Addtime： 2020-07-16 Browse times： 2334
Microwave tube needs high vacuum environment: microwave tube is an electrical vacuum device to amplify microwave signal, mainly including five basic functional components: electron gun, slow wave system, bunching system, energy transmission device and collector.
The function of electron gun is to generate electron beam by cathode, then use focusing pole and anode to gather and accelerate the electron beam to form electron beam with certain cross-section shape, size and energy.
The slow wave system is a device used to slow down the propagation speed of electromagnetic wave along the tube. The interaction between the electron beam and the high frequency electromagnetic field can convert the energy of the electron beam into the high frequency microwave energy, so that the microwave tube can complete the signal amplification.
The beam bunching system uses magnetic field to restrain the divergence of electron beam and makes it pass through the slow wave system smoothly. The energy transmission device is the inlet and outlet of the amplified microwave signal.
The collector is used to collect the electron beam current after energy exchange through the slow wave system.
It can be seen that the microwave tube uses the electric field and magnetic field to control the movement of the electron beam in its interior, so that the electron beam can generate, accelerate, move, collect and interact with the signal, so as to achieve the purpose of signal amplification. Therefore, it is absolutely necessary to avoid the collision between electrons and gas molecules, to prove the movement law of electrons in the microwave tube, to stop the cathode oxidation poisoning of emitting hot electrons, and to pump the microwave tube to a certain vacuum degree to ensure the normal operation of the microwave tube. It is required to maintain high vacuum or ultra-high vacuum during the whole life of microwave tube. The good vacuum affects the reliability of microwave tube directly.
In order to obtain the high vacuum environment and maintain the vacuum degree in the microwave tube, the exhaust process is used to extract the gas in the tube to obtain a certain vacuum degree. Due to the complex structure of microwave tube and various parts, there are more than 100 kinds of vacuum gas tight welding seams, generally dozens of welding seams. Therefore, the exhaust gas needs to be heated to a high temperature to conduct outgassing treatment on metal parts and welding joints. In this way, in the working state of microwave tube, the temperature of internal parts will be high, and there will be no outgassing, so that the high vacuum degree can still be ensured in the working state.
The common exhaust systems are oil diffusion pump exhaust system and turbine molecular pump exhaust system, which are mainly composed of the front stage pump (mechanical pump, roots pump, etc.) for rough pumping. Then, the main pump (oil diffusion pump, molecular pump, etc.) is used to configure necessary vacuum pipeline, certain high vacuum valve and required vacuum gauge (resistance vacuum gauge, ionization vacuum gauge, B-A vacuum meter, etc.) to pump the vacuum to a high level Vacuum. When the oil diffusion pump is used, an oil trap should be added to prevent the pollution of the pump oil on the internal system of the vacuum device.
After the exhaust, the vacuum can reach 106pa, which can meet the requirements of microwave tube. In order to improve the vacuum degree of microwave tube during exhaust and reduce the amount of exhaust gas in the following process, we use the ion pump to exhaust the microwave tube, and the vacuum degree can reach 2 x 10-7 Pa. after the exhaust, the microwave tube and the exhaust system are cold sealed off. During the sealing process, the metal parts are deflated by mechanical extrusion. At the same time, after the microwave tube is placed for a period of time, the vacuum degree can reach 2x10-7pa In high vacuum, the internal material will deflate, thus reducing the vacuum in the tube. At this time, we use a small ion pump (commonly known as titanium pump) to ensure the internal vacuum. At the same time, in the process of microwave tube aging and testing, the parts in the tube will produce outgassing after high-speed electron beam bombardment, which needs titanium pump to remove. The current of the titanium pump can also detect the vacuum degree inside the microwave tube.
After the microwave tube is aged and tested, the vacuum in the tube will be stable. As the volume of titanium pump itself is relatively large, affecting the overall size of microwave tube, we will generally remove (for the shape of no requirements can not be removed).
