to different working states. Microwave heating equipment mainly works in continuous wave state, so continuous wave magnetron is often used.

Magnetron is an electric vacuum device used to generate microwave energy. It is essentially a diode placed in a constant magnetic field. Under the control of mutually perpendicular constant magnetic field and constant electric field, the electrons in the tube interact with the high-frequency electromagnetic field to convert the energy obtained from the constant electric field into microwave energy, so as to achieve the purpose of generating microwave energy.

There are many kinds of magnetrons. Our industrial microwave equipment uses multi cavity continuous wave magnetrons. Magnetron is composed of tube core and magnetic steel (or electromagnet). The structure of the tube core includes anode, cathode, energy output device and magnetic circuit system. High vacuum is maintained inside the tube. The following describes the structure and functions of each part.

1 Anode

Anode is one of the main components of magnetron. It and cathode together constitute the space where electrons interact with high-frequency electromagnetic fields. Under the action of constant magnetic field and constant electric field, electrons complete the task of energy conversion in this space. The anode of magnetron not only collects electrons like the anode of ordinary diode, but also plays a decisive role in the oscillation frequency of high-frequency electromagnetic field. The anode is made of metal materials with good conductivity (such as oxygen free copper) and is equipped with multiple resonators. The number of resonators must be even. The higher the working frequency of the tube, the more the number of resonators. The anode resonators are usually hole slot, sector and slot fan. Each small resonator on the anode is equivalent to a parallel 2C oscillation circuit. Taking the slot sector cavity as an example, it can be considered that the slot part of the cavity mainly constitutes the capacitance of the oscillation circuit, while the sector part mainly constitutes the inductance of the oscillation circuit. According to the theory of microwave technology, the resonant frequency of the cavity is inversely proportional to the geometric size of the cavity. The larger the cavity, the lower the operating frequency. Therefore, we can estimate the operating frequency band of the cavity according to its size. The anode of magnetron is coupled by many resonators to form a complex resonant system. The resonant frequency of this system mainly depends on the resonant frequency of each small resonant cavity. We can also estimate the working frequency band of magnetron according to the size of the small resonant cavity. The anode resonance system of magnetron can not only produce the required electromagnetic oscillation, but also produce a variety of electromagnetic oscillations with different characteristics. In order to make the magnetron work stably in the required mode, a "spacer strip" is often used to isolate the interference mode The spacer belt 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 also have a certain amount of energy, these electrons are connected to the anode to increase the anode temperature. The more electrons collected by the anode (i.e., 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 capacity Generally, the power tube adopts forced air cooling, and the anode is equipped with a heat sink High power tubes are mostly water-cooled, and there is a cooling water jacket on the anode.

2 Cathode and its lead

The cathode of magnetron is not only the emitter of electrons, but also an integral part of the interaction space. The performance of cathode has a great impact on the working characteristics and service life of the tube, and is regarded as the heart of the whole tube. There are many kinds of cathodes with different performances. Direct heated cathode is commonly used in continuous wave magnetron. It is wound by tungsten wire or pure tungsten wire into a spiral shape. It has the ability to emit electrons when heated to the specified temperature by current. This kind of cathode has the advantages of short heating time and strong resistance to electron bombardment, and has been widely used in continuous wave magnetron. The cathode heating current is large, so the cathode lead shall be short and thick, and the connecting part shall be in good contact. The cathode leads of high-power tubes work at high temperature, so forced air cooling is often used to dissipate heat. The cathode of magnetron is connected to negative high voltage during operation, so the lead part shall have good insulation performance and meet the requirements of vacuum sealing. In order to prevent the anode from overheating due to electron flashback, the cathode current shall be reduced as required after the magnetron works stably to prolong 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 through the microwave without loss and breakdown, so as to ensure the vacuum sealing of the pipe. At the same time, it should be easy to connect with the external system. Low power CW magnetrons mostly use coaxial output at the place where the high-frequency magnetic field in the anode cavity is the strongest. A coupling ring is placed. When the magnetic flux passing through the ring surface changes, high-frequency induced current will be generated on the ring, thus leading high-frequency power to the outside of the ring. The larger the area of the coupling ring, the stronger the coupling. High power continuous wave magnetrons are commonly used as axial energy output devices. The output antenna is connected to the anode fin through the pole shoe hole. The antenna is generally made into a strip or a round rod, or 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 that microwave energy passes through without loss and has good vacuum tightness. Forced air cooling is often used in the output window of high-power tubes to reduce the heat generated by dielectric loss.

