| Traveling wave tube is a microwave electronic vacuum device that realizes the microwave amplification function by the interaction of electron beam and microwave.Because of its high output power,high efficiency and wide bandwidth,it is widely used in radar,electronic countermeasures,millimeter wave communication and other fields.The electronic gun is the only device to produce the electron beam in the traveling wave tube.Its performance plays a vital role in the whole traveling wave tube.The cathode emission in the electron gun is the key factor affecting the mass of the electron beam,and the effect of the heat initial velocity in the cathode emission model will seriously affect the shape of the electron beam,especially the effect of the heat initial velocity in the low conductivity electron gun.Therefore,it is necessary to study the thermal muzzle velocity effect of electron gun in simulation design.In this paper,the thermionic emission model of traveling wave tube electron gun is studied.The thermal cathode emission model in the numerical calculation is derived.According to the distribution function of the heat initial velocity and the discrete method in the MICHELLE software,the thermal initial velocity distribution function is normalized and the continuous function is transformed into a discrete point which is convenient for numerical calculation by the method of regional discrete.The cathodes in the traveling wave tube electron gun mostly use the spherical structure,and all the electrons in the grid on each cathode surface in the simulation calculation are represented by a macro particle trajectory,which represents the overall effect of multiple electrons.The electron emission of the triangle emitter surface is regarded as a flat surface for launching.Therefore,the model needs to take into account the effect of the shape.In this paper,the similarity between the motion of electron in the electromagnetic field and the propagation of light in the medium is used to calculate the emission angle under the confinement of the spherical cathode,which is equivalent to the focus lens of the center as the focus lens,and the discrete model of the heat initial velocity in the spherical cathode structure is obtained.The influence of spherical cathode structure electron gun on thermal initial velocity is simulated and analyzed.The influence of the parameters of the electron gun on the effect of the thermal initial velocity is analyzed.It is found that the greater the ratio of the cathode radius to the radius of the cathode,the greater the conductivity,the greater the anode voltage and the smaller the working temperature of the cathode,the smaller the effect of the thermal initial velocity effect in the heat of the electron gun.Through the simulation and analysis of the grid controlled electron gun,it is concluded that the existence of grid will increase the influence of thermal initial velocity effect.In CST software,the energy is divided into uniform distribution and Maxwell distribution.The simulation results of the electronic gun in MTSS software are compared with the two transmission methods,and the reasons for the error are analyzed.The reasons are the difference of grid division,the mode of launching and the angle setting.A method for suppressing thermal initial velocity effect in TWT gun is presented.The discrete model of the thermal initial velocity in the electronic gun is an approximation of the actual physical model.The error is unavoidable.If it can design a small electron gun which has a small influence on the effect of the thermal initial velocity in the case of the design requirements,the agreement between the design and the machining assembly can be improved.According to the previous research,it can be found that the influence of the thermal initial velocity effect can be suppressed by increasing the conductivity coefficient,increasing the anode voltage or increasing the radius ratio of the cathode to the radius of the cathode,and an optimization example of the electron gun is given. |
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