The working principle of electron gun（HCD）
One.An overview of the electron gun
Electron gun is the accelerator of electronic injector, it emits a certain energy, must have strong stream, a beam diameter and emission Angle of electron beam  (the direction and strength of the electron beam can be controlled, usually by hot cathode anode, etc), control electrode and several accelerated into accelerating tube, used to provide electron accelerator beam electron gun is generally divided into two kinds of thermal emission and field emission; The function of the electron gun is to give the required electron beam, and the material and process structure of the electron gun must consider the convenience of processing and maintenance. In general, the design of an electron gun should consider the following aspects:
1. The injected electrons have a certain amount of energy, and the structure of the gun should have enough compressive strength to withstand a certain acceleration voltage.
2. It should have enough transmitting capacity to give sufficient pulse current.
3. Beam diameter and emission Angle of electron beam should be within a given range.
4. Simple structure, easy processing, installation and maintenance.
Guns have a long service life.
Two.The basic parameters of an electron gun
In the discussion of electron gun, people often use some special terms. Here we give a brief introduction to them:
1. Conductivity coefficient
When the anode current is limited by the space charge, the anode current (emission current) of the electron gun is related to the voltage between the anode and the cathode.
Under the limitation of space charge, regardless of the shape of the electrode system, the law of the third power of two is generally applicable. When the electrode shape is constant, the conductivity coefficient is a constant in general, independent of temperature. The conductivity coefficient indicates the ability of electron gun to emit electrons. In other words, the conductivity coefficient is a measure of the electron beam intensity, which represents the size of the electron beam space charge.
2. Beam waist
In the axisymmetric convergent electron gun, the electrons are emitted from the cathode, and under the action of the electrostatic field formed by the space charge of the electrodes and the electrons themselves in the gun, a certain shape of electron beam is formed.The area with the smallest section radius in the electron beam is called the beam waist.
3. Area compression ratio
Area compression ratio refers to the ratio between the area of the cathode and the area of the cross section of the waist, as well as the ratio between the average current density of the waist and the average current density of the cathode emission. For convenience, the area of the cathode ball is usually replaced by the area of the cathode cross section.
4. Shot range
Shot Range represents the distance between the anode head of the gun and the injection waist formed by the gun. In general, it is desirable that the range be large enough for the electron beam to enter the high frequency field under optimal injection conditions.
The so-called laminar flow of electronic injection is only a qualitative concept, and it is usually judged by whether the trajectories of electronic injection cross or cross severity. The electron beam with good laminar flow can be focused with a lower magnetic field, and the defocus of the electron beam caused by high frequency field is smaller. On the contrary, the electronic injection with poor laminar flow will result in poor flow rate and larger defocus.
Three.The structure of the electron gun
Regardless of the type of electron gun, they are composed of the electron emitter - the cathode, the shape of the electron beam restrictions - the focus of the electron and the acceleration of the lead - the anode three parts. The structure of an electron gun used in different environments may vary, but the basic components remain the same. In operation, the potential of the focusing electrode is usually equal to or close to the cathode potential to limit the shape of the electron beam, and an accelerating voltage (anode voltage) is added between the cathode and the anode. When the electron is emitted from the cathode, it will interact with the electrostatic field established by the space charge of the electrode and the electron beam itself to form an electron beam with a certain shape, which will be ejected from the anode hole for use. The electron gun works like a diode, so people call it a diode gun.
The most common type of electron gun is the two-electrode Pierce type. The structure is shown in figure 2.1-2. It is mainly composed of cathode, focusing pole and anode. There is an opening in the middle of the anode through which electrons are injected into the accelerator chamber. Pierce type electron gun, also known as axisymmetric spherical convergence injection Pierce electron gun, it is to take two concentric spherical part of the formation of a diode. In order to make the electron trajectory along the direction of the radius of cathode curvature, the edge of the spherical electrode needs to be modified.
Four.The cathode of an electron gun
The cathode is one of the key parts of the electron gun. At present, the cathode of the electron gun used in the electron linear accelerator has various forms, which can be divided into two methods:
Direct - heating cathodes mostly use pure tungsten as cathode material. Interthermal cathodes generally use thorium-tungsten, oxides, scandium salt and lanthanum hexaboride cathodes, which can be divided into two types: bombardment type and heating type.
1. Bombarding type: the heating method is to add hundreds or even thousands of volts to the bombarding voltage between the theron (filament) and the cathode. Under this voltage, the electrons emitted from the theron bombarding the cathode will emit a large number of electrons from its surface after the cathode is heated to a certain temperature.
