In addition to generating the required electromagnetic oscillations, the magnetron anode resonance system can generate multiple electromagnetic oscillations with different characteristics. In order to stabilize the magnetron in the required mode, the "separation strap" is often used to isolate the interference mode. The spacer strip connects the anode fins one at a time to increase the operating mode and the adjacent interference mode. Frequency interval between. In addition, since the energy exchanged electrons also have a certain amount of energy, these electrons are applied to the anode to raise the anode temperature, the more electrons are collected by the anode (ie, the larger the current), or the higher the energy of the electron (the lower the energy conversion rate) ), the higher the anode temperature, therefore, the anode needs to have good heat dissipation.

Under normal circumstances, the power tube is forced air-cooled, and the anode has a heat sink. The high-power tube is mostly water-cooled, and the anode has a cooling water jacket. Such a cathode heating current is large, and the cathode lead is required to be short and thick, and the connecting portion is in good contact. The cathode lead of the high-power tube has a high temperature during operation, and is often forced to cool by air. When the magnetron is working, the cathode is connected to a negative voltage, so the lead portion should have good insulation properties and can meet the requirements of vacuum sealing. In order to prevent the anode from overheating due to electron re-bounce, the cathode current should be reduced according to the regulations after the magnetron is stable to prolong the service life.

The cathode of the magnetron, the electron emitter, is an integral part of the interaction space. The performance of the cathode has a great influence on the working characteristics and life of the tube and is considered to be the heart of the entire tube. There are many types of cathodes with different properties. In the continuous wave magnetron, a direct-heating cathode is commonly used, which is wound into a spiral shape by a tungsten wire or a pure tungsten wire, and has the ability to emit electrons after being heated to a predetermined temperature by a current. The cathode has the advantages of short heating time and strong anti-electron bombardment capability, and is widely used in continuous wave magnetrons.

When the magnetron is working normally, a strong constant magnetic field is required, and the magnetic field induction intensity is generally several thousand Gauss. The higher the operating frequency, the stronger the applied magnetic field. The magnetic circuit system of a magnetron is a device that generates a constant magnetic field. The magnetic circuit system is divided into two major categories: permanent magnet and electromagnetic. The permanent magnet system is generally used for a small power tube, and the magnetic steel and the die are firmly combined to form a so-called package type. The high-power tube uses a magnet to generate a magnetic field, and the die and the electromagnet are used together. The upper and lower pole pieces are provided in the die to fix the distance of the magnetic gap. When the magnetron is working, it is convenient to adjust the output power and operating frequency by changing the strength of the magnetic field. In addition, the anode current can be fed into the magnet wire package to improve the stability of the pipe operation.

Correct use of magnetron
The magnetron is the heart of the microwave application equipment. Therefore, the correct use of the magnetron is a necessary condition for maintaining the normal operation of the microwave equipment. The following problems should be noted when using the magnetron:

First, the load should match.

No matter what equipment requires the output load of the magnetron to be matched as much as possible, that is, its voltage standing wave ratio should be as small as possible. The standing wave not only has a large reflection power, but also reduces the actual power obtained by the material to be treated, and causes the magnetron to jump over the mold and the cathode to overheat. In severe cases, the tube is damaged. When the mode is skipped, the anode current suddenly drops. The reason for the mode hopping is that the mode separation of the pipe itself is small, mainly in the following aspects: (1) The internal resistance of the power supply is too large, and the no-load high 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 current, and cannot maintain the normal π mode oscillation. (3) Insufficient heating of the filament, causing insufficient emission, or insufficient emission due to cathode poisoning due to deflation in the tube, and the tube current required for π-mode oscillation cannot be provided. In order to avoid the occurrence of mode hopping, it is required that the internal resistance of the power supply should not be too large, the load should be matched, and the heating current of the filament should meet the requirements of the specification.

Second, cooling.

Cooling is one of the conditions for ensuring the normal operation of the magnetron. The anode of the high-power magnetron is usually water-cooled, and the cathode filament lead-out portion and the output ceramic window are simultaneously forced air-cooled. Some electromagnets are also air-cooled or water-cooled. Poor cooling will overheat the tube and will not work properly. In severe cases, the tube will burn out. It should not be forbidden to work under conditions of insufficient cooling.

Third, reasonable adjustment of cathode heating power.

After the magnetron starts to oscillate, the cathode is overheated due to the unfavorable electron repulsive cathode, and the cathode overheating will exacerbate the evaporation of the material, shorten the service life, and burn the cathode in severe cases. The way to prevent the cathode from overheating is to reduce the cathode heating power according to the regulations.

The microwave energy required to heat and cook food in a microwave oven is generated by the core component, the magnetron. Currently widely used in microwave ovens is a continuous wave forced air-cooled magnetron, the basic structure of which is shown in the figure. Right
It can be seen that the magnetron is composed of a cathode (filament), an anode, a toroidal magnet, a coupling ring, an antenna (ie, a microwave energy output device), a radiator, and a filament plug. The anode is cylindrical, usually made of copper. The plurality of fins divide the anode into a dozen fan-shaped spaces. Each sector is an anode cavity, and its resonant frequency is the operating frequency of the magnetron. Generally it is around 2450MHz. A pair of annular permanent magnets are nested in the outer casing of the anode, and the magnetic field formed by the magnetic steel is used to control the microwave oscillation energy in the anode cavity. The microwave energy output from the anode is transmitted to the antenna through an annular metal tube (ie, a coupling loop), and the microwave energy is sent from the antenna to the furnace to heat the food.

The working voltage of the filament of the microwave oven is generally 3.3V AC and the current is about 10A; the anode (to cathode) voltage is about 4000V DC. When the magnetron is energized, the filament is heated, and a high-voltage electric field is formed between the cathode (filament) and the anode. Under the action of the electric field, the cathode emits electrons to the anode, and the anode receives electrons to generate an anode current. When the electron reaches each sector-shaped anode cavity, it oscillates at its resonant frequency, and the constant magnetic field generated by the toroidal magnet is perpendicular to the direction of the high-voltage electric field. Under the action of the magnetic field, electrons are made along the circumferential space between the cathode and the anode. The balance curve moves to form a rotating electron cloud that accumulates energy and continuously delivers it to the anode, thereby obtaining a stable microwave oscillation energy of about 2.45 billion vibrations per second on the anode. The magnitude of the microwave energy depends mainly on the level of the anode voltage and the strength of the magnetic field. Since the magnetic field strength of the toroidal magnet is constant, the microwave output power is mainly related to the anode voltage. However, if the magnetic steel is broken or the magnetic properties are obviously degraded, the output power of the magnetron will be reduced, the heating effect of the microwave oven will be deteriorated, and the failures such as slow heating and insufficient firepower will occur, and this problem must be paid attention to during maintenance. The dynamic conduction internal resistance of the magnetron is small, and the fluctuation of the anode voltage has a great influence on the microwave output power, which will obviously affect the heating performance of the microwave oven. In order to avoid unstable operation of the microwave oven due to fluctuations in the power supply voltage, the anode voltage of the magnetron is usually provided by a power supply circuit composed of a leakage inductance transformer, which stabilizes the anode current of the magnetron and stabilizes the output power of the microwave oven.

The microwave conversion efficiency of the magnetron is about 70%. The remaining 30% of the power in the working process becomes heat, which is dissipated on the tube. Because of the large power and high temperature rise, the cooling fan is installed in the microwave oven. The magnetron performs forced air cooling to prevent overheating damage.