Research On High-frequency Geometry Of A 170GHz Coaxial Gyrotron

Gyrotron oscillators(or gyrotrons) are a kind of relatively rapid developmental tubes in the family of tubes. From the birth of the first gyrotron to now, their frequency, power, and efficiency have been a huge boost after 50 years of development. Due to the conventional gyotrons with hollow cylinder as a cavity, though making great progresses, they encounter some difficalties in developing forward higher frequency and higher power capacity. First, in order to increase the output power, the higher order modes must be taken and the radius of cavity must be increased. However, with the radius of cavity increasing, mode spectrum becomes very denser and mode competition becomes more intense in cavity, which will reduce the stability and efficiency of gyrotron. Secondly, with the output power of gyrotron increasing, the cavity wall Ohmic loss increases. The existing cooling technology requirements that the density of cavity wall Ohmic loss must be less than 4KW/cm2, when the gyrotron operates at long pulses or continuous wave. All of these restrictions severely hamper the increase of output power of gyrotron. In order to resolve these problems, some scholars proposed coaxial cavity to rarefy the mode spectrum and reduce mode competition. Comparing to the conventional gyrotrons, there is a coaxial inner conductor with radii gradually changing smaller along longitudinal in a coaxial gyrotron. For improving the performance of coaxial gyrotrons, some coaxial inner conductors have axial grooves on surface. Owing to structural changing, coaxial gyrotrons show some different advantages comparing to the conventional gyrotrons. First, in coaxial cavity, the mode eigenvalue(for TEmn modes, eigenvalue is xmn and TMmn mode vmn.) is no longer constant, but the function of the ratio of the outer to inner conductor radius C = R/Rin. The diffraction quality factor Qd of each mode can be changed by setting a suitable inclination of inner and outer radius. Therefor, the start oscillation current of operating mode can be reduced and that of competition modes increase(the start oscillation current of mode is inversely proportional to the Qd), which play a role of mode selection. Furthermore, since the inner conductor is fixed in the vicinity of the moving electron beam, it is possible to lessen the constrant of voltage depression and limiting currents. Thus, the study of coaxial gyration is receiving increasing attention of many researchers around the world.The study in this dissertation was combined with the XXX RESEARCH of "NATIONAL MAJOR SPECIAL PROJECTS OF SCIENCE AND TECHNOLOGY" of our team and the characteristics of 170 GHz improved gradually tapered cross-section open-cavity were studied at first. From a simple and effective method of mode selection, the TE31,12 mode was selected as operating mode and a set of source codes was developed using matlab software. The effects of circular or elliptical arc on the diffraction qulity Qd value of a 170 GHz improved gradually tapered cross-section open-cavity were studied thoroughly. In order to verify the correctness of conclusions and computer codes, another 170 GHz similar type resonator operating with TE0,1 mode was simulated by HFSS and the developed computer codes, respectively. The same conclusions were abtained. Thus, the correctness of the conclusions was proved and the developed computer codes were also verified.Based on the study of improved gradually tapered cross-section open cavity, the start oscillation current and coupling coefficient of a 170 GHz smooth coaxial megawatt-level gyrotron were studied in the paper. Taking into account Ohmic loss and velocity spread and basing on the single-mode approximation, a high computationally efficient, fully vectorized program was developed using matlab software according to the self-consistent nonlinear theory. Applying the developed program, the beam-wave interaction of a 170 GHz, TE31,12 smooth coaxial megawatt-level gyrotron was simulated systematically. Then the effects of velocity spread of electron beam, resistivity, orbital-to-axial velocity ratio, magnetic field, beam current, and beam voltage on the efficiency of interaction were investigated. In addition, the effects of the geometric characteristics of inner rod on resonant frequency, quality factor Q, and beam-wave interaction efficiency were researched.Due to the limitation of precision of processing and equipment, a very small misalignment of inner rod is inevitable. The tiny misalignment of inner rod completely changed the boundary conditions of electromagnetic fields so that overall performances of the gyrotron have changed, which are often encountered in the practical engineering applications. Therefore, the dispersion equation and nonlinear self-consistent equations of smooth coaxial gyrotrons with a misaligned inner rod are derived and established in the dissertation. The effects of the misalignment of inner rod on the eigenvalue, quality factor Q, beam-wave interaction efficiency, and resonant frequency are deeply studied. Some valuable conclusions that can provide a reference to the applied research were acquired.Comparing to the smooth coaxial gyrotrons, the corrugated coaxial gyrotrons have stronger ability of suppression of mode competition and more stability of operating, theoretically. Thus, on the basis of the study of smooth coaxial gyrotrons, a 170 GHz, TE31,12 corrugated coaxial megawatt-level gyrotron is studied and designed in the paper. A set of source codes was developed and the start oscillation current of corrugated coaxial gyrations was calculated as well as the coupling coefficient. Then the initial operating parameters were identified. On the basis of considering velocity spread and Ohmic loss, a high computationally efficient, fully vectorized program was developed according to the nonlinear self-consistent theory. Applying the developed program, the beam-wave interaction of a 170 GHz, TE31,12 corrugated coaxial megawatt-level gyrotron was simulated systematically. Then, the effects of velocity spread of electron beam, resistivity, spread of guiding centers radius, orbital-to-axial velocity ratio, magnetic field, beam current, and beam voltage on the efficiency of interaction were comprehensively investigated. Besides, the effects of the geometric characteristics of inner rod on resonant frequency, quality factor Q, beam- wave interaction efficiency were studied.The geometry of corrugated coaxial gyrotrons is more complicated. So the processing and equipment of corrugated coaxial gyrotrons are more difficult. Because of the processing errors, the misalignment of inner rod is inevitable. Therefore, the dispersion equation and nonlinear self-consistent equations of corrugated coaxial gyrotrons with a misaligned inner rod are derived and established in the paper. The effects of the misalignment of inner rod on the eigenvalue, quality factor Q, beam-wave interaction efficiency, and resonant frequency are studied in detail. The conclusions of research provide a valuable reference to the application research.