2. Heating type: this cathode, the compound layer is fixed on a thin wall base (nickel tube or molybdenum tube), under the bottom of the base is placed the heat insulation of spiral tungsten wire. The current flows through the filament, which heats the cathode. When the cathode reaches the temperature at which it emits electrons, it emits them.
The material of cathode and its working temperature have a decisive effect on the emission ability and life of electron gun. The cathode must be made of materials with low work - out. The outer electrons of the atoms on the surface of the cathode, excited by a certain amount of heat or electric energy, will jump out of the bondage of the orbit and become free electrons.
The filament cathode of heat emission type electron gun is made with tungsten wire commonly, must rely on electric current to heat filament to 1000 degrees of above, filament emission current density and filament temperature and the energy of filament material escape are concerned.
Take the tungsten wire temperature as an example, its escape work is 4.55 electron volts, at the working temperature of 2500K, J=0.5 ampere/cm 2.
The filament temperature has a great effect on the emission intensity of electrons. If the cathode material with less work of escape is adopted, the filament temperature can be greatly reduced under the condition of obtaining the same emission intensity. In order to prolong the cathode life as long as possible, the material is required to have higher melting point and smaller evaporation rate, and it is not easy to be poisoned by air erosion. The melting point of tungsten wire is 3655K, when the working temperature is 2750K, the evaporation rate is 0.0043 mg/cm • second, the tungsten wire has a strong corrosion resistance. The release work of oxide cathode is lower, for example, the release work of barium oxide is only 2.8 electron volts, but its corrosion resistance is poor, generally only suitable for 10-5-10-6 MMHG under the high vacuum, at 10-4 MMHG, its emission power decreased significantly, at 10-3 MMHG, even serious poisoning, can not continue to use.
The field emission gun requires a strong electric field of more than 106 volts/cm near the cathode surface, so improving the field intensity on the cathode surface is an effective way to increase the emission intensity.
Generally speaking, the current intensity of electron gun is always above 1 milliampere, pulse electron flow can reach ampere level, and its life is over 100 hours.
Five.working principle of electron gun
1. Working principle of diode gun
The physical processes occurring near the cathode of an electron gun are very similar to those occurring in an electron diode. It is known that the flow of current in a diode is realized by the movement of electrons emitted by the cathode. If a positive voltage of Ua was added between the cathode and anode of the diode, the cathode was heated gradually (Uf heating voltage of the filament was gradually increased), and the corresponding anode current Ia was recorded. A Ia/Uf relation curve could be obtained. Change Ua could get another relation curve of Ia/Uf When Uf is low, that is, when the cathode temperature is low, the anode current Ia increases rapidly with the increase of heating voltage.When Uf exceeds a certain value, anode current Ia does not increase with the increase of filament heating voltage Uf. We know that at a certain cathode temperature, the cathode has a certain emission current, and the higher the cathode temperature is, the larger the emission current will be. When the cathode temperature is high enough, the cathode temperature continues to increase (at this time, the emission current of the cathode is still increasing) while the anode current remains unchanged, which indicates that the current emitted by the cathode does not reach the anode completely at this time. If we look at the curves corresponding to different Ua, the situation is the same. As a positive voltage Ua is added between the anode and the cathode of the diode, a certain electric field distribution will be formed between the anode and the cathode. When the cathode is not heated, the electric field distribution is stable. When the cathode was heated, the cathode began to emit electrons, which flew to the anode under the action of Ua.Due to the presence of space charge, the potential at each point between the anode and the cathode will decrease. When the cathode temperature is not too high and the emission of electrons is not too much, there is still an acceleration field between the anode and the cathode, and electrons can reach the anode under the action of this electric field. With the increase of electrons emitted from the cathode, the change of potential gradient on the surface of the anode and the cathode is different. Because the electrons on the cathode surface are not only attracted by the anode accelerating electric field, but also repelled by the front space charge, the force on the electrons is smaller than that without space charge, that is, the potential gradient decreases. In addition to being attracted by the anode acceleration field, the electrons on the anode surface are also pushed by the space charge behind, so the potential gradient increases. If the temperature of the cathode continues to increase, the density of the space charge will continue to increase. Due to the effect of the space charge, the potential gradient on the cathode surface will drop to zero. At this time, the electric field generated by the space charge on the cathode surface is exactly equal to the acceleration field. It is known that electrons must have a certain initial velocity to escape from the cathode. Even if the cathode surface potential gradient drops to zero and continues to increase the cathode temperature, the space charge density can still increase. At this time, the cathode surface potential gradient becomes negative. In other words, the acceleration field generated by the space charge near the cathode is greater than that generated by Ua at the anode voltage.