4 Magnetic circuit system

Magnetron requires a strong constant magnetic field when it works normally, and its magnetic field induction intensity is generally thousands of Gauss. The higher the operating frequency, the stronger the applied magnetic field. The magnetic circuit system of 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. The magnetic steel and the tube core are firmly integrated to form the so-called packaging type. High power tubes use electromagnets to generate magnetic fields. The tube core and electromagnets 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 works, it can easily adjust the output power and working frequency by changing the magnetic field strength. In addition, the anode current can be fed into the electromagnetic wire package to improve the stability of the tube.

5 Correct use of magnetron

Magnetron is the heart of microwave application equipment. Therefore, the correct use of magnetron is a necessary condition to maintain the normal operation of microwave equipment. The magnetron shall pay attention to the following problems during use:

1、 Load to match.

No matter what equipment, the output load of magnetron should 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 has a large reflected power, which reduces the actual power of the treated material, but also causes mode jump of magnetron and overheating of cathode, and even damages the tube in serious cases. The anode current suddenly drops during mode tripping. Apart from the small mode separation of the pipe itself, the main reasons for mold jumping are as follows:

(1) The internal resistance of the power supply is too large, and the no-load is high, causing non π mode.

(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, but cannot maintain the normal π mode oscillation.

(3) Insufficient heating of filament leads to insufficient emission, or insufficient emission due to cathode poisoning caused by gas discharge in the tube, which cannot provide the tube current required for π mode oscillation. In order to avoid mode skipping, the internal resistance of the power supply shall not be too large, the load shall be matched, and the filament heating current shall meet the requirements of the manual.

2、 Cooling.

Cooling is one of the conditions to ensure the normal operation of magnetron. The anode of high-power magnetron is usually water-cooled. The cathode filament leading out part and output ceramic window are forced air-cooled at the same time. Some electromagnets are also air-cooled or water-cooled. Poor cooling will overheat the pipe and make it unable to work normally. In serious cases, the pipe will be burnt out. It is forbidden to work under the condition of insufficient cooling.

3、 Adjust cathode heating power reasonably.

After the magnetron starts to vibrate, the cathode temperature will be increased due to the adverse electron back detonation of the cathode, which will lead to overheating. The overheating of the cathode will aggravate the evaporation of the material, shorten the service life, and burn out the cathode in serious cases. The way to prevent the cathode from overheating is to adjust and reduce the cathode heating power according to the regulations.

4、 Installation and commissioning.

At present, magnetron in microwave heating equipment is placed on the excitation cavity to directly excite the transmission system. The excitation cavity is not only an energy excitation device, but also a part of the transmission system. Therefore, the performance of the excitation cavity has a great impact on the work of magnetron. The excitation cavity shall be able to effectively transmit the microwave energy generated in the tube to the load. In order 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 impact on the stability of the work. During normal operation, there is a large high-frequency current passing 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 ignition. The depth of the antenna inserted into the excitation cavity directly affects the transmission of energy and the working state of the pipe, and it should be carefully assembled according to the instructions.

5、 Preservation and transportation

The electrode materials of magnetron are oxygen free copper, Kovar, etc., which are easy to oxidize in acid and alkali moisture. Therefore, magnetrons should be kept away from moisture and acid-base atmosphere. Prevent high temperature oxidation. Since the packaged magnetron is equipped with magnetic steel, the magnetic change of the magnetic steel shall be prevented. If it exists, there shall be no ferromagnetic substance within 10cm around the pipe. The pipes shall be put into special anti vibration packaging box during transportation to prevent damage due to vibration impact.