Studt On Electron Optical System And Beam-wave Interaction In Tapered Cavity For THz Gyrotron

Terahertz science and technology are topics of great contention today for thesuperiorities in fields of medical care, safety inspection, information technology andbasic research. Terahertz Gyrotron oscillators are one of promising sources of highpower of millimeter wave and sub-millimeter wave electromagnetic radiation. In orderto combine with the projection of our team, in this dissertation, several key issues, suchas no-operating mode suppression, high-order harmonic terahertz gyrotron withhigh-power, high-efficiency from single mode, the problem of two cyclotron beamswith equal velocity in the same magnetic field distribution and electron optical structure,the influence of beam velocity spread, the electron optical system optimal design, andthe connection between the high frequency structure and competition mode, are furtherresearched by studying on the relative theories of gyrotrons and the difficulties indeveloping the terahertz gyrotrons. Some innovative achievements are made.Based on the self-consistent nonlinear theory for gyrotron, a kind of graduallytapered cavity with the ability to effectively suppress the parasitic modes is designed,which can improve the purity of the operating mode up to27dB. On account of the stateof the need of the terahertz wave at present, the gradually tapered cavity is adopted toset up a0.4THz second harmonic gyrotron, the results, which is of numerical simulationbased on the self-consistent nonlinear theory, show that the output power could be over150kW with the efficiency of about30%at an electron beam current of9A and avoltage of60kV. A0.6THz second harmonic gyrotron with the same type of structureand operating mode is also designed, whose interaction efficiency and output power areabout21%and over100kW, respectively. Moreover, there are some researches in thetheory of beam-wave interaction of gyrotrons, the electron optical system and theprogram development in the paper. The main work and contributions are shown asfollows:1. Based on the coupled-wave theory, a set of generalized telegrapher’s equationsin the first-order differential form for studying the high frequency structure andbeam-wave interaction of gyrotrons is built after the generalized telegraphist’s equationsfor the cylindrical cavity with gradual tapers are studied. Considering the character ofthe gyrotron cavity, the first-order differential equations are adopted to study the high frequency structure and beam-wave interaction of gyrotrons, which will make the theorymore perfectly.2. Based on the muti-mode self-consistent nonlinear theory, the theory and methodare advanced to explain the parasitic modes suppression. The high-order TE modeshould be used as the operating mode of a terahertz gyrotron for size effect, which leadsto strong mode competition. Competition modes suppression becomes the point. A kindof gradually tapered cavity with the ability to effectively suppress the parasitic modes isdesigned by analyzing the coupling coefficient and studying the first-order differentialtelegraphist’s equations.3. After dealing with the telegraphist’s equations with numerical method, theprograms are designed for cold cavity and muti-mode, self-consistent nonlinearinteraction simulation, and electron optical system optimal design. The main works ofstudying the millimeter wave and sub-millimeter wave gyrotrons are focus on the modelbuilding and efficiently solving by computer. Based on the differential equations forcold cavity, the muti-mode self-consistent nonlinear theory and vector programming forsteady-state self-consistent nonlinear interaction simulation, and PIC for electron opticalsystem optimal design, the corresponding programs are designed.4. The models of velocity spread and guiding centre spread of the electron beamare set up. And they are written into the muti-mode self-consistent nonlinear interactionprogram to study the influences to the interaction efficiency. There are lots of factorsaffecting the characters of gyrotrons, besides the high frequency. Beam quality willmake great diffenence to gyrotrons’ action. To make the simulation close to practice, themodels of velocity spread and guiding centre spread of the electron beam are set up.And the effects of the spreads have been discussed.5. In order to ensure the validity of the theoretical derivation and the reliability ofthe programs, a great deal of verification activities have been carried out. After theprograms for cold and hot cavity are designed, design and experiments of gyrotrons aredone to prove the correction of the codes besides seting up the calculating models fromthe authority papers and contrasting with professional software.6. By studying the theory of Magnetron Injection Gun, the fact of two cyclotronbeams designed with equal velocity in the same magnetic field distribution and electronoptical structure is proved. And a W-band, single-anode Magnetron Injection Gun isdesigned, which is entirely consistent with the demand for engineering. 7. By the programs, the studies of mode selection, high-frequency structure, modecompetition and nonlinear interaction of the gyrotrons operating at the frequency of94GHz,0.4THz and0.6THz have been respectively carried out in detail.