Investigation Of A Microwave Source Based On Cylindrical,Two Dimensional Periodic Surface Lattice Structure

With the development and the increasing demands of high power microwave technology,higher operating frequency and power capacity become important targets for high power microwave sources.For devices working in the high frequency range,their small tranverse sizes impose heavy restrictions on higher power capacity.An effective solution is to enlarge tranverse sizes and use oversized annular electron beams.However,microwave sources based on those overmoded structures often encounter problems like modes competition,complex spectrums,etc.Researches seeking solutions for maintaining mode selection under high frequency and high power conditions have become research hotspots in recent years.Two dimensional periodic surface lattice structure is a novel structure which can maintain mode selection under overmoded conditions,so that it has the potential of high power capacity.Researches into microwave sources based on cylindrical,two dimensional periodic surface lattice structure are preliminarily performed in theory and simulations.Therefore,in this dissertation,a microwave source based on cylindrical,two dimensional periodic surface lattice structure is further explored and studied.Firstly,theoretical research is performed.A mathematical model of cylindrical,two dimensional periodic surface lattice structure is established.The related theoretical expressions of dielectric cylindrical waveguide model are deduced.Main conclusions are as follows.The Bragg condition serves as the coupling condition for the eigenmode.Parameters of the cylindrical dielectric waveguide model are related to the operating frequency and lattice parameters.According to the amplitudes of voltage,operating states can be divided into high and low voltage states and different operating states need to match the geometrical structures of the devices.Then,high frequency characteristics of this structure are studied.The results show that the eigenmode in the structure can be described as a superposition of a volume wave TM0 l and a surface wave HE7 n.With the eigenfrequency elevating,radial variations of volume and surface waves increase while azimuthal variations remain unchanged.Besides,volume waves in eigenmodes are near cut-off waves;Two dimensional periodic surface lattice structure can be simplified as a cylindrical dielectric waveguide model to analyze.Surface waves and volume waves are coupled via the surface currents on the surfaces of lattice.The coupling can offer an explanation for this structure to maintain mode selection under large overmoded ratio conditions.Finally,the VSim software is used to conduct 3D Particle-in-Cell simulations.The results indicate that,with the voltage of 170 kV,the current of 20 A,the guiding magnetic field of 1.8T,the electron beam radius of 2.0mm,a microwave power output of 50 kW at the frequency of 923 GHz can be observed.The longitudinal momentum space distribution of the electron beam is chaotic,whereas the current modulation phenomenon still exists.The electron beam's azimuthal motion is not negligible in the cavity.Although there exist disparities between the theoretical derivations of beam-wave synchronization condition and the results of simulations,working characteristics of the device can still be revealed.By studying the influences of voltage,magnetic field,electron emitter position on the microwave outputs,guidance for the design and optimization of the device can be obtained.