Application and maintenance of electron gun |classification | basic parameter
The electron gun is the device that emits and accelerates the electron beam. It emits an electron beam with a certain energy, a certain current intensity, a certain beam diameter and a certain Angle of emission into the accelerator tube. The electron guns used to provide electron beams for electron accelerators are generally divided into thermal emission and field emission.
A. according to the shape of the electron beam
1. Parallel electron gun
2. Axisymmetric convergent electron gun
3. Planar symmetric convergent electron gun
B.By the number of electrodes
1. Two pole gun
2. Three pole gun
· Basic parameters
1. The conductivity coefficient P
represents the size of the space charge of the electron beam.
2. Emittance epsilon.
Characterizing the quality of electronic injection
3. Brightness B
Beam intensity per unit area and per unit solid Angle
4. Note the waist rm
Rmin is the radius at the smallest section radius in the electron injection
5. Compression ratio M2
The ratio of the cathode area to the cross-sectional area of the cathode beam is also the ratio of the average current density Jm of the waist beam and the average current density Jk of the cathode beam.
6. Gunshot Zmin
Range represents the distance between the anode head of the gun and the beam waist.
· 7. Laminarity
· Application and maintenance
1. Try to operate in a high vacuum degree to reduce ion anti-detonation and oxidation, avoid "poisoning" of cathode, and improve the life of cathode and thermion.
2. The theron current of the electron gun is added to the rating value step by step (the factory has set it). Due to the low temperature of the theron resistance, current, to prevent sudden large current, burn the theron.
3. Reduce the working temperature of the cathode as far as possible under the condition of meeting the dose.
4. The cathode shall be connected with "negative high voltage" to prevent high voltage striking and damaging the heat element.
5. For "grid-controlled gun", the gate negative bias is connected first, then the filament is preheated, and the negative high voltage of the cathode is added after preheating, and finally the gate pulse is connected.
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 in the world has various forms, which can be divided into two methods:
The direct-heated cathode mostly USES pure tungsten as the cathode material, and the heating current passes through the cathode directly. 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), the base under 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.
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