However, microwave tube and other vacuum devices generally need a certain long life, and the general index is required to be 3 years (space microwave tube requires 20 years). In this long-term work, the defects of parts (pores, cracks, etc.) and the welding dead space may cause chronic air leakage or deflation, which will damage the internal vacuum. We provide the soft water hardness testing agent for the drainer here and the hardness testing agent prepared by some domestic water softener manufacturers. One of their common characteristics is that they only serve as indicators. That is, it can only qualitatively indicate whether the hardness of the water sample is within a specified value (such as 0.03mmol / L). The author has tested several indicators, and the error is relatively large, one of them is up to 300%. 4 the use of KS soft water hardness tester has the advantages of fast testing speed and high efficiency, and it only takes a few seconds to determine a water sample.
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Under the same exhaust process, the vacuum data of 5 microwave tubes were calculated.
Place getter. Absorb the residual gas in the tube and the gas released from the parts in the tube again to maintain the vacuum in the tube.
It can be seen from the principle of adsorption vacuum pump that we use ion pump, titanium pump and getter in the manufacture of microwave tube. Their specific working principles are as follows.
Ion pump and titanium pump: a honeycomb anode is installed between the two cathode plates, and a magnetic field is applied in the direction perpendicular to the cathode plate and parallel to the honeycomb axis. The electron moves in a spiral motion in the magnetic field line and ionizes with the gas molecules. The electrons generated in the ionization process will continue to separate from other gas molecules. During ionization, the electrons give up energy and gradually reach the anode, while the positive ions are accelerated to reach the negative electrode made of titanium. The interaction between ions and titanium metal can be realized in three ways: ① the ions eject out of the titanium cathode material to deposit the degassing film and adsorb the reaction gases (nitrogen, hydrogen and oxygen); ② the energy of some ions will be large enough to make them buried in the electrode, so as to be absorbed by ion implantation, which is effective for all ions including noble gas and inert gas. ③ The titanium emitted from the cathode can bury the gas atoms on the target electrode and improve the pumping speed of inert gas. The typical working voltage of large ion pump is 7kv, and that of small ion pump (titanium pump) is 3.5kv. The main application of getter in microwave tube is non evaporable getter. Porous sintering structure is formed by using zirconium and zr-v-fe alloy. It has excellent strength and density, and can withstand certain impact and vibration. The microwave tube was activated after normal baking at 400-500 ° C, and its porous structure had a strong absorption capacity for CO2, 2, N2 and water vapor.
The application effect of U4 adsorption vacuum pump on CO2 and H2 adsorption curve of a certain inspiratory ion pump as a high vacuum pump, the ion pump has obvious pumping effect on inert gas which is difficult to be pumped out by molecular pump, and can improve the vacuum degree of microwave tube after exhaust. The figure below shows our comparison of two vacuum systems. The first system is mechanical pump + molecular pump, and the second system is mechanical pump + molecular pump + ion pump.
It can be seen that the application of ion pump can improve the vacuum degree of microwave tube, reduce the amount of outgassing in the following aging and testing processes, and the current drops to almost zero when the vacuum degree is detected by the small titanium pump, which reduces the time of these processes and improves the production efficiency and the reliability of the tube.
In the microwave tube, small ion pump (titanium pump) is added at the discharge pipe and the getter is added at the electron gun. The exhaust system is required to be baked at high temperature to activate the getter. During the exhaust process, the titanium pump is also vented. After cold sealing, the residual gas in the tube and the gas released from the parts in the tube are removed by both the titanium pump and the getter, Maintain the vacuum in the tube. In the follow-up work permit process, the titanium pump current can also help to detect the micro leakage point, which facilitates the production and reduces the scrap rate of microwave tubes. In the microwave tube with added getter, the probability of high-pressure ignition of electron gun is greatly reduced. After long-term storage, the microwave tube is directly electrified without deflation or mass phenomenon.
It can be seen that the vacuum pumps such as ion pump, titanium pump and getter can improve and maintain the high vacuum degree in the production and manufacture of microwave tubes, so as to improve the reliability and service life of microwave tubes.
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