Electrons emitted from the cathode have different initial velocities. Only the electrons whose initial kinetic energy is greater than the minimum potential energy can overcome the negative electric field near the cathode surface and cross the minimum potential, enter the accelerating field and fly to the anode. The electrons, which had less initial kinetic energy, returned to the cathode with the space charge. At equilibrium, the number of electrons per unit time running to the anode plus the number of electrons returning to the cathode equals the number of electrons emitted into space by the cathode. If the cathode temperature continues to increase, the cathode emission will increase and the space charge density will also increase, which is equal to strengthening the negative electric field near the surface of the cathode. The number of electrons returned to the cathode by the negative potential will increase, while the number of electrons going to the anode will increase slightly.
When the heating temperature of the cathode is low, all the electrons emitted from the cathode can reach the anode. At this time, the anode current depends on the emission temperature of the cathode, which is called temperature limitation. In this case, the cathode heating temperature has a great influence on the emission current. When the heating voltage continues to increase, the space charge effect plays a major role, and the anode current is limited by the space charge. The electron gun of the accelerator mainly works under the condition of space charge limitation.
2. Working principle of Pierce type electron gun
The optical system of this kind of electron gun mainly includes cathode, anode and focusing pole. Some of them are equipped with gate control pole. The potential of the focusing pole is usually equal to or close to the cathode potential. The cathode and the anode form a diode. The cathode is heated and baked by the heater (filament). The heater is powered by an ac power supply. The electron beam is driven by the focusing pole towards the anode hole and finally enters the acceleration system through the anode hole.
3. Grid-controlled gun working principle
With the improvement of accelerator tube, the technique of low pressure injection becomes possible. At the same time, current medical accelerators, according to the needs of radiotherapy, require to change the injection current to meet the requirements of both X-ray and electronic line. In this way, the anode voltage of the gun can be reduced to 7-15kv, and the injection flow can be varied in the range of 200-1000ma. The injection energy can be gradually reduced to a very low value at the time of electron line exit (the beam intensity required by medical accelerator in X-ray treatment and electron line treatment differs greatly by more than 100 times). The solution of low pressure injection technology lays a foundation for the use of grid-controlled gun. Low-voltage gun can greatly reduce the size of the electron gun, reduce the electrode insulation porcelain pressure requirements, reduce the ion back cathode energy, more importantly, the power supply volume, weight and efficiency can be greatly improved.For the design of the gate gun, people usually add a control pole (gate pole) on the basis of the design of the diode gun. When a small negative voltage (cut-off bias - Egc) is applied to the cathode by the grid, the cathode emission is cut off. This is equivalent to stopping the emission in the gap period of the pulse, and the control pole of the duration of the pulse adds zero or a small positive voltage to the cathode to make the cathode emit electrons. By adjusting the positive voltage, the control of the electron injection flow can be achieved. The anode to cathode voltage, however, can always be added to a stable dc high voltage. Obviously, the amplitude stability of dc power supply voltage is much easier than that of high voltage pulse modulator.It also reduces the pressure on the power supply design. People adding control cabinet, usually have three forms, their structure has been given in the first part of their way to work, now are briefly as follows: the first hole gate gun: it is a diode in structure design of the gun is focused on the pole and the cathode insulating, appropriate modification is focused on the design of the pole, can make it in relatively cathode and the absolute value of the bias (eog) try to lower case, realize electronic note. General experience is that when P < 0.5, gun compression is small, | -- Ego| / Va can be less than 25% of the case, the realization of the hole gate gun design. The second type of needle grid gun: it is placed in the center of the cathode and the cathode insulation and perpendicular to the cathode surface of a small needle, the needle as the control pole, its cut-off voltage, can do or slightly lower than the level of the hole grid gun.However, as for the needle gate gun, its cathode and needle gate are more complex in structure design, which are rarely used in China at present. However, Russia is more mature in this aspect, and they can cover the needle gate with a layer of anti-emission substance to reduce the gate emission. However, it remains to be proved whether such control pole is feasible for accelerators with strong ion anti-detonation. The third type of grid gun, which is in the diode gun from the cathode 1% to 3% of the equipotential surface set a grid, when the grid to the cathode and equivalent to the net in the plane of the potential, does not change the original diode gun potential distribution. The cut-off bias of the gate can be designed very low. This will be beneficial to the fabrication of grid-controlled power supply.
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