Study On Beam Wave Interaction Of Terahertz Backward Wave Oscillator

In recent years,vacuum electronic devices are gradually developing towards the goal of "five high and one small",that is,high frequency,high power,high gain,high efficiency,high reliability and miniaturization.Especially,the working frequency of the required electromagnetic wave emission source gradually moves towards terahertz frequency band.The traditional solid-state devices are limited by their inherent characteristics.The vacuum electronic devices have attracted more and more attention,mainly including the traveling wave tube,the backward wave oscillator and other electromagnetic wave radiation sources.As a classical vacuum radiation source,backward wave oscillator is also a kind of terahertz radiation source,which is widely used at present.The interaction between the propagated electromagnetic wave and the electron beam in the slow wave structure of the backward wave oscillator is also the focus of the research about backward wave oscillator in recent years.The efficiency of the beam wave interaction directly affects the performance of the backward wave oscillator.At present,many researchers study beam wave interaction through particle simulation by large-scale electromagnetic simulation software.However,particle simulation needs a lot of time to calculate,and the calculation efficiency is very low.Therefore,the theoretical study of beam wave interaction in backward wave oscillator is of great significance for the development of backward wave oscillator.In this paper,the beam wave interaction process of the backward wave oscillator is studied based on the large signal theory of the backward wave oscillator.Firstly,the physical model and mathematical model of the large signal theory of backward wave oscillator are introduced,and the formula of the large signal theory of backward wave oscillator is derived.Then,the program of the large signal theory of backward wave oscillator is written with C++ programming language,and the realization of the large signal theory of backward wave oscillator is carried out.Finally,the 1.03 THz,0.67 THz and 0.22 THz backward wave oscillator are simulated with the current mainstream electromagnetic simulation software.The simulation data and the calculation results of large signal theoretical program of backward wave oscillator are compared and analyzed.The main work and innovations are as follows:1.The physical model of large signal theory of backward wave oscillator is established.This paper introduces the process from the beginning to the stable operation of the backward wave oscillator,and analyzes the characteristics of the stable operation of the backward wave oscillator,which mainly include average power,electric field strength and electron beam state.Finally,it is concluded that studying the beam wave interaction of the backward wave oscillator only needs to be concerned about the beam wave interaction at the steady state of the backward wave oscillator.Therefore,the large signal theory of the backward wave oscillator in this paper is based on the basis that the work of the backward wave oscillator reaches the steady state.2.The mathematical model of large signal of backward wave oscillator is established.The physical model is transformed into a mathematical model.The electron beam is divided into macro particles in an electromagnetic wave period.The length of each slow wave structure period is divided into several spatial steps.The motion of the electron is discussed in the time domain,the change of the field is discussed in the frequency domain,and the relationship between the change of the electron kinetic energy and the intensity of the electric field is obtained.The motion of electron is described by one-dimensional equation of motion of electron.The circuit field can be obtained by the definition of power combined with coupling impedance on the circuit field,and the space charge field can be obtained by combining the large signal theory of Rowe.3.The large signal theory of backward wave oscillator is realized by program.First,the physical model of the large signal of backward wave oscillator is analyzed by the program flow chart,and a self consistent process of solving the electric field intensity is obtained.Then,the program of the large signal theory of backward wave oscillator is obtained by using C++ programming language according to the established mathematical model.4.The backward wave oscillator of 1.03 THz,0.67 THz and 0.22 THz are simulated by CST.The high frequency characteristics of each backward wave oscillator are calculated.The normalized phase velocity,average coupling impedance and loss per unit length of each backward wave oscillator are obtained.CST pic particle simulation is carried out for each backward wave oscillator,and the convergence of simulation grid division is considered.By changing the period number of slow wave structure,the optimal saturation tube length of the backward wave oscillator is obtained.When the length of the backward wave oscillator is the optimal saturation tube length,the voltage is tuned,and the output power of each backward wave oscillator is obtained at different oscillation frequencies.5.The calculation results of large signal program of backward wave oscillator are compared with CST simulation results.It mainly includes the comparison of coupling impedance and average coupling impedance with the same output,the comparison of starting length and starting current of backward wave oscillator,the comparison of output power before and after program second harmonic separation,the influence of space charge field on output power,etc.Through detailed analysis,the output power of the 1.03 THz folded waveguide backward wave oscillator calculated by the large signal program of the backward wave oscillator differs from the CST simulation result by 24%-51% under the optimal saturation tube length.The 0.67 THz grid-loaded rectangular waveguide backward wave oscillator is 21 times the output power of the CST simulation under the optimal saturation tube length.The 0.22 THz folded waveguide backward wave oscillator differs from the CST simulation result by 0.51%-38%.The difference of output power before and after the harmonic separation of the large signal program of the backward wave oscillator is about 20-30%,and the output power will be reduced after the harmonic separation.The difference in output power before and after the space charge field is added is about 5%-10%,and the output power will be increased after the space charge field is added.