Analysis And Design Of The Photonic Band Gap Cavity

In high-power gyrotron devices, to overcome the difficulty of small dimensions and high thermal loading when operating at the fundamental mode, devices often use an overmoded structure. The use of photonic band gap structures has been experimentally shown to be a promising approach to realize the mode-selective cavity. In the paper, we researched the issues for the Q value and mode competition of the photonic band gap cavity further. Then, we summarized the method of designing photonic band gap cavity with specific eigen-frequency.Firstly, two methods are proposed to solve the problem of the high Q value in PBG cavity. The result shows that the Q value can be effectively controlled by loading media structures or inserting dielectric perturbation in PBG cavity. Then, the mode selection is calculated respectively for the two methods. It is shown that the two methods both can control the Q value without reducing the mode selection or changing the electric field distribution of TE04 mode. In addition, two competing modes are cleared in the method of dielectric perturbation, which improves the mode selection at the operation frequency of TE04 mode.Then, for the problem of mode competition, it was achieved to optimize the operational point in PBG cavity and reduce the competing modes of TE04 from 5 to 2 based on analyzing the global band gaps of TE modes and mode distribution characteristics. The optimized design reduces the mode competition substantially, but also radius of metal rods is expanded much to bear more thermal loading, which is helpful to increasing power capacity. On the other hand, the operational parameters, such as guiding radius of electron beam and the density of magnetic field, are set at the optimum value for single-mode operation after analyzing the nonlinear wave interaction. Finally, more integrated electric field distribution of TE04 is made to verify the validly of the design.In the end, the TE modes distribution of high-order photonic band gap is calculated. The results show the photonic band gap structure has mode selection, but there also exist some competing modes at the high frequency from 200 to 300Ghz. Therefore, we combine the photonic band gap structure and the bean-wave interaction nonlinear theory in gyrotron to study 270GHz third harmonic PBG gyrotron of TE15 mode. The single mode gyrotron is achieved with the out put power 43kW and the wave-beam interaction efficiency 14.